Sign and geometric meaning of curvature

M. G R O M O V Institut des n a u t e s ]~tudes Scientifiques

SIGN

AND

GEOMETRIC

MEANING

OF

CURVATURE

(Conferenza tenuta il 14 giugno 1990)

SUNTO. Q u e s t a m o n o g r a f i a ~ u n a v e r s i o n e e s t e s a di u n a m i a C o n f e r e n z a t e n u t a i n M i l a n o n e l l ' a m b i t o delle Lezioni L e o n a r d e s c h e . E ' u n t e n t a t i v o di r i v e l a r e ai n o n i n i z i a t i il m e c c a n i s m o di sviluppo della g e o m e t r i a R i e m a n n i a n a s e g u e n d o le t r a c c e di r e l a t i v a m e n t e poehe idee dalle f o n d a m e n t a al t e t t o dell'edificio. -

-

CONTENTS. w 0.

T h e second f u n d a m e n t a l f o r m a n d c o n v e x i t y in t h e E u c l i d e a n space . Pag.

w 89

Generalized

w 1.

Recollection on l e n g t h , d i s t a n c e a n d R i e m a n n i a n m e t r i c

convexity

.

.

.

.

.

10

>>

18

>>

29

w 2.

E q u i d i s t a n t d e f o r m a t i o n a n d t h e sectional c u r v a t u r e K(V)

>>

38

w 2 89

I n f l u e n c e of K(V) on s m a l l b a l l s i n V .

>~

46

w 3.

M a n i f o l d s w i t h p o s i t i v e sectional c u r v a t u r e .

~

49

w 3 89

Distance function and Alexandrov-Toponogov theorem .

>>

59

w 3~.

S i n g u l a r m e t r i c spaces w i t h K _>_ 0 .

>>

61

w 3%.

T h e s p h e r e t h e o r e m a n d e q u i d i s t a n t d e f o r m a t i o n of i m m e r s e d hypersurfaces . . . . . .

w

N e g a t i v e sectional c u r v a t u r e

w

Ricci c u r v a t u r e .

.

.

.

.

.

.

63 70 ~

w

Positive scalar curvature

w 6 89

Spinors and the Dirac operator .

.

w

The curvature operator and related invariants

.

.

.

.

.

75

>>

87

>>

95

>~

101

w 7 89

H a r m o n i c m a p s of s u r f a c e s a n d t h e complexified c u r v a t u r e K r

w 7~.

H a r m o n i c m a p s i n t o m a n i f o l d s w i t h K r ___ 0

>>

117

w 7%.

M e t r i c classes defined b y i n f i n i t e s i m a l convex cones

>>

120

106

10

M. GROMOV

The c u r v a t u r e tensor of a Riemannian manifold is a little monster of (multi)linear algebra whose full geometric meaning remains obscure. However, one can define using the curvature several significant classes of manifolds and then these can be studied in the spirit of the old-fashioned s y n t h e t i c g e o m e t r y with no appeal to the world of infinitesimals w h e r e c u r v a t u r e tensors reside. A similar interplay between infinitesimal quantities and visual f e a t u r e s of geometric objects a p p e a r s in all corners of geom e t r y and analysis. The simplest example is provided b y tht equivalence of the t w o definitions of a m o n o t o n e function df > 0 ~ dt . .

f ( t l ) < f(t2) f o r t~ < to . . . .

.

Then the infinitesimals of t h e second order bring along a geometrically more interesting phenomenon of c o n v e x i t y .

d~f > 0 ~ dt ~ -

f

(1 -

(tl + t:)

)

1

< -2- (f(tl + f(t2))

_

.

Our next example lies at the v e r y verge of the Riemannian domain so we look at in a g r e a t e r detail.

w 0.

THE

SECOND

FUNDAMENTAL FORM

AND

CONVEXITY

IN

THE

EUCLIDEAN SPACE.

The basic infinitesimal invariant of a smooth h y p e r s u r f a c e W c ~ " ( means codim W = n - - d i m W = l ) is the de]

f o r m 17 = 17 W which is the field of q u a d r a t i c on the tangents spaces T ~ ( W ) c T ~ ( ~ ' ) = ~ defined as

second fundamental

forms//~ follows.

AFFINE DEFINITION OF H. Move w to the origine of ~ " by a parallel translation of W in ~ " and compose the resulting embedding W c ~ with the linear quotient m a p ~ ' - - > ~ ' / T w ( W ) ---- N w . Identify the a,j

(one dimensional linear) space Nw with ~ and thus obtain a function, s a y p ~ pw : W--> ~ , whose differential D p vanishes at w (by the definition of T ~ ( W ) ) . T h e n define the second d i f f e r e n t i a l De p t h a t is a quadratic form on T ~ ( W ) , such t h a t f o r every pair of vector fields 01 and 02 on W the (second Lie) derivative of p at w satisfies 01(02 p) ( w ) =

(D: p) (01(w), (0~(w)).

SIGN AND GEOMETRIC MEANING OF CURVATURE

11

(The existence of such De p follws f r o m Dp = 0 by a simple computation). This construction applies to all w E W and gives us o u r / / f o r / / w = D e P, t h o u g h t of as a quadratic f o r m on T(W) with values in the normal bundle N = T w ( 1 R ~ ) / T ( W ) over W, where Tw(1R~) denotes the restriction T(IR~)IW. Here is the f a m i l i a r picture for n = 1.

W

N w "-R F i g . 1.

Geometrically speaking, Hw measures the second order infinitesimal deviation of W f r o m the a f f i n e subspace T w ( W ) c ~ . In particular, if W is connected, then the vanishing of / / on W is equivalent to W lying in a hyperplane, as everybody knows. A more interesting relation between H and the (affine) geometry of W reads

The farm II is positive semi-definite if and only if W is convex. To m a k e it precise we have to choose a coo~ientation of W t h a t is a w a y to distinguish between two components into which W locally divides 1R~. This is customary done with a transversal (e.g. normal) vector field v along W. Such a p r e f e r r e d field, once chosen, is called

12

Yl. GROMOV

interior looking and the p a r t of ~ " w h e r e v looks into it is called the interiar of W.

w

terior

r

W

w

F i g . 2.

Notice t h a t such a field also defines an orientation of the normal f i b e r s N~ and so one can speak of t h e sign of the f o r m s llw with values in N ~ . N o w we invoke the following DEFINITION OF CONVEXITY. W is called convex at w if it is contained in the interior half-space T + c ~ " bounded b y the hyperplane T~(W) ~ "IR". F o r example, the curve W in Fig. 2 is convex at w b u t not at w'. Yet it becomes locally convex at w' if w e change the coorientation b y inverting the sign of ~. In the language of the projection p : W--~ N~ ~ ~ the convexity claim is p >__0 which well agrees with the positivity of I I ~ - - D e p . In fact, positive definiteness of H~ obviously implies local convexity (i.e. the convexity of small neighbourhood U c W of w) of W. B u t it is slightly h a r d e r to derive local convexity of W f r o m positive semidefiniteness of H in a neighbourhood of w (rather t h a n at w alone). Also, the proof of the global convexity of W is not quite trivial. Recall t h a t the global convexity, (i.a the convexity

SIGN AND GEOMETRIC MEANING OF CURVATURE

13

at all w ~ W) follows f r o m H >_ 0, provided W is a closed connected hypersurface, w h e r e 1Rm (not only embeddings) and it generalizes to nonEuclidean ambient spaces endowed with affine connections. However, the affine n a t u r e of this definition m a k e s it poorly adapted to the needs of the Riemannian geometry w h e r e the m a j o r object of study is the distance function associated to the Riemannian structure. With this in mind we t u r n to our second definition o f / / which is based on the following important notion.

EQUIDISTANT DEFORMATION. Let W be a cooriented h y p o r s u r f a c e in 1R" and denote by J(x), x ~ 1R" the signed distance function to W. That is ~(x)-----dist(x, W) for the exterior points x and 5 ( x ) ~ - - - d i s t ( x , W) in the interior. Notice t h a t in general the distinction between interior and exterior points makes sense only locally n e a r W and then J(x) is defined only in some small neighbourhood of W. Also recall that dist(x, W) ~ - infdist(x, W) def

wE ~r

f o r the Euclidean distance dist(x, w ) ~

l]x--wll ~

< x --w, x --w

>

1/2.

Then we look at the levels of ~, t h a t are

w~ ~--~-l(e)~ {x ~ 1R"I~(x)-- s}, and call them e-equidistant hypersurfaves or e-equidistaz~t defo~vnatians of W ~ Wo. It is easy to show that f o r small e the manifolds W~ a r e smooth if Wo is smooth, b u t W~ m a y become singular f o r large c. In fact w e shall see in a minute t h a t the i~ward (i.e. e < 0) equidistant deformation necessarily produces singularities f o r every convex initial h y p e r s u r f a c e Wo. (See fig. 3 below). F o r example such a deformation of the round sphere W o ~ S ~ - l ( r ) c 1R" of radius r

14

M. G R O M O V

brings Wo to the center of the sphere f o r E - ~ r . S ' - l ( r ~- ~) f o r all ~ >__~ r ) .

(Here W =

r

We

Wo

interior

N~~~~ w

Fig. 3.

N e x t we consider the lines Nw in 1R" normal to W at t h e points w E W. It is easy to show that every such line meets each W~, f o r small e, a t a single point denoted w, or (w, e) E W, and the resulting m a p d~ : W--> W~ for d ~ ( w ) = we is smooth. (In fact, d~ is a diff e o m o r p h i s m ; moreover, Nw is normal to W, at w , , as elementary differential g e o m e t r y tells us). N o w w e a r e going to define the second quadratic f o r m as the rate of change of the lengths of curves C c Wo as we pass f r o m Wo to infinitesimally close h y p e r s u r f a c e W~. W e recall t h a t the length of C is determined by (integration of) the length of the t a n g e n t vectors of C which is given, in turn, b y the first f u ~ t a l f o r m g on W t h a t is j u s t the restriction of the Euclidean scalar product (which is a quadratic f o r m on ~ ) to the t a n g e n t spaces T~(W) c T~( IR ~) = 1R", w E W. In other w o r d s g is the Riemannian metric on W induced f r o m the standard Riemannian metric on 1R~+1. Geometricly speaking, _ 0,

where 2, denote the principal curvatures of W. This is equivalent in terms of We to the monotonieity of the volume element of W~ rather than of the induced metric. Geometrically this monotonicity says that the (n--1)-dimnsional volume of every domain U c W E W0 increases as we pass to W~ with E > 0. More precisely the domains U, ~ - d, (U) c W~ satisfy the relation dVol U~ _> 0 at E ~ O . dE The positivity of the mean curvature of the boundary W ~ aV of a domain V c IRn implies the following property of the signed

SIGN AND GEOMETRIC MEANING OF CURVATURE

19

distance function 5(v)

----- - -

dist(v,

W ) ~-- - -

inf dist(v,

w).

~'E W

The function 5(v) /s subharmonic, Aa(v) >_ 0 for all v E V. Notice, t h a t the function ~ is not everywhere smooth in V and at the singular points the sign of the Laplace operator A m u s t be understood in an appropriately generalized sense. The inequality A5 >_ 0 at the smooth points of 5 can be easily derived by applying the t u b e formula to the levels of J which are j u s t our equidistant h y p e r s u r f a c e s W~, and at the singular points one needs an e x t r a approximation argument. (We shall come back to t h e positive mean c u r v a t u r e in the more general f r a m e w o r k of manifolds V D W with Ricci V _ 0 w h e r e the equidistant deformation of h y p e r s u r f a c e s provides the m a j o r tool f o r the study of such V (see w 5). k-CONVEXITY. A cooriented h y p e r s u r f a c e W in ~ " is called k-convex for some integer k ~-~ 1, 2 ..... n - - 1, ~ dim W, if among its n - - 1 principal c u r v a t u r e s ~ at least k are >_ 0. Then W is called strictly k-cinvex if k among ~ are > 0. F o r example, ( n - - 1 ) convexity is the same as the o r d i n a r y convexity. Notice t h a t k convexity is invariant under pro]ctive transformation of ~ which allows us to extend the notion of k-convexity to the sphere S ~ and the projective space P~ which a r e locally projectively equivalent to ~ . Then we observe t h a t k-convexity is stable under small inward equidistant deformations of W in ~ " , as follows f r o m the tube formula. (This is also t r u e for large deformation with an a p p r o p r i a t e generalization of k-convexity to non-smooth hypersurfaces). F u r t h e r m o r e , the i n w a r d equidistant deformation performed in S n with respect to the spherical metric also preserves k-convexity since S ~ has (constant) positive c u r v a t u r e w h e r e the generalized t u b e formula (see (**) in w 2) leads to t h e desired conclusion (compare the discussion in w 2 following (**)). Moreover, since t h e c u r v a t u r e of S ~ is strictly positive, an a r b i t r a r i l y small equidistant deformation in S " m a k e s every k - c o n v e x W strictly

20

M. GROMOV

k-convex. As both notions are projectively invariant, we conclude t h a t every k-convex hypersurface in ~ admits a strictly k-convex approximation. So an elementary Riemannian geometry of positive curvature leads to a purely Euclidean conclusion. More interesting global properties of closed k-convex hypersurfaces can be obtained with elementary Morse t h e o r y of linear functions f : ~ n _ > ~ restricted to W. I f the critical points of such f are non-degenerate, which is the case f o r generic linear functions f, then all critical points of f on W have indices either >_ k or _ n - - 1 ~ k. Therefore W admits a cell decomposition with no cell of dimension e in the interval n - - 1 - - k < e < k. EXAMPLES. (a) I f k ~ n - - 1 , the only possible cells are of dimension 0 or n - - 1 and the above reduces to the s t a n d a r d p r o p e r t y of locally convex closed h y p e r s u r f a c e s W in ~ . - 1 : these are homeomo~phic to disjoint unions of spheres (we do not assume W is connected). (b) Let k - n - - 2 . Now the above cell restriction becomes nonvacuous s t a r t i n g f r o m n ~ 5. I t says t h a t the cells have dimensions 0, 1, n . - 2 and n - 1. An obvious consequence of t h a t f o r n _ 5 is the freedom of the f u n d a m e n t a l group and the vanishing of He(W) f o r i ~ 0 , 1, 2, n - - 2 , n - - 1. general, the Morse t h e o r y of W tells us something non-trivial only if n _ 2k + 1. However, there are non-trivial restrictions for all k_> 1 on the domain V c ~ bounded by W. In

/f a bounded domain V in ~n has smooth k-convex boundary, then V admits a homotopy retraction onto an e-dimension subpolyhedron in V for e ~ - - n - - l - - k . This is immediate with the Morse t h e o r y applied to f on V which is a compact manifold with boundary. I t is not h a r d to see t h a t the converse is also true. I f V can be built in ~ n by sequentially a t t a c h i n g handles of indices H e (V) is in]ectiv fo~" e ~ n - - 1 - - k and codim X ~ d _< n - - k. EXA~PLP.. I f k ~ n - - 1 and d , ~ - - n - - 1 the above says, in effect, t h a t the intersection of every connected component of V with a line is connected. In other words, a connected domain with locally convex b o u n d a r y is convex in the o r d i n a r y sense. It is easy to see t h a t the Lefschetz p r o p e r t y is characteristic for k-convexity. If a compact domain V c 1R~' w i t h smooth boundary has Hr injective for all affine (~ + 1)-dimensional subspaces X in ~n then (the boundary of) V is k-convex for k ~ n - - e - - 1 . Now, one can accept the above injectivity as the definition of k-convexity w i t h o u t any smoothness assumption on W E 0V. The f i r s t (obvious) theorem of the resulting theory reads I f V1 and V,_ are k-convex in ~ is also k-convex.

then the intersection V1 N V,2

Notice t h a t the homological definition of ( n - 1)-convexity allows disconnected domains in ~ n with locally convex boundaries. The connectedness condition generalizes for k < n - 1 by the requir e m e n t t h a t Ho_(V ~ X) ~ 0 f o r e ~ n - - 1 - - k and for all (e + 1)dimensional subspaces X c ~n. F o r example, in the next-to-convex case of k ~ n - - 2 this requires the intersection of V with every plane X c ~ to be simply connected.

22

M. GROMOV

Finally w e indicate y e t another k-convexity condition by the analogy with the classical convex case, for every point x in ~ outside V there is a k-dimensianal affine subspave passing through x and missing V. This is a t r u l y global p r o p e r t y of V c ~ which is much stronger than the above k-convexity and which becomes indispensable if w e w a n t to reconstruct V b y its linear projections to ( n - - k ) - d i m e n sional subspaces in ~n. IMMERS]~) k-CONVEX HYPERSURFACES. H e r e our W in ~ is allowed to have self-intersection. This means, W is a smooth ( n - 1)-dimensional manifold which comes along with an i~mersion W--> ~ n that is a locally diffeomorphic map. If W is oriented as an a b s t r a c t manifold, then the immersed W c ~ " becomts cooriented if w e fix once and f o r ever some orientation of ~ . In this case we can define the second fundamental f o r m and the notion of k-convexity of W. One sees in fig. 5 below a locally convex immersed closed curve in ~2. Notice t h a t the image of this immersion is singular at the double points and in no sense convex

Fig. 5.

A classical convexity theorem claims t h a t every locally convex closed connected h y p e r s u r f a c e in ~ is embedded (i.e. has no double

SIGN AND GEOMETRIC MEANING OF CURVATURE

')3

point) for n _> 3 and thus bound a convex domain in ~ - . The latter statement generalizes to the k-convex case as follows. Let W c ]R" be a closed immersed k-convex hypersur]ace for n

k > - ~ . Then W bounds an immersed manifold V in IRn of d i m e ~ i o n n(i.e. W bounds V as an abstract manifold and the immersion o] W~V to IR" extends to ~n immersion V--> ?R"). The construction of V is achieved by following the levels of a linear functions on W that are the intersections W fl Xt for a family of parallel hyperplanes Xt c ~". These intersections are ( k - - 1 ) convex for non-critical t where X~ is transversal to W and the intersection W t ~ W N X t is a smooth immersed hypersurface in X ~ - - - ~ , - 1 . As we move t in a non-critical interval this hypersurface moves by a rgular homotopy (i.e. remaining immersed) but the selfintersection pattern of W~ m a y change with t. However, the inequan lity k __E ~ 0 implies V is compact, whenever it is complete as a metric space. This gives an (effective !) partial solution to the above diameter problem f o r coverings V--> V.. Conversely, if the sectional c u r v a t u r e of 11o (and hence of V) is everywhere negative then V is non-compact and its diameter is infinite (see w167 2 and 4). Unfortunately, most manifolds have sectional c u r v a t u r e of variable sign and t h e above criterion does not apply. B u t the constant sign of the sectional c u r v a t u r e does occur f o r certain interesting examples including some homogeneous Riemannian manifolds (yet solvable groups mentioned above do not fall into the constant sign category). RECAFTURING g

FROM T H E

METRIC. T h e r e

is

the following simple

w a y to reconstruct g f r o m the corresponding distance function. F o r a given point v we define the function ~(v') on V by ~ ( v ' ) - ~ (dist (v, v')) 2. W e observe t h a t ~ is smooth at v ' ~ v and the differential

SIGN AND GEOMETRIC MEANING 0 F CURVATURE

33

Dq vanishes

at v. Then the second differential D20 is a well defined quadratic f o r m on Tv(V) which equals (by an easy a r g u m e n t ) t o g on Tv.

Thus we established the equivalence of the three basic viewpoints on a Riemannian s t r u c t u r e : infinitesimal, f o r g a quadratic f o r m on T(V); path-theoretic, for the length function on curves and the metric (or distance function) point of view. Often one does not distinguish between the three structures and apply the expression to all of them. It should be noted t h a t although the three s t r u c t u r e s a r e formally equivalent t h e y represent objects f r o m quite different worlds. F o r example, g is a tensor, specifically a quadratic form, whose positivity m a y be sometimes forfaited. B u t then nothing remains of the metric s t r u c t u r e attached to g (except f o r some residual terminology such as >). On the other hand one m a y have a ( n o n - R i e m a n n i a n ) m e t r i c space with r a t h e r n a s t r y singularities w h e r e the infinitesimal approach becomes h a r d to persue. So it seems we are quite lucky in having so diverse notions and ideas merging together in the Riemannian stream. RIEMANNIAN VOLUME. Each Riemannian manifold V of dimension n carries a canonical measure, which is (uniquely) characterized b y the following t w o axioms. IVf0NOTONICITY. If t h e r e exists a distance decreasing surjective map between t w o n-dimensional manifolds, say f : V1 --> V2, then Vol V2 _ denotes the total volume (or mass) of t h e m e a s u r e on the manifold in question. NORMALIZATION. The unit cube in ~ " has volume ~--- 1. Notice t h a t the above definition makes sense on the level of the length s t r u c t u r e and of the Riemannian n o r m on T(V) as well as for the distance function. In fact, the following three conditions on f a r e obviously equivalent,

8omi~r~o

Matemati.co

9 Fi.~t, co - 3

34

M. G R O M O V

(i) f is distance decreasing, (ii) f decreases the length of curves, (iii) t h e differential of f decreases the length of the t a n g e n t vectors (here we m u s t additionally assume f differentiable). The existence and uniqueness of the Riemannian measure for the continuous structures g on V immediately follows f r o m the obvious infinitesimal ~pproximation of g at each point vo E V by a Euclidean metric go. Nomely, if we t a k e some local coordinates u~ ... u~ in V n e a r vo, then g defines a Euclidean metric go on the coordinate neighbourhood U by go(a~(u), O,(u)) = g (a,(v~), 0j(vo)),

a w h e r e 0 ~ - - - - ~ - and u r u n over U while v~ E U is fixed. It is clear t h a t (U, go) is isometric to ~ and t h a t go approximates g a~ Vo wi$h zero order. T h a t is f o r every E :> 0 t h e r e exists a neighbourhood U~ c U of v, such t h a t ( 1 - - e ) go ]12 which is computed at each v ~ V with the (Euclidean !) metrics g l ( v ) on To(V) and g2(w) on T~(V2), w = f ( v ) E V2, as [Det I of t h e differential Dr: T,,(V~)--> T,~(V2). T h a t is t

]Jac f] ~ ]Det f] ~ (Det DD*) ~ , w h e r e D ~ D f and D* is t h e adjoint of D with respect to gl(~) and g2(w). Then one defines t h e Riemannian volume of every small neigh-

SIGN AND GEOMETRIC MEANING OF CURVATURE

35

bourhood U c V b y taking a diffeomorphism f : U' -~ U f o r some U' c ~ and b y setting IJac fl du'

Vol~ U ~ j U"

f o r the Euclidean volume element du'. Notice that the Riemannian s t r u c t u r e restricts to submanifolds W c V of dimension k < ~ ~ dim V and then w e have the Riemannian volume Vol~ on W. In particular, f o r k ~ 1 we come up again with the length of curves, t h a t is Vol~ W f o r dim W - ~ 1. F I R S T ORDE~ I N F I N I T E S I M A L APPROXIMATION OF g

BY g o .

Since g is

Euclidean at v0 with zero order one might t h i n k t h a t non-flatness of g (i.e. the deviation f r o m being locally Euclidean) can be measured b y the f i r s t derivatives of g~j--g(O~, aj) in some local coordinates. Then, surprizingly, this does not w o r k because t h e r e always exist particularly nice local coordinates u~, ..., u~ around v, called geode~ic coordinates, such that

~ g~j(Vo) ~ 0 f o r all i, ], k ~ 1, ..., n, w h e r e we need the metric g to be Cl-smooth at vo. In fact, a little thought explains how this could happen. When w e change coordinate systems which a r e n-tuples of functions ~ : V--> ~ , we observe that the change of ak g~j(vo) is determined by the second derivatives of u~ at vo. Altogether, there are

n~(n + 1) 2

of these derivatives (for n

functions ~ ) which a r e (somewhat miraculously) the same in n u m b e r as the first derivatives of n(n 2~ 1) functions g~. at v~. Then it is easy to believe (and not hard to prove) t h a t one can a d j u s t the second derivatives of % such t h a t O~g,s become zero. In fact. one can prove t h a t these second derivatives a r e uniquely determined b y the condition O~g~s(v~)~-O. Namely, if u~ and u~ a r e geodesic coordinates, such t h a t O~ u~ ~ 6!~, then the second derivatives of u~ with respect to ~ vanish at vo.

36

M. GROMOV

The Euclidean metric go on U constructed as earlier, b u t now with geodesic coordinates u~, approximates g at Vo with the fixst

order, --e2go 0 (see, e.g. [Bu-Kar]). But we are concerned with more subtle scalar invariants which are not automatically positive and whose sign conveys a non-trivial geoxnetric information about V. As a m a t t e r of comparison we m a y look again at the second f u n d a m e n t a l f o r m [I v of V c 1R"+1 whose n o r m ]]IlVll measures non-flatness of V in 1R~+1 but where the sign of the eigen-

values ~ of [iv (which are scalars i n v a r i a n t under rigid motions tells U S much more t h a n the sheer size of H. ( N o t i c e of V in ~,§ n

that ~

~.~=

III/11~).

i~l

In the Riemannian case there are m a n y similar scalar invariants (which correspond to functions on the space of c u r v a t u r e tensors on T(V) i n v a r i a n t u n d e r fiberwise orthogonal t r a n s f o r m a t i o n s ) but only few of t h e m have found a meaningful geometric interpretation so far. Most studied among these are the sectional curvature K(V), which is, in fact, a function on the Grassmann bundle of t h e t a n g e n t 2-planes in V, the Ricci curvature, which is a quadratic f o r m on V and the scalaz curvature which is a function on V. These curvatures are defined and studied in the following w167 2-6.

38

w 2.

M. 6ROMOV

EQUIDISTANT DEFORMATIONS AND THE

SECTIONAL CURVATURE

g(v). F o r every h y p e r s u r f a c e W in a Riemannian manifold V : - (V, g) (recall t h a t r h y p e r >> signifies dim W E n - 1 f o r n ~ dim V) we define t h e equidistant h y p e r s u r f a c e s We as the levels of the signed Riemannian distance function dista(v, W) exactly as w e did it f o r h y p e r s u r f a c e s in IR~ in w 0. W e can also define the m a p d : W - - - Wo --> V, which moves Wo to W~ f o r small ~ using geodesic e-segments normal to W instead of straight segments in ~ + ~ (see w 0). In order to have good t h e o r y of geodesics in V w e a s s u m e g is C2-smooth (in some coordinate system). Then one k n o w s since t h e w o r k of Riemann t h a t f o r every unit t a n g e n t vector ~ E T~(V) t h e r e exists a unique geodesic issuing from v in the direction 7. H e r e the word r geodesic >> means a locally isometric m a p of IR or of a connected subset of ~ into V, w h e r e r locally isometric 9 refers to the distance function in V. Namely, every sufficiently small subsegment of a geodesic m u s t be a minimizing segment in V t h a t is of the length equal the distance between its ends in V. ( F o r example, the geodesics in t h e unit sphere S ~ c ~ + ~ a r e t h e g r e a t circles or r a t h e r lines and segments going around these circles with the unit speed. They a r e minimizing in-so-far as their length does not exceed ~). I f V is a complete (e.g. compact) manifold w i t h o u t boundary, then it is classically known (probably, already to Riemann) then one has a geodesic ray ~, : ~ + - - > V with r ( 0 ) ~ v and 7 ' ( 0 ) ~ r f o r all v E V and unit 7 E T~(V). I f V has a boundary, t h e r a y m a y hit the b o u n d a r y at some finite moment. Similarly, if V is non-complete the r a y m a y reach the V w i t h 7 ( 0 ) ~ - v, 7 ' ( 0 ) ~ z (for a given unit T E T~(V) which is a minimizing geodesic segment between v ~ ( 0 ) and v ' ~ r ( D E V. Thus the local g e o m e t r y of geodesics in V is v e r y much the same as t h a t in IR ~ (where t h e geodesics a r e straight lines, r a y s and segments). N o w we define d~ with an exterior unit normal field v to W b y sending each w E W to the e-end of the geodesic segment issuing f r o m w in the direction v(w). (If E < 0 w e use the segment

SIGN AND GEOMETRIC MEANING OF CURVATURE

39

7: [ - - e , 0] -->V with 7 ( O ) - ~ w and 7'(O)~-v). I f V is complete we thus obtain a m a p (called the normal exponential map) d: W X 1R-->V f o r d(w, D ~ d, (w) in the above notation. I t is characterized by the geodesic p r o p e r t y of d on the lines w X 1R and by the initial conditions

d(w, O ) ~ w and - 0~ d ( w , O ) ~ v ( w ) f o r all w E W. (In the non-complete case we have such a m a p on some neighbourhood U c W X 1R of W X 0 c W X 1R, provided W lies in the interior of V). T H E SECOND FUNDAMENTAL FORM OF W IN V. Using d~ one defines the second f u n d a m e n t a l f o r m //w which measures how much W is curved inside V (exactly as in the Euclidean case, see w 0), by

(+)

//w . . 1. . d

,

2 de g~=o, where g* is the metric on W induced f r o m g by t h e m a p de : W -> V. There is another w a y to define H w by using geodesic coordinates u l , . . . , u ~ at a point w E W c V where we w a n t to evaluate H W. Nomely, these ooordinates i d e n t i f y the coordinate neighbourhood U with a domain U' c ~ * (with the Euclidean coordinates ul .... , u,), such t h a t w ~ 0 and W N U becomes a h y p e r s u r f a c e W' c U' c ~ " passing t h r o u g h the origin. Then we define the f o r m H W of W c V at w as t h a t of W' in ~ * at O,

I

nW l To(W)

where the t a n g e n t spaces Tw(W) and To(W') are identified by the differential of t h e implied diffeomorphism U ~ U" (sending W N U to W' and Tw(W) onto To(W')). A little t h o u g h t shows this definition to be independent of the coordinates u~ and with a m i n o r e x t r a e f f o r t one can see this f o r m is the same as defined by the above ( + ) . An offshot of the second definition is the existence, f o r every t a n g e n t hyperplane S c T~ (V), of a smooth h y p e r s u r f a c e W c V passing t h r o u g h v, t a n g e n t to S (i.e. having T ~ ( W ) ~ S) and being geodesic at v which means, by definition, TIWI T ~ ( W ) ~ O. F o r example, one m a y t a k e W corresponding to the Euclidean hy~er-

40

M. GROMOV

phtne W' c 1R~ in geodesic coordinates, such t h a t W" is t a n g e n t to S at To(1R") ----- T,(V). d

,

Now, as -~e g ~ = 0 ~ 0 a t v f o r the above W and the corresponding equidistant We, we w a n t to look at the second derivative. This is better done (for the reason which will become clear later o n ) w i t h the operators A~* on T(W) defined as the pull-backs of the shape operators A~ on W~ under the differentials of the maps d~ : W --> W~ (compare w 0; here as in the Euclidean case d~ is a diffeomorphism of W on W~ f o r small ~ and A~is defined by llW~(r~, ~2)~---g(A~7~, 72)----< As 7~, 72 > v). Namely we set

(++)

d , Bs=--~ A~=o]S.

This Bs is a symmetric operator on S ~ T~(W) which depends only on S and on g but not on a choice of W with T ~ ( W ) ~ S , as a simple infinitesimal computation shows. Also notice t h a t Bs does not depend on the choice of coorientation of S (and t h u s of W) since the second deriative is i n v a r i a n t under the sign change of the variable. The operators Bs on the t a n g e n t hyperplanes S c T(V) c a r r y the same infinitesimal information as the c u r v a t u r e tensor and t h e r e are simple algebraic f o r m u l a e expressing one in t e r m of the other. On the other hand, one can define with Bs the sectional curvature K(o) f o r all t a n g e n t 2-planes o c T,(V) as follows. Take an a r b i t r a r y hyperplane S c T~(V) meeting o a t a line ~ ~ S N o c T~ and being normal to o (i.e. normal to the line ~l c o normal to e ). Then we t a k e a unit vetor 7 6 e and define

(*)

K(o) ~ - - g(Bs(7), 7).

Again one should go t h r o u g h some simple algebraic computation to see w h y the result does not depend on the choice of S and r. As f o r the minus sign, this comes about because we w a n t the round sphere in 1R"+1 to have positive curvature. In fact, let V equal the unit sphere S ' ~ - - - S ' ( 1 ) c 1R"§ with the induced Riemannian metric g and W c V = S ~ be an equatorial hypersphere. Then W clearly is geodesic at all points w 6 W and so is suitable for computing Bs for

SIGN AND GEOMETRIC MEANING OF CURVATURE

41

S~-~ T(W). The e-equidistant concentric spheres We c V clearly are smallev than W. More precisely, the metrics of these spheres W brought to W-~-Wo are given by the following well known (and obvious) formula g** ~ (cos: s) go. Therefore the second derivative of g* measured by B is negative (definite) and the sectional curvature K is positive in accordance with the usual convention. Also notice htat the diminishing of g* for frowing Is] agrees with the behavior of the second form llW~: the smaller of the two balls in S n bounded by We is convex and the larger ball is concave. Thus the interior equidistant deformation of W (for a given coorientation) makes W convex and the exterior deformations We are concave. Now, let us compute K for W ~ - S ' ( 1 ) . First, d d-~ g ~ ~ - - 2 sin Ecos E go, and so

A*

EIa,

d , (formally, A~ equals -~e g~ divided by g~*). Then d A*=(--1--tge) ds

Id,

for the identity operator Id on S ,-~ T(W). Thus B s - - - - I d K ( o ) - - 1 for all tangent planes o in W ~ S ' ( 1 ) .

and

Notice that K scales quadraticly under the scalar change of the metric. For example, the sphere Sn(R) of radius R has K = R -2. (This can also be seen directly for g* wcos2(ER -1) in this case). In general, we denote by R V the manifold with the new distance defined by dist .... - - R disto~,~, which correspond to g.e..~-R 2 go~d. Then the scaling formula for K reads K(RV)-.~ R-'-' K ( V ) .

42

M. GROMOV

SECTIONAL CURVATURE FOR SURFACES. If dim V ~ - 2 then the curvature tensor reduces to a function on V (as the n u m b e r of components n2(n ~ - - 1) becomes 1 f o r n ~--- 2, see w 1) which adequately is expres12 sed by t h e sectional c u r v a t u r e K ( v ) ~ K(o ~ Tv(V)) f o r the points v E V. The famous Gauss' formula expresses K for surfaces in ~3.

TEOREMA EGm~IUM. The sectional curvature K(v) of V c ~ equals the Jacobia~ of the Gauss map V--> S o at v, or equivalently to the product of the principal curvatures (eienvalues of H v or of the shape operato~ A) at v. Of course, the proof is trivial b y the standards of t h e modern infinitesimal caulculus. Yet, the m a j o r consequence of t h e theorem looks as r e m a r k a b l e as it appeared 200 y e a r s ago: the Jacobian of t h e Gauss m a p does not change if we bend V in ~ , t h a t is if we apply a deformation preserving the length of the curves in V. F o r example, when we s t a r t bending an initially flat sheet of paper it does n o t remain flat in ~ 3 b u t its intrinsic geometry does not change and so the Jacobian of the Gauss m a p s remains zero. A n o t h e r corollary of Gauss theorem reads Convex (and concave) surfaces have K >__0 while saddle surfaces home K __ 0 and K _< 0 in term of equidistant hypersurfaces W~. LOCAL CONS/F_~ITYCRITERION. If K(V) >_ 0 then the inward equidistant deformations W~ of every convex hypersurface W c V remain convex, and if K(V)_< 0 then the outward deformation is convex. Conversely, if the inward equidistant deformation preserves convexity of all convex hypersurfaces in V then K(V) >_ 0 and if this happens for the outward deformation then K(V) _ O. The equidistant deformation in question is only considered f o r small E so t h a t the normal geodesic m a p d~ : W--> V is a diffeomorphism of W onto W~ (as is needed f o r our version of tube formula). Then the claims

K(V) > 0---> inward deformation preserves convexity and

K ( V ) _< 0

> o u t w a r d deformation preserves convexity

become obvious as K>O(

.',B < 0

KB>_O.

and

To prove the opposite implication, from preservation of convexity to the sign of K(V), one needs sufficiently many convex hypersurfaces W in V whose equidistant deformations W~ are non-convex whenever the c u r v a t u r e sign is wrong. Such a W m u s t have quite small second fundamental form (and hence, small IIAII) to be sensitive to t h e B-term in (**). This can be easily a r r a n g e d b y using h y p e r s u r f a c e s W corresponding to (pieces of) Euclidean spheres of large radius in geodesic coordinates. (We suggest the r e a d e r to actually produce these W and conclude the proof following t h e above hint.). The above convexity criterion makes sense for quite general metric spaces (e.g. for Finsler manifolds), w h e r e one can define convexity b u t where our infinitesimal definition of the c u r v a t u r e

SIGN AND GEOMETRIC MF_,ANING OF CURVATURE

~5

does not work. On the other hand, the usefulness of the (infinitesimally defined) conditions K >_ 0 and K ~ 0 in the Riemannian fram e w o r k owns v e r y much to the possibility of several different geometric interpretations which by no means follow one f r o m another f o r non-Riemannian manifolds. F o r example, one does not know how to extend the implication K ( V ) >_ 0

y K ( W ) >_ 0

to convex h y p e r s u r f a c e s W in non-Riemannian spaces V. Even for non-smooth convex h y p e r s u r f a c e s in 1R" the only simple proof of K ( W ) >_ 0 uses an approximation by smooth h y p e r s u r f a c e s followed b y the application of Gauss' teorema egregium. We conclude this w by relating the sectional c u r v a t u r e to the c u r v a t u r e tensor defined in w 1. SECTIONAL C U R V A T U R E A N D T H E C U R V A T U R E OPERATOR. The sectional curvature at each point v E V is a function on the Grassmann manifold Gr2 I R ' ~ Gr2T~(V) of the planes in ~ (Tv(V),g,,). To understand the nature of this function w e use the standard (Plficker) embedding of G ~ IR ~ into the unit sphere of the exterior power A: ~ which assigns to each plane o ~ Gr~ IR ~ the bivector l?~Xl/kX2 for an orthogonal basis (xl, x2) in o c IR~. This fi does not depend on the choice of xl, x2 (here w e need o oriented and the basis respecting this orientation) and the n o r m [[~II (naturally defined with the Euclidean norm in ~') equals one. N o w a simple algebra shows that the sectional curvature function o~->K(o) on Gr2 IR~c A 2 IR ~ is quadratic: there exists a (necessarily unique) quadratic form Q on A21~ ~, such that K(o)~--- Q(o, o) for all o~Gr21R ~. Following the established tradition one often uses instead of Q the correspoding symmetric operator R defined by (Roe, ~)~-Q(u, ~) for the scalar product on A ~ T(V) induced by g on T(V). This is called the curvature operator R : A 2 T~(V)--> A 2 T~(V). n ( n - - 1) Notice t h a t d - dim A s 1R~ - and so quadratic forms 2 Q on A e 1R~ constitute the space of dimension

d ( d Jr 1)

n ( n - - 1) (n ~- 1) (n - - 2)

2

8

This is more than the n u m b e r of the independent indices in the c u r v a t u r e tensor ( w h i c h is n ~ ( n12 ~ - - l ) , see w 1 ) and in fact, the

46

M. GROMOV

form Q satisfies some s y m m e t r y relation, called the Bianchi identity, nS(n~ - - 1) which reduces the dimension to 12 . Then the f o r m Q (and the c u r v a t u r e operator R) can be identified with c u r v a t u r e tensor of (V, g). w 2 8 9 INFLUF~CB OF K ( V ) ON SMALL BALLS IN V.

W e want to give here another geometric criterion for the sign of K(V), n o w in terms of the size of small balls in V. Namely we shall show that small concentric balls grow slower in V with K(V) >_ 0 than the balls in IR". O n the contrary, if K(V) __ 0 then for every point v E V there exists a number ~ > 0 such that every two concentric balls B (v, ~) and B (v, 2~) with ~ _ 0 t h a t S(v, e) g r o w slowe~" with e than the corresponding spheres in ~ (where K ~ 0 and there is no negativt B-term in the tube formula), while initially, for infinitely small >> e the spheres S(v, e) a r e ( a s y m p t o t i c a l l y ) E u c l i dean. In other words, the (eigenvalues of the) shape operator on the sphere S(v, e) are smaller in V than in ~ * and so the spheres do g r o w slower in V. This implies the required 2-inequality on f comparing B(v, ~) and B(v, 2,~), f o r K _> 0 and t h e case K _< 0 follows b y a similar argument. I f dim V ~

2, then the converse statement giving the sign of

K(V) in t e r m s of the balls follows f r o m w h a t we have j u s t proved an n e a r each point v w h e r e K(v) ~ 0 the c u r v a t u r e is either positive or negative (because there is a single 2-plane o at v). Then this extends to n _> 2 by looking at the growth of the small balls B(v, ~) intersected with a geodesic surface 2: at v t a n g e n t to the plane oE T~(V) w h e r e we study the (sign of the) c u r v a t u r e K(o). The details here are not hard to fill in and this is suggested to the reader. Notice, t h a t the as a definition of K corresponding theory does not know when hypersurfaces.

inequality (0) for the balls in V can be used _> 0 f o r an a r b i t r a r y metric space V b u t the has not been t r u l y developed. F o r example, one this definition agrees with t h a t using convex

Another r e m a r k is that the above a r g u m e n t gives us besides a comparison between concentric balls in V also a comparison of the small 0-balls B(v, 5 ) c V, with the Euclidean ball B ' ( ~ ) c ~ . Namely, K(V) >_ 0 ~ B(v, ~) _ B'(J).

48

M. GROMOV

Again this can be used as a definition of K _> _< 0 b u t for nonRiemannian V this is quite different f r o m the above definition using the inequality (0) f o r concentric balls in V. EXAMPLE. Let V be a finite dimensional Banach space t h a t is an n-dimensional linear space with a norm ]] II and the corresponding dist(vl,v2)---~-Ilvl--v2]]. This V, like 1R~, admits a similarity t r a n s f o r m a t i o n at each point v E V by

v'-,v

+ ~(v'--v)

f o r variable v' and each s E ~ § This establishes the metric equality

B(v, ~ ) ---~~B(v, ~) f o r all balls and thus suggests the vanishing of the c u r v a t u r e K(V). On the other hand if a ball B ~ B ( v , 6) in such a V is comparable with the Euclidean 5-ball B" b y either of the t w o inequalities B >_ B' or B _< B', then necessarily B ~ B' and V is isometric to 1R". (This is a simple exercise f o r the reader). One m a y ask at this point w h a t is the deep rtason which makes various geometric definitions of the sign of the c u r v a t u r e coincide f o r the Riemannian manifolds. F i r s t of all, b y their very definition, Riemannian manifolds are infinitesimally Euclidean and so their basic g e o m e t r y is similar to that of 1R~. F u r t h e r m a r e , as w e assume t h e Riemannian structure g smooth, w e tremendously restrict the infinitesimal geometry at each point v E V. F o r example, all infinitesimal information of the second order (which is reflected in the curvature) is defined by finitely many parameters at each point of V (that are the values of the f i r s t and second derivatives of g~j), and so there are plenty of algebraic relations between these parameters. When integrated, these infinitesimal relations acquire a geometric meaning such, f o r example, as the equivalence of different geometric definitions of (the sign of) the curvature. On the other hand the infinitesimal geometry, say, of a Finsler manifold at a given point involves infinitely m a n y p a r a m e t e r s as these a r e needed to specify a general (Banach) n o r m at every tangent space. However, t h e r e exist some non-Riemannian spaces with finite dimensional infinitesimal geometry. Among them most known a r e those called sub-Riemanni~n or C~rnot-Caratheodory spaces b u t their g e o m e t r y has not been studied as deeply as in the Riemannian case (compare [ Str] ).

SIGN AND GEOMETRIC MEANING OF CURVATURE

w 3. MANIFOLDS

WITH

49

POSITIVE SECTIONAL CURVATURE.

As w e already know the c u r v a t u r e condition K ( V ) >__0 is characterized b y preservation of convexity of small i n w a r d equidistant deformations W, c V of convex h y p e r s u r f a c e s W in V. N o w we w a n t to establish the convexity of W , f o r all negative E ( corresponds to with our conventions, see w 0) and we need f i r s t of all a definition of convexity suitable f o r non-smooth hypersurfaces. We s t a r t with the following basic notion of CONVEX BOUNDARY. Let V' be a Riemannian manifold with b o u n d a r y called W'. We say that W' is (geodesically) convex of in the interior Int V' ~ 1 1 ' - W', every two points can be joined by a minimizing segment provided such a segment exists f o r the t w o points in question in the ambient space V' D Int V'. (The latter condition is satisfied f o r all complete, in particular compact manifolds V'). In other words, non-convexity of W" is manifested by the minimizing segments between vl and v2 in Int V' which meet W' at some point w between vl and v2. See Fig. 8. W,

V,

Fig. s. Notice t h a t such a minimizing segment [vl, v2] in V' typically at the points w w h e r e it meets W'. F o r example if V' is p a r t of a larger manifold V D V" with dim V ~ dim V" then Ira, v2]

8em'~lr~rio

M a t e m o , t,{co e F ~ d , eo . 4

50

M. GROMOV

m a y be (and typically is) non-geodesic at w in V. With this in mind one can see t h a t W" is convex if and only if the second fundamental f o r m H w" is positive semi-definite, provided W' is C2-smooth in order to have I I w" defined. It follows that the convexity is a local p r o p e r t y of W and this locality remains valid (for the above reason) for non-smooth W' as well. (Notice t h a t the above a r g u m e n t which appeals to t h e length minimization inside V' gives us a v e r y quick p r o o f of the classical result on the convexity of c o n n e c t e d locally c o n v e x subsets in ll~". We challenge the reader to find a purely elementary proof of this classical local to global convexity criterion f o r finite polyhedra V' in 1~). N o w a hypersurface W in V is called c o n v e x if near each point w E W it can be made into the (part of the) b o u n d a r y of a convex domain IT' c V where t h e latter convexity refers to the b o u n d a r y onvexity of IT' defined above. Again, if W is smooth this is equivalent to 11 W >_ O, b u t n o w no a p p a r e n t global convexity of W follows f r o m our local definition, as is seen in Fig. 9 below.

Fig. 9.

We shall also apply the notio~a of geodesic convexity to subsets Vo c V as follows. Vo is called geodesically c o n v e x if f o r every two points vl and v2 there exists a path between vl and v2 which is length minimizing among all paths between vl and v2 in Vo and which is also geodesic in t h e ambient manifold V. Notice, t h a t convex hypersurfaces W a r e not convex in this sense b u t w h a t is bounded b y W m a y be convex. On the other hand every connected totally geodesic submanifold Vo c V is geodesically convex. (Recall t h a t Vo is called

SIGN AND GEOMETRIC MEANING OF CURVATURE

totally geodesic if every geodesic in point is necessarily contained in V,). k-dimensional convex V0, f o r k _< n inside a totally geodesic submanifold

51

V which is t a n g e n t to Vo at a In fact, one can think of every ~ dim V, as a convex domain of dimension k in V.

INWARD DEFORMATIONOF THE BOUNDARY.Let V be a compact manifold with b o u n d a r y a V ~ W and set V 7 -~- {v ~ V [ dist(v, W) >_ e}. I f V7 happens to be a manifold with b o u n d a r y then

OV-[~ W _ ~ { v

~ V [ dist(v, W ) ~ e } ,

(where the minus sign at ~ is due to our coorientation convention, see w 0). If W is smooth then also W_~ is smooth f o r small e b u t as e g r o w t h s W_, m a y develop singularities. There are t w o slightly different reasons f o r the appearance of singularities. First, t w o diff e r e n t p a r t s of W m a y meet inside V as they move inward, see Fig. 10 and 11 below.

W

W'

W

W-t.

W- t W-c

Fig. 10.

Fig. 11.

V is non-regular at w, i.e. the differential of d, is non injective at w. (Recall that d~ moves each w to the e-end of the geodesic e-segment normal to W at w). Notice t h a t the f i r s t moment so w h e r e a focal point appears is characterized by the blow-up of the second fundamental f o r m of W mapped to V byd. I[//[[ ~

=

f o r e-->eo.

This is clearly seen, f o r example, in the inward deformation of the sphere of radius so in ~n. N o w the reader m a y appreciate the elQgance of the following basic theorem by Gromoll and Meyer (see [Ch-Eb]). CONVEX CONTRACTION FOR K >_ O. L e t V be a compact connected manifold with convex boundary and non-negative sectional curvature. Then the subsets V T c V are convex fo~ all s >__O. (We assume v is connected to satisfy our c u r r e n t definition of convexity).

IDEA OF THE PROOF. Assume f o r t h e m o m e n t t h a t W ~ 3 V is smooth. Then V 7 remains smooth, and hence convex, in-so-far as the normal geodesic m a p d, : W - - > V is a smooth embedding. F u r t h e r m o r e , if d , ( W } develops a self-intersection w i t h o u t focal points, then VT becomes locally represented as an intersection of smooth convex subsets and so again it is convex. Then it is easy to believe in the convexity at the focal points as well as these are j u s t r infinitesimal >> double points (vanishing of the differential of a m a p at a t a n g e n t vector E T ( W ) brings together the corresponding to the 0) whose 9 interior region 9 locally contains W as in Fig. 13 below.

W

W, F i g . 13.

One might think t h a t t h e r e is little point in fussing a b o u t nonstrict convexity as a small perturbation could m a k e e v e r y convex h y p e r s u r f a c e W strictly convex. In fact, this w o r k s if V has sf/rictly p o ~ t i v e c u r v a t u r e (K(~) > 0 f o r all ~ c T ( F ) ) w h e r e a small inward equidistant deformation leads to strict convexity. Similarly, if K < 0, one obtains strict convexity with the o u t w a r d deformatiorL Also in V~-~n convex h y p e r s u r f a c e s can be approximated b y strictly convex ones (see w 1/~). B u t if we look a t a product manifold such as V ~ Vo X 1R~, w h e r e Vo is a closed manfold with dimVo ~ 0 and t a k e W ~ Vo X S ~-1 f o r the round (and strictly convex l) sphere S ~-1 in ~ , w e shall see t h a t this W is convex b u t not strictly convex, nor can it be approximated b y anything strictly convex.

SIGN AND GEOMETRIC MEANING OF CURVATURE

55

The g e o m e t r y of the equdistant h y p e r s u r f a c e s is very simple in the above product example. Namely, W_, ---~ Vo X S~-l(e n D w h e r e is the radius of the original sphere S ~-1 ~ S ~-1 (~) c ]R~. F u r t h e r more the region V7 c V bounded b y W_~ equals Vo X B~-I (~ - - s) f o r the balls B k - l ( e - - s ) bounded b y t h e spheres in ~ . Thus V~- and W , disappear at the moment s ~ ~ and at t h e last moment V7 and W_, equal V~ X 0 in V---~Vo XTR ~. A similar picture is observed f o r all manifolds V with K(V)>_O. As we deform the b o u n d a r y W E aV i n w a r d t h e r e is the f i r s t moment &]

: ~ inrad V ~

sup dist(v, W),

vEv

such t h a t dim V7 ~ - n ~- dim V f o r s < ~ and dim V~-< n. If K ( V ) > 0 or if W ~ OV is strictly convex, then the only possibility f o r Vo is to be a single point since no totally geodesic submanifold V~ of positive dimension in V can be strictly convex at a non-boundary point v~ E V with our definition of strict convexity given above f o r singular points. On the other hand, in the non-strict case, the inward equidistant deformation m a y t e r m i n a t e with a subset V~ c V of positive dimension which is, as w e know, convex. This VT, is itself a compact manifold with or w i t h o u t boundary. If VT, has a boundary, call it W t ~---3V7,, one can shrink V/, f u r t h e r with t h e i n w a r d deformation W ~ of W ~ in V~,~. I f the process stops at a closed (i.e. without boundary) manifold we are through ; if not we go to yet lower dimensional manifolds (VT,)7,, ((VT,)7, )~ .... unless we do arrive at a closed totally geodesic submanifold Vo ~ V without boundary, called the soul of V. Then it is not very hard to show t h a t V is homeomorphic to ~ bundle of balls over Vo. F o r example, V is homeomorphic to the n-ball in the strict case i.e. w h e r e either K > 0 or W ~ aV is strictly convex. (The strict case is due to Gromoll-Meyer and the general one to Cheeger-Gromoll (see [Ch-Eb]). This indicates that manifolds V with K _> 0 tend to have a r a t h e r simple topology and whenever this topology approaches the critical level of complexity compatible with K >_ 0, then the geometry of V becomes very special. F o r example, if t h e above V with K(V)>_O

56

M. G R O M O V

and convex boundary has non-trivial homology in dimension k, then V contains a closed totally geodesic submanifold (the above soul) of dimension >__ k. It is worth noticing t h a t the existence of a totally geodesic submanifold of dimension k f o r 2 _< k _< n - 1, n ~ dim V, is an exception r a t h e r than a rule: t h e r e is no such submanifold for a gene~ic Riemannian metric g on V. The above discussion also shows that the homotopy classification of manifolds V with K ( V ) >_ 0 admitting a convex b o u n d a r y reduces to t h a t f o r closed manifolds. (Notice t h a t a soul V ~ c V h a s K(Vo)>__O as it is totally geodesic in V). This result extends to non-compact complete manifolds V w i t h o u t b o u n d a r y : every such V f i b e r s over its soul which is a closed totally geodesic subrnanifold Vo c V and the f i b e r s a r e homeomorphic to some 1R~. (This is shown b y constructing an exhaustion of V by compact convex domains with convex boundaries, see [Ch-Eb]). Then one m a y ask w h a t is a possible homotopy t y p e of a closed manifold V with K ( V ) >_ O. On the positive side, one knows t h a t every compact homogeneous space, V ~ G / H for a compact Lie group G, admits a metric with K _> 0. In fact, every bi-invariant metric g on G has K ( g ) >__0 according to the following formula (see [Ch-Eb]) which expresses the value of K at the span o ~ x / k y of t w o orthonormal vectors x and y at the t a n g e n t space Te of G at the identity,

1

II Ix, y] I I ,

where [, ] is the bracket in the Lie algebra L ( G ) ~ T 6 . Then g descends to the metric g on V defined by the following condition: the differential of the projection G--.~ V isometrically sends the horizoq~tal subbundle of (T(G) g) to (T(V), g) (where the horizontal subbundle consists of the vector g-normal to the fibers of the projection which are also the arbits of H in G). One knows, that the 3

curvature of ~ satisfies K(x-Ay-)~---4-[[[x,y]~e~t]] 2, where x and y are orthonormal horizontal vectors in Te and x, y are their images in T(V), (see [Ch-Eb]), and so K(g) >_ 0.

SIGN AND GEOMETRIC M E A N I N G OF C U R V A T U R E

57

Among homogeneous manifolds with K _> 0 the most r e m a r k a b l e are compact symmetric spaces V, w h e r e f o r each point v E V there is an isometric involution I : V - - > V fixing v and having the differential D I - - - - I d I T ~ ( V ) . In fact one m a y think that the symmetric examples provide the m a j o r motivation f o r the study of K>_0. There are some non-homogeneous manifolds with K >_ 0 b u t they do not influence much f u r t h e r our intuition. F o r example, one believes that the topologically r largest >) n-dimensional manifold with K >_0 is the n-torus T ~ (which admits a metric with K ~ 0 as T" ~ ~ / Z ' ) . One k n o w s in this r e g a r d that, indeed, the fundamental group ~ (V) f o r K ( V ) >_ 0 cannot be much greater than ~," as it is commensurable w t h Z ~, (this is already t r u e for Ricci _> 0, see w 5) and one also k n o w s that the Betti numbers b~(V) are bounded by universal constants b~,,. Yet one is unable to bound b~(V) by b ~ ( T ' ) ~ (~). (See [CheJ about it). The above bound on ~1 (V) becomes radically b e t t e r if we assume K ( V ) is strictly positive (i.e. K ( o ) > 0 f o r all ~ E T(V)). Namely nl(V) is finite in this case by the following classical BONNET THEOREM. I f K(V) >_ u: then the diameter of V is bounded by D i a m V _< ~ / ~ ,

where Diam V ~

sup

v~,vsE V

dist(vl,v2).

IDEA OF THF. PROOF. T a k e a minimizing segment between t w o points in V, say [vo, vii between vo and vl and look at the spheres S(e) of radius e around v~ near the points v E Iv0, vl]. (See Fig. 14 below). ~

~176

vo "..::__,,],] ]

I

I

o ~176 ..~~

Fig. 14.

! I I I

58

M. GROMOV

The minimizing property of [vo, v~] implies t h a t every sphere S(D is smooth at the point vE [vo,vl] with d i s t ( v 0 , v ) = E for all < dist(vo, vl). (This is a simple general f a c t which is t r u e without any c u r v a t u r e condition). On the other hand a simple analysis of the t u b e formula (**) in w 2 shows that A~* must blow up f o r some finite negative E. Namely if we s t a r t with some A~, then A~* becomes infinite f o r some E in the interval [ - - u / x , 0]. Thus the length of [vo, vl] cannot exceed - - ~ / z and the theorem follows. The critical case for the Bonnet theorem which clarifies the picture is t h a t of V equal to the round sphere S'(~) c .~n+l which has constant c u r v a t u r e ~.~___e-2. H e r e t h e ball B(vo, ~) of radius E around vo E S" is convex in S" until the moment E ~ n ~ / 2 and for larger ~ the boundary sphere S(",7~ -----W_~ of this ball becomes concave. As e--> ne the c u r v a t u r e of W_~ (measured by A*~) blows up to infinity while the complementary region S * - - B ( v o , D becomes r infinitely convex ~>f o r s--> nQ and blows out of existence f o r E > ~e. N o w the tube formula shows that all this happens even f a s t e r for K(V) >_ ~2. Namely the spheres S ( D a r e more concave in V than in S*(#) and the complement V - - B(vo, e) is more convex. In particular this complement must become e m p t y f o r e > ne as is claimed b y the Bonnet theorem. Now, the finiteness of n~(V) follows from the Bonnet theorem applied to the universal covering V--> V which has finite diameter and t h e r e f o r e is compact. It is also w o r t h looking at the case w h e r e K is non-strictly positive and ~ is infinite. F o r example if nl is isomorphic to ~ , then V (isometrically!) is a flat torus, i.e. V = ?~"/L f o r some lattice in ~ " isomorphic to Z" (see [Ch-Eb]). There is no comparable result of this n a t u r e f o r b,(V) f o r i _> 2. REMARK. The conclusion of Bonnet's theorem remains valid with the following (weaker) assumption on Ricci(V), Ricci >_ ( n - - 1) r2, and the above characterization of flat tori by Ul-----Z" also remains valid f o r Ricci ___ 0 (see w 5). On the other hand, the u~-corollary to

SIGN AND GEOMETRIC MEANING OF C U R V A T U R E

59

Bonnet's theorem is sharpened by Singe's theorem (see w 71/~) which says t h a t if n ~ dim V is even and K > 0, thtn the fundamental group is either trivial or Z o, w h e r e the latter happens if V is nonorientable.

w 31/@ DISTANCE FUNCTION AND ALEXANDROV-TOPONOGOV

THEOREM.

If one looks at a metric space V from a finite combinatarial point of view then one w a n t s to know the properties of the (N X N)m a t r i x of t h e pairwise distances between the points in every subset in V containing N elements. In other words, one m a y t r y to characterize V b y the set of those metric spaces with N elements which isometrically embed into V. Yet another w a y to see it is by considernig the m a p of the Cartesian power V N = V X V.X ... _X V into N 11 ~' f o r N ' = N ( N 2~ 1) , say MN: V~--> ~N,, which relates to each N-tuple of points in V the set of the mutual distances between these points. Then our invariant of V is the image M ~ ( V N) c 11N'. (If there is a natural m e a s u r e on V as in the Riemannian case one should look at t h e MN-push-forward of this measure to 1RN'). One obvious universal restriction on M a ( V 3) c 11~ is expressed b y the triangle inequality. Then one laso k n o w s how to characterize the Euclidean and (Hilbert) spaces in terms of M~ (express the scalar products a~j between the vectors x o - x~ in 1R", i ~---1 ..... N - 1, in t e r m s of the squred distances and observe t h a t the m a t r i x a~j is positive semidefinite). N o w we w a n t to state the Alexandrov-Toponogov theorem which characterizes the manifolds V with K ( V ) >_ 0 by t h e image M 4 ( V 4) c ~ . To abbreviate the f r m u l a e we shall w r i t e below I v 1 - v21 f o r d i s t ( v l , v2). We consider t h r e e points Vo, vl and v2 in V and also a point v3 between vt and v~. This means

Iv1 - - v~l + Iv~_-- v.~i = Iv1 - - w.I. Then w e observe t h a t t h e r e exist f o u r points in ~ - , v~, v l , v~ and s v 2 between v~ and v~, such t h a t

I v ~ - v~ I ~ ~--- Iv` - - vjlv for i, ] ----- 0, 1, 2 and also

Iv1--v~lv = I v l - - v ;

I~,.

60

M.

GROMOV

Then automatically

and t h e Euclidean distance Iv o - v ~ l ~ 2 c a n

be expressed by a (well

known) formula in t e r m s of the f o u r numbers Ivo--vil, Iv,,--wl, Ivl--v21 and Ira--V31. H e r e is the picture which helps to keep everything in mind. t

vI

V3

Vl

~

v2

'

/

"~ v2

!

Vo

vo Fig. 15.

THEOREM. If V is

complete with K(V) >_ 0, then

We call this AT-inequality as it was discovered by Alexandrov for n = 2 and extended b y Toponogov to n >_ 3. IDEA OF THE PROOF. One can think of the AT-inequality as a kind of concavity relation f o r the function do(v) ~ distv(v~, v) on V restricted to the segment [vl, v2] as it gives a lower bound on do(v3) in t e r m s of do(v1) and d0(ve). More precisely, the theorem says that do is more concave on each segment in V than the Euclidean distance function on the corresponding segment in ~2. Although the function distv(Vo,. ) is non-smooth, the concavity type inequalities on geodesic segments follow f r o m the corresponding local concavity which, in the smooth

SIGN AND GEOMETRIC MEANING OF CURVATURE

61

case, can be expressed with the Hessian of do ~ - d i s t ( v o , .). Now we invoke the tube formula (**) in w 2 and apply it to the concentric spheres S(v~), D c V for all ~ > 0. We can see with this formula (with a little a d j u s t m e n t at non-smooth points of the spheres) t h a t these spheres are less convex (or more concave) t h a n the e-spheres in ~ . ( F o r example the spheres in the unit sphere V ~ S ' ( I ) c ~'+1 become concave f o r ~ > ~/2). This of the spheres tog e t h e r with the obvious relation ]lgrad do(v)l I -----1 translates into a certain concavity of the function do(v) and by inspecting this translation one sees t h a t it gives exactly the desired locall version of the AT-inequaliity on each geodesic segment in V. Q.E.D. REMARK. It is easy to see t h a t the AT-inequality, when applied to the points v l , v~o and va which are infinitely close to v~, yields K __ 0 at v~. Thus A T is equivalent to K __ 0. One m a y wonder if t h e r e are f u r t h e r universal metric inequalities related to the curvature, but none besides A T is known today. However, the inverse AT, for complete manifolds V tionally assume t h a t V is with Ricci V _> 0 satisfy these depend on dim V.

namely Ivy--v~lv 0 and 0 < c < 1. H e r e one thinks of the constant ~ as (*) This analysis was started by Otsu and Shioya and it is rather subtle, (**) This is proven by Otsu and Shioya and also appears in the final version of [B-G-P].

64

M. GROMOV

1

of the curvature of the round sphere of radius ~-T and c, called the pinching constant, measures the allowed amount of non-constancy in K(V). Notice, that by scaling one can reduce the general case to that of c ~ 1 and then the inequality

c 3 b y the following FILLING LEMMA (compare w 1/~). Let V be a complete Riemannian manifold of dimension n >_ 3 with K ( V ) > 0 and let e : S - > V be a (topolagival) immersion of ~ closvd connected ( n - - l ) - d i m e n s i o n a t manifold S into V; I f the immersed hypersurface is locally convex

68

M. GROMOV

in V, then it is diffeomorphic to S n-1. Moreover, there exists a bedl B' which bounds this S n - l ~ S and an immersion B' --) V extending e : S --> V, such that the immersed ball B' fills in e(S) t o m the oenvex side.

REMARK. Notice t h a t the image e(S) c V does not have to be convex in any sense. The local convexity m e a n s t h a t each point s E S has a neighbourhood W c S on which the m a p e is one-to-one and whose i m a g e / s (locally) convex in V. A typical example is provided b y the locally convex immersed curves in the plane as in Fig. 17 below. (Compare Fig. 5 in w 1/2 and Fig. 9 in w 3).

Fig. 17.

Also notice that a closed immersed curve S in ~ 2 does not bound a n y immersed disk unless S is embedded (i.e. has no double point). This does not contradict Filling L e m m a w h e r e w e assume n >__3. IDEA OF THE PROOF. Given an immersed locally convex h y p e r s u r f a c e in V, we m a y t r y the i n w a r d locally equidistant deformation which applies simultaneously to all small embedded neighbourhoods of e(W) ~ V, see Fig. 18 below. The equidistant deformation sketched in Fig. 18 develops a (cuspidal) singularity at a certain moment to and cannot be continued beyond eo. However, an elementary a r g u m e n t as in w 1/2 shows t h a t no such singuarities appear f o r deformations of locally convex (pieces of) h y p e r s u r f a c e s in ~ f o r n >_ 3. This conclusion extends to all Riemannian manifolds V ~dth local geodesic coordinates systems a t the points x ~ e(S) w h e r e one is afraid of singularities. Thus one can

SIGN AND GEOMETRIC MEANING OF CURVATURE

69

continue the equidistant deformation as long as the local convexity is preserved. (In fact one needs here s t ~ z t convexity as it is stable under small perturbations and goes along as one passes to the Euclidean picture in geodesic coordinates). Now, since K ( V ) > 0, the

e(S)

f Fig. 18.

convexity only improves in the course of the inward deformation and so S eventually shrinks to a single point v' E V. The totality of the deformed hypersurfaces form a multiple domain in V filling in e(S) which is a manifold B' with a B ' ~ S immersed into V, and our locally equidistant hypersurfaces

S, ~ {b E B' ] dist(b, aB')~-~e} for the Riemannian metric in B' induced by the immersion B'--> V. In this case we know by the Gromoll-Meyer theorem (see w 3) that B' is homeomorphic to the ball B" (this homeomorphism is easily constructed with the family of convex S~ shrinking to a point in B') and the proof of Filling Lemma is concluded. (See [Esch] for a detailed argument). REMARK. Coming back to the theorem, we observe that the simply connectedness of V implies that the (covering) map S" ~ B U B'--> V is injective. So the exponential map e : B--> V is injective after all!

70

M. GROMOV

w 4. NEGATIVE SF_~TIONAL CURVATURE.

Here

V~-(V,g)

is a complete Riemannian manifold with K(V) 0 of convex h y p e r s u r f a c e s W in V, for small e, see Fig. 19.

W

We

Fig. 19.

This is quite similar to thc case K _> 0. But w h a t happens here for large e ___ 0 is different, Namely, the tube f o r m u l a shows t h a t the normal geodesic m a p d : W--> V~ is regular (i.e. an immersion) for all ~ > 0 and d~(W)r W is a locally convex immersed hypersurface. The only problem comes f r o m possible self-intersections of this hypersurface. The simplest case to consider is w h e r e W~ are concentric e-spheres a r o u n d a point Vo E V. In other words we look at the exponential marp e : H~0(V)-->V which locally isometrically sends each straight r a y ~ in t h e t a n g e n t space T~0 (V) to the geodesic r a y in V issuing f r o m vo and t a n g e n t to ~ at vo. This e (obviously) maps every Euclidean r-ball B (0, r ) c T~~ (V) onto t h e r-ball B (vo, r) c V (this is t r u e for all complete V with no assumption on the curvature) and for K _< 0 the tube formula (applied to the spheres) shows t h a t e is an immersion. Moreover, by t h e tube formula the m a p e is infinitesimally distance increasing, i.e. the metric g ' o n T~0 (V) induced by e f r o m g on V is (non-strictly) g r e a t e r t h a n

SIGN AND GEOMETRIC MEANING OF CURVATURE

71

the Euclidean metric go on T~0 (V). Since go is complete the (greater) metric g is also complete which then implies b y an easy a r g u m e n t that e is a covering map and since T~ ( V ) ~ 1g~ is simply connected this is the universal covering. F r o m this one immediately derives the classical THEORIgM OF CARTAN-HADAMARD. The universal covering of a corn

plete n-dimensional manifold V with K V is a bijective diffeomorphism for each v ~ V. It follows t h a t every t w o points v and v' in V can be joined b y a unique geodesic segment in V, which (because of uniqueness) must be minimizing in V and t h a t the distance function d i s t ( v l , v2) is smooth f o r v~ : ~ v 2 . Then, b y applying the tube formula to the s-neighbourhoods of the diagonal d --- V c V X V, one can easily prove that dist is a convex function on V X V. This means dist is convex on every plane in V X V which is the Cartesian product of t w o geodesics in V. In particular, every ball B(v, e) c V in convex. The above discussion indicates a certain duality between manifolds with K _> 0 and simply connected manifolds with K _ 0. This duality shows up even better for the monotonicity of the balls

B(v, he) > ~B(v, e) f o r all v E V, e _> 0 and ). >_ 1 (compare w 2 8 9 and the AlexandrovToponogov inequality

IVo-- v3iv _< Iv0 - (see Fig. 15 in w 31/~), which is a sharpening of the convexity of the function d i s t ( v o , . ) on V. Notice t h a t both inequalities need nl ~ 0 as well as K _< 0. B u t the duality does not seem to extend much further. In fact the essential features of manifolds V with K ( V ) _< 0 and n ~ ( V ) ~ 0

72

M. GROMOV

are seen (if we know w h e r e and how to look) asymptotically as we go to infinity in V, which has no counterpart f o r K _> 0. Now, if we t u r n to closed manifolds V with K < 0 then they a p p e a r as quotient spaces of the universal covering ~ b y the Galois g r o u p F ~ - ~ I ( V ) which isometrically acts on ~. Even if we fix the v a r i e t y of different F c Iso V and the corresponding V ~ ~ / r m a y be quite astounding. Tht richest source of examples is the 3-dimensional space ~ H 3 of constant negative curvature which can also be defined as PSL2 C/S0(3) with an invariant Riemannian metric. Discrete subgroups F c PSL2G conformcally act on the Riemann sphere S 2 and their study under the name of t h e Kleinian groups has been conducted f o r m a n y y e a r s in the f r a m e w o r k of complex analysis. A new development emphasizing K ~ 1 was started about 12 y e a r s ago by Thurston w h o has created (or discovered) a magnificent geometric world in dimension 3. There is nothing comparable to this f o r K >_ 0. Notice t h a t negative c u r v a t u r e accompanies every non-compact semi-simple group G. Namely, if w e divide G by the maximal compact s u b g r o u p H c G then, by the compactness of H, the spact V ~ G/H admits a G-invariant metric g. Such a metric is co~nplete (this is elementary) and by a well known theorem of E. C a f t a n K ( g ) < O. This c u r v a t u r e is strictly negative if and only if r a n k ~ G ~-~ 1, and K(g) is constant if and only if G is locally isomorphic to O(n, 1). The compact manifolds V covered by V a r e associated to discrete subgroups/~ c G which are usually produced b y arithmetic constructions. The above mentioned examples b y no means exhaust all compact manifolds with K _< 0. In fact, t h e r e is no slightest chance o~ any meaningful classification of such manifolds (but there m a y exist a classification of compact manifolds with K _< 0 modulo those with K < 0). On the other hand f o r K _> 0 a rough description of all compact manifolds looks quite feasible. ( F o r example, every convex subset V c ~ is roughly equal to some solid [0, ~1], X [0, ~ ] X ... 9.. X [0, ~ ] and every fiat t o r u s roughly is the Riemannian Cartesian product of the circles S~ of certain lengths ~). The situation here is vaguely similar to the classification of algebraic varieties (e.g. sur-

SIGN AND GEOMETRIC MEANING OF CURVATURE

73

faces). The varieties of general t y p e look kind of hyperbolic (which corresponds to K _< 0) and are unclassifiable. On the contrary, special varieties (such as F a n o varieties) sometimes can be classified in every fixed dimension. These correspond to manifolds with K _> 0. In order to have a better idea of a possible classification for general manifolds with K > 0 one may look a t t w o examples where the problem was, in principel, solved. The f i r s t example is t h a t of constant curvature 1, where V,-~ S"/F f o r some finite isometry group F freely acting on the unit sphere S n c ~ + ~ . The second class of examples is given by the flat Riemannian manifolds (K ~ - 0 ) which are ~ ' / F f o r so-called crystallographic groups F (isometrically acting) on ~ . In both cases one has a good overall picture of such groups F as well as a possibility of classification f o r every fixed n (but such classification quickly becomes a mess f o r large n and is not very appealing).

HYPERBOLIC GROUPS. The main topological problem concerning the spaces with K _< 0 is a characterization of t h e groups F which m a y serve as f u n d a m e n t a l groups of such spaces. I f we only assume t h a t V with K __ 0 is complete then it is unclear if t h e r e is a n y nonobvious restriction on F~z~I(V). (The condition is the existence of a free discrete action on a Euclidean space as ~ is diffeomorphic to ~ ) . On the other hand, if V is compact, then there are m a n y specific properties of F ~ u l ( V ) . F o r example, F contains a free group on two generators unless V is a flat manifold. (In the latter case F contains a subgroup F ' ~ Z ~ of finite index). The idea of the freedom theorem goes back to Felix Klein who proved it for groups acting on the hyperbolic space H 8 with K - - - - - - 1. The generalization to the subgroups F c SL,, acting on the manifold S L J S O (n) with K _< 0 is a famous result by J. Tits. The extension to the variable strictly negative c u r v a t u r e is due to P. Eberlein and in the general case the freedom theorem was recently proven by W. Ballmann on the basis of a deep analysis of the n a t u r e of r nonstrictness >>f o r K _< 0. To get some idea on the difference between strictly and nonstrictly negative c u r v a t u r e we f i r s t recall the old result by Preissmann saying t h a t every Abelia~ subgroup A c F ~ n l ( V ) is free cyclic,

74

M. GROMOV

assuming V is a closed manifold w i t h K < 0. Furthermore, if we only assume K < 0 and insist on the existence of a free Abelian subgroup A in nL(V) of rank k >_ 2, then V contains an isometrically and geadesically i m m e r s e d flat torus T k r V and 8o K(o) ~--- 0 on all planes tangent to T k. This example shows how the 0 i ~ 1 , 2 , then K ( V - - V 1 X V2) vanishes on m a n y 2-planes ~ E T ( V ) . Namely, K vanishes on the Cartesian products of geodesics ~'1 X 7: c V f o r ~,+ c V+, as these products a r e isomtric to 1R: geodesically immersed into V. (This is alweys true for Riemannian products with no r e g a r d f o r the curvature). Some results similar to the splitting theorem were more recently established f o r manifolds V with K ( V ) ~ f o r n--> ~ , but an i m p o r t a n t p r o p e r t y of S ~ with m a n y implications, see [Mi-S]). MANIFOLD WITH Ricci V _ 0. I f Ricci > 0 (i.e. positive semidefinite), then the traced tube f o r m u l a shows t h a t the second derivative (va-

78

M. QROMOV

riation) of logVol, is negative or. equivalently d2 dea log J(v, e) _ 0. One can derive a s h a r p e r inequality f r o m the t r a c e d tube formula ( § by observing t h a t - - T r a c e A ~ _< ( n - - 1 ) -1 (TraceA) 2 for n - - 1 ~ dim W ~ dim V - - 1. Then d f o r m u a f o r - y - J yield

(§

and the preceeding

~E

(+§

a~logJ _ 0. In t e r m s of the mean c u r v a t u r e M of W~ the equivalent inequalities a r e (+,y

d~ log VoL d~

--

dM de _< m

(n--

1 ) -1 M 2

-

-

Ricci

and (§

where

d 2 log Vol. dM d@ - - de - < - ( n - 1 ) M d log VoL

2 for Ricci_>0,

is identified with M according to (,).

In order to emphasize the similarity of the above inequalities with those f o r K >_ 0 in w167 2, 3 we introduce the following terminology. A cooriented h y p e r s u r f a c e W c V is called m e a n conve~ if M ( W ) >_ 0. Then a domain Vo c V with smooth boundary is meun

SIGN AND GEOMETRIC M~,ANING OF CURVATURE

79

convex if the b o u n d a r y is mean convex. Clearly, convexity implies mean convexity as the latter requires the positivity of all principal curvatures not only of t h e i r mean. Also notice, t h a t with our terminology, round spheres in ~ " looked upon as boundaries of balls B c ~ " are convex, but the same spheres which bound the complements 1 R " - - B are regarded concave.

Observe t h a t the round e-sphere St c 1R" has mean c u r v a t u r e ( + . ) ' becomes an equality. Thus (_{_.)t the i n w a r d d e f o r m a t i o n (i.e. f o r e _< 0) at every w E W f a s t e r t h a n i t h a p p e n s the same mean c u r v a t u r e as W a t the particular, i f W is m e a n convex t h e n

( n - 1)e -1 and so the above shows t h a t if Ricci _> 0 then m a k e s W ~ W~ m e a n convex to the r o u n d p h e r e in ~ " of point w E W in question. In so is W~ f o r e V to send W diffeomorphically onto We. This assumption, as we know, is satisfied f o r smooth W and small lel but it is usually violated f o r large ]eI. However, the above mean convexity p r o p e r t y for Ricci _> 0 remains valid f o r all e with an appropriate generalizations of mean c u r v a t u r e and mean convexity to non-smooth hypersurfaces (notice t h a t exactly a t this point the geometry t r u l y comes into play. The above f o r m u l a e for small e would remain a futile infinitesimal exercise if t h e y were not valid globally f o r all W ). The idea of such a generalization comes f r o m the r e m a r k t h a t the intersection of two mean convex domains 111 and V2 in V should be mean convex, though the b o u n d a r y of V1 N V2 m a y be (and usually is) non smooth. Thus one can enlarge the class of mean convex dom a i n s with smooth boundaries by t a k i n g finite and (with some precaution) infinite intersections. Then one defines mean convex nonsmooth h y p e r s u r f a c e s as those which locally are the boundaries of such domains. Alternatively, one can m a k e a definition of mean convexity of W a t a given point w E W with an (( a m b i e n t >) smooth mean convex W' touching W at w f r o m outside as in Fig. 13 used earlier in w 3 to define convexity. F r o m this m o m e n t on we assume we know the meaning of the relation M ( W ) >_ 0 for non-smooth W and we t a k e a similar a t t i t u d e

gO

M. GROMOu

t o w a r d t h e relation M(W) >_ ~ f o r all real 3. We also introduce the notions of strict mean convexity, denoted M ( W ) > 0, as we did earlier in the convex surrounding of w 3. Now let us apply the above principle of the f a s t mean convexity r a t e to concrete geodesic situations. PUSHING-IN MEAN CONVEXBOUNDARY. Let V be complete with compact mean convex b o u n d a r y W~---OV (V itself may be non-compact but look like W X [0, ~ ) ) . Then we define as earlier V7 = {v 6 V [ dist(v, W) >_ e} and observe t h a t the above discussion f o r Ricci >__ 0 implies t h a t V~-c V is mean convex for all e > 0. Then we can estimate the ( n - - D - d i m e n s i o n a l volume of W_~ ~ aV7 since the derivative of Vol W_~ e q u a l s - - M ( W _ ~ ) integrated over W, with a positive weight. Thus we see t h a t Vol W_~ is monotone decreasing in e. Finally we recall t h a t Vol W_~ integrated over E gives the n-dimensional volume of the r band >> V - - V7 which we can now bound by e Vol W. Suppost f u r t h e r m o r e t h a t either W ~---0V is strictly mean convex (i.e. M ( W ) > 0) or Ricci > 0. In this case the inequality (-~*)" integrated over We shows t h a t Vol W~ becomes zero at some finite moment s. It easily follows t h a t V is compact in this case. In particular, it can not be hameomorphic to W X [0, ~). One knows more in this regard. For example, the splitting theorem of Cheeger and Gromoll (who generalized the earlier splitting theorem of Toponogov f o r K >_ 9) implies t h a t if a complete manifold V with Ricci >__ 0 has no boundary and has more than one end (i.e. V - - V o is disconnected f o r all sufficiently large compact subsets Vo in V) then V splits into

the isometric product by V-----W X 9 for some closed manifold W. Recall t h a t the Laplace operator Af on smooth functions f : V--> ~ is

!VIEAN CUKVATURE AND T H E LAPLACE OPERATOR.

Af = div grad f, de/

where t h e vector field g r a d f is defined with t h e Riemannian metric

SIGN AND GEOMETRIC MF_,ANING OF CURVATURE

81

g as the dual of the differential df, i.e. ( g r a d f, ~) ~ df(T) for all ~ E T ( V ) , and the divergence is defined as the L i e d e r i v a t i v e of the Riemannian volume (density) with respect to the gradient. Now let f ( v ) - - - d i s t ( v , W). This function obviously has Ilgradll ~ 1 and the divergence of the g r a d i e n t equals the variation (i.e. derivative with respect to s) of the volume densities Vol, of W , , since the volume of the region between W~1 and W~, in V equals the integral of Vol W~ over [ ~ , e2], and the same remains t r u e f o r all subdomains U~ c W~. Thus we obtain the equality between A f a t v and the mean curvature of W, passing t h r o u g h v. This equality, A f ( v ) ~ M ( M , , v),

makes sense so f a r only where W~ and f are smooth, but with our earlier discussion we can extend the story to all points v E V. In particular, if Ricci V _> 0 and W ~ OV is mean convex, then - - A dist(v, w) _ 0, t h a t is the which m a y function /s is done f o r

minus distance to W is a s u b h a r m o n i c function on W be called m e a n convex in our language. Notice t h a t this convex f o r K _> 0 (compare w167 0 and t/~ where all this V c ~').

VOLUME MONOTONICITY FOR BALLS. Consider concentric balls B(D in V around a fixed point v~ E V and observe with our tube inequalities f o r Ricci V >_ 0 t h a t these balls are (( less mean convex ~> t h a n the corresponding balls in ~ ' , t h a t is M(OB(e)) 0, as f o r Ricci >__ ( n - - 1 ) ~ - : the rate of g r o w t h of spheres and balls in V is dominated b y t h a t in the round sphere S~(#) c ~n+l. It follows, t h a t the volume VolB(D does not increase at al f o r e >_ ~ and so the diameter of V is baunded by ae, which generalizes Bonnet theorem (see w 3). In partcular, the universal covering of every closed manifold V with strictly pos@ive Rieci curvature is compact and ~1 (V) is finite exactly as in the case K > 0 w e studied earlier in w 3. Moreover, even in the non-strict case the s t r u c t u r e of t h e fundamental group

SIGN AND GEOMETRIC MF_,ANING OF CURVATURE

83

zl(V) f o r Ricci ~ 0 is similar to t h a t f o r K _> 0. Namely, nl contains a free Abelian subgroup of rank_< dim V having finite index in ~1. This follows f r o m the Cheeger-GromolI splitting theorem applied to the universal covering of V. (This theorem, in its general form, claims t h a t every complete Riemannian manifold X without b o u n d a r y and with R i c c i X >_ 0 admits an isometric splitting, X ~ Y X ~ , provided X contains a line ~ t h a t is a geodesic which minimizes the distance between each pair of points in ~ ). On the other hand, there seems to be no restriction on the simply connected p a r t of the topology forced by Ricci _> 0. (An exception is discussed in w 6 in the f r a m e w o r k of the positive scalar curvature). F o r example, a recent construction by Sha and Yang (refined by Anderson) provides manifolds V of a given dimension n _> 4 with Ricci > 0 and with arbitrary large Betti numbers. (These manifolds can not have metrics with K >_ 0 by the discussion in w 3). To obtain some perspective one m a y compare the manifolds with Ricci ~ 0 to subharmonic functions while K >_ 0 goes parallel to convexity. This analogy suggests Ricci flat manifolds (i.e. with Ricci ~ 0) as the c o u n t e r p a r t s to harmonic functions (which a r e by f a r more numerous t h a t linear functions corresponding to K - - 0 ) and one m a y expect t h a t a g r e a t deal of simply connected manifolds admit Ricci flat metrics. But even a single example of this kind is not a t all easy to produce. Yet, Yau has proven the existence of these f o r all even n ~ dim V, as he has produced a Riemannian (even KRhler) metric with Ricci ~ 0 on every smooth complex projective h y p e r s u r f a c e V of degree m ~ 1 in (l: pro. (Notice t h a t all such V are mutually diffeomorphic f o r a fixed m and they are simply connected f o r m > 3). Yau has also shown t h a t the h y p e r s u r f a c e s of degree _ m have metrics with Ricci > 0 and those of degree ~ m ~- 2 with Ricci < 0. Moreover, in the latter case Yau has proven the existence of Einstein metrics g on V satisfying the equation R i c c i , - - J g. Among more elementary examples of manifolds with Ricci _> 0 we mention again homogeneous spaces G / H f o r compact G, whose homogeneous metrics often have Ricci > 0 while K is, typically, only non-strictl.v positive. The simolest among them a r e Cartesian products, like those of spheres; S k X S ~ f o r k, ~ >_ 2, and compact semisimple Tie groups G with biinvariant metrics (only S U ( 2 ) and S 0 ( 3 ) have K > 0). Notic t h a t the tube related inequalities provide non-

84

M. GROMOV

trivial information on the geometry of G expressed by Paul Levy's isoperimetric inequaliby which generalizes the classical isoperimetric inequality in S% This inequality is especially interesting when Ricci G --> ~ along with dim G --> ~ as it implies t h e following remarkable concentration of values of functions f~ : G~--> ~ with [[gradf~[[a~ _< const.

The pushforwards of the Haar measures on Gi weekly converge to the Dirac 5-measure on ~ for dim Gi--> ~, provided the biinva. riant metrics are normalized to have Diam(Gi,g~)~-~-I and fi are normalized by

f l ~ 0, (See [Mi-S]). *]

Gi

SINGULAR SPACES WITH Ricci >_ 0. The theory of these spaces does not exist yet. It seems h a r d (if at all possible) to express adequately the inequality Ricci >_ 0 by some universal distance inequality similar to Alexandrov-Toponogov f o r K _> 0. Although the condition Ricci _> 0 does imply some relations on t h e distance, e.g. those related to balls inside a larger ball (see above), these are not strong enough to characterize Ricci _> 0. Let us state a finer inequality of this kind, t h a t a p p e a r s v e r y much similar to A - T b u t still does not furnish a characterization of Ricci _> 0. We consider, as in the case K >_ 0, f o u r points v~ E V, i----- 0, 1, 2, 3 w h e r e v3 lies on a minimizing segment [ v l , v2] (see Fig. 15 in w 31/2) and we w a n t to give the lower bound on t h e distance Ivo--v~] between vo and v3. Denote by E the excess in the triangle inequality f o r (v~, v~, v2), t h a t is

E ~ Iv0 - - vii + I v o - - v ~ l -

I v 1 - v_~l

and let ~--~

~in(Ivo--

v, I, Ivo - - v~]).

Then, if Ricci >_ 0, the distance I v o - v3[ satisfies the following A B R E S C H - G R O M O L L INEQUALITY. 1

Ivo--v31 >_ (s(E/4).-~)'. (See [Che] f o r a proof). Recall t h a t the distance inequalities, in general, characterize the image of the distance function on V N, denoted M , ( V N) c ~ N', N'

N ( N - - 1 ) (see the discussion preceeding Alexandrov-Toponogov 2

SIGN AND GEOMETRIC MEANING OF CURVATURE

85

in w 31/~). Now, the Ricci c u r v a t u r e manifests itself via t h e tube inequalities f o r the volume behavior of the distance function. Thus one m a y expect the desired abstract characterization of Ricci _> 0 in t e r m s of the p u s h - f o r w a r d of the Riemannian m e a s u r e to ~N,. ( F o r example, the Bishop inequality ( + ~-) is of this nature). Notice that f o r general metric spaces there is no distinguished measure, and so the (hypothetical) theory of Ricci >_ 0 m u s t include a m e a s u r e as a given element of the s t r u c t u r e along with t h e metric. An i m p o r t a n t f e a t u r e of the theory of spaces with K >__ 0 defined by A-T is the good behavior under the Haussdorf limits of sequences of spaces, w h e r e the H a u s s d o r f convergence V~--> V roughly corresponds to the convergence, for every N ~ 2 , 3 ..... of the subsets

MN(V~) c ~N, to MN(V N) for the H a u s s d o r f metric (see [G-L-P] more about it). Now, in the Ricci c u r v a t u r e case one probably should allow weak Haussdorf limits corresponding to w e a k limit of the M , - p u s h f o r w a r d measures on ~N,. There is another option f o r the abstract theory of Ricci >__ 0 w h e r e instead of the metric ont emphasizes the heat flow (diffi~ion) on V, b u t at this stage it is unclear w h e t h e r the t w o approaches are equivalent and if not which one is better f o r applications. Let us indicate a specific problem giving more substance to the above discussion. We recall that smooth manifolds with Ricci >_ ~ > 0 satisfy Paul Levy's inequality which implies, in turn, certain bounds on the spectrum of the Laplace operator A and on t h e heat kernel on V. (See [Mi-S], [Gall). Now we ask if similar bounds remain valid on singular spaces with K _> 0 w h e r e the e x t r a condition Ricci _> e is enforced in an appropriate way. F o r example one can strengthen the A-T-inequality in w 31~ to m a k e it equivalent to K >__e / n - - 1 which would imply the above bound on Ricci. ( F o r the meaning of the spectrum of A etc. on an a b s t r a c t metric space we r e f e r to [Gr]l). ON RIccI _< 0. The traced tube formula does not provide much information if Ricci _< 0 (unlike the case K _< 0) and in fact nothing is known on the geometric meaning of this condition. It seems t h a t the only known result is the old theorem by Bochner saying t h a t a closed manifold V with Ricci < 0 must have finite isometry group.

86

M. GROMOV

Also no topological restriction on V seems to issue from the negative Ricci. One believes nowadays that every manifold of dimension n _> 3 admits a complete metric with Ricci < 0. ( F o r n ~ 3 this is a thearem of Gao and Yau) (*). EXAMPLE. If V Js immersed into ]R m f o r some m as a minimal subv a r i e t y then the induced metric has Ricci _< 0 (by an easy computation) and one m a y expect t h a t every open manifold V admits a complete minimal immersion into some Euclidean space. R E M A R K S ON PSEUDOCONVEXITY AND POSITIVE BISECTIONAL CURVATURE. The notion of ps~doco~vexity of domains and hypersurfaces is (~" (see w I/~)interpolates between convexity and mean convexity. Then one m a y ask what are the manifolds endowed with complex structures and Riemannian metrics where the inward equidistant deformations preserve pseudoconvexity. Notice, that a priori there m a y be no such manifolds at all, but, in fact, they do exist. They are K~hler manifolds with a certain inequality on the curvature tensor called the positivity of the bisectional curvature. ( F o r example every K~ihler manifold with K _> 0 has bisectional c u r v a t u r e >__ 0). A theorern of Siu-Yau claims t h a t every closed manifold with strictly

positive bisectional curvature is diffeo~no~phic (even biholomo~phic) to (~ P~. Yet t h e r e is no direct p r o o f of this result using pstudoconvex deformations. (Siu and Yau use hwrmonic maps S 2 --> V, compare w 7 ~ . There is another approach due to S. Mori w h o appeals to algebraic g e o m e t r y over finite (!) fields). On the other hand the pseudoconvexity considerations a r e v e r y useful in the s t u d y of complex subvarieties in V with positive bisectional curvature. F o r example, f o r every complex h y p e r s u r f a c e H c (~ P~ the regions IT:- ~ {v e r P~ I dist(v, H ) >_ e} are pseudoconvex and, hence, by easy Morse theory G p m H has the homotpy type of an m-dimensional polyhedron. This is (the special case of) the famous theorem of Lefschitz. Finally w e notice f o r experts that similar positivity conditions can be introduced f o r other manifolds with restricted holanamy, b u t the usefulness of these is limited by the list of known examples. (*) A strengthened version of this conjecture, namely the h-principle for Ricci ~ 0, is proven by Lohkamp for all n >_ 3.

SIGN AND GEOMETRIC MEANING OF CURVATURE

87

w 6. POSITIVE SCALAR CURVATURE. The formal definition of the scalar c u r v a t u r e Sc(V) is easy,

Sc (V) ~- Traceg Ricci V. Then, if we recall the definition of Ricci in t e r m s of t h e sectional curvature, we can compute Sc at a given point v E V by using an orthonormal f r a m e ~,1 ..... v, in T~(V) and adding together the sectional curvatures of the planes o~j, spanned by ~ and ~j f o r all 1 0 t h e n e v e r y s u f f i c i e n t l y small ball B(V, s) h a s Vol B(V, s) < s'* Vol B ~ ~ Vol B ( 1R", s). Conversely, if Vol B (V, e) _< e" Vol B ~

90

M. G R O M O V

for all sufficiently small E, then Sc~(V) >_ O. (Notice, that our discussion here and earlier only m a k e s sense f o r n _> 2. If n ~ 1 all Riemannian manifolds are locally isometric to ~ and t h e r e is no c u r v a t u r e to speak of). It m a y seem at this stage that we have achieved a certain understanding of the scalar c u r v a t u r e of V. Yet the above infinitesimal relations f o r Sc do not integrate the w a y it was happening f o r the sectional c u r v a t u r e and Ricci. In fact, we a r e still nowhere as f a r as the g e o m e t r y and topology of manifolds with Sc >_ 0 (or Sc _ O. F i r s t we observe that the scalar c u r v a t u r e is additive f o r the Cartesian product of manifolds. Thus, if a manifold V has inf Sc > M co, (e.g. V is compact) then t h e product of V with a small round sphere S~( O. The f i r s t global result for Sc > 0 was obtained by Lichnerowicz in 1963 who proved the following LICHNEROWICZ THEOREM. I f a closed 4k-dimensional spin manifold V

admits a metric w i t h Sc > 0, then a certain chwracteristic n u m b e r of V, namely the .~-genus, vanishes. The meaning on along with the the Atiyah-Singer w e only indicate applies.

of _ m ~- 2, then A ( V ) ~ 0, and so such a V cannot have a Riemannian metric with S c V > 0. The simplest such manifold is quartic (i.e. d~-~-4) in Gp3, which is a 4-dimensional simply connected manifold V' which by Lichnerowicz's theorem admits no metric with Sc > O. (By Yau's

SIGN AND GEOMETRIC MEANING OF CURVATURE

91

theorem mentioned in w 5 this V 4 admits a metric with Ricci ~ 0, and hence with Sc ~ O. On the other hand, even if we a r e content to show r no metric with Ricci :> 0 >> or even less, 0 ~, we are still unable to do it geometrically w i t h o u t an appeal to the deep analysis underlying the proof of the Lichnerowicz theorem). The method of Lichnerowicz was extensively developed by N. Hitchin, w h o has shown, among other things, t h a t there exists an exotic 9-dimensional sphere V (i.e. a manifold which is hameomo~phic but not diffeomorphic to S 9) which admits no metric with Sc > 0. (In fact, half of the exotic spheres in dimension 1 and 2 (mod. 8) c a r r y no such nmtrics by Hitchin's theorem). H e r e again, t h e r e is no alternative geometric approach even with Sc > 0 replaced by K > 0. SCALAR C U R V A T U R E AND MINIMAL HYPERSURFACES.

The first geometric

insight into Sc >_ 0 was achieved by Schoen and Y a u in 1979 with the following innocuously looking modification of the traced tube formula (see (**) in w 5) for surfaces W in a 3-dimensional manifold V. At every point v ~ W w e consider the tangent plane o~ ~ T~(W) and the unit normal vector v~ to W. ( W e assume W is cooriented and stick to inward looking ~). First w e observe that our formulas expressing Ricci and Sc in terms of K imply, that

Sc~ ~---2K(o~) + 2 Ricci (~, ~). (In general, for hypersurfaces W in V ~ for ~ ~ 3 the term 2K(o~) must be replaced by the s u m of K(mj) for some orthonormal basis vl .....~'~-i in T~(W)). Then w e bring in the principal curvatures ~i and 42 of W at v and recall that the sectional curvature of W with the induced metric is expressed according to Gauss' formula (Teorema egregium in w 2) as

K ( W , o~) ~ K ( V , o,~) ~- ~1 ;~2, which is equivalent to 1 (M 2 _ TraceA'-) K ( W , o~) ~ K ( V , ~ ) + --~ where A is the shape operator of W (whose eigenvalues a r e exactly ~1 and ~2) and M ~ Trace A ~ ~t + )~., is the mean c u r v a t u r e of W.

92

M. GROMOV

T h e t u b e f o r m u l a (see (**) in w 5) e x p r e s s e s t h e d e r i v a t i v e of M u n d e r t h e n o r m a l e q u i d i s t a n t d e f o r m a t i o n W, of W ~ as follows

dM de

--

Wo a t ~ ~ 0

T r a c e A'-' - - Ricci (~, ~,).

T h e n w e recall t h a t M equals t h e ( l o g a r i t h m i c ) d e r i v a t i v e of t h e v o l u m e d e n s i t y on W~ a t ~ 0 (see w 5). I t f o l l o w s b y i n t e g r a t i o n o v e r W, t h a t t h e d e r i v a t i v e - -

A r e a W~ a t s ~ - 0

equals t h e t o t a l

m e a n c u r v a t u r e of W

~ f M dw. W

( W e s a y r A r e a >> r a t h e r t h a n since dim W ~ 2). Then we observe that

dM dE - -

f ~ dM d w - ~ fM:dw, )y w

w h e r e t h e second s u m m a n d is due t o t h e v a r i a t i o n of t h e v o l u m e ( a r e a ) e l e m e n t d w ~ Vol~ = ~ e x p r e s s e d b y t h e m e a n c u r v a t u r e . T h e n

dM

w e s u b s t i t u t e Ricci in t h e a b o v e t u b e f o r m u l a f o r ~

by

1

2 Sc(V) + K ( V I T ( W ) ) a n d t h e n w e use t h e Gauss f o r m u l a 1

K ( V I T ( W ) ) - - - K ( W ) + - ~ ( T r a c e A -~- - M : ) . T h u s w e o b t a i n t h e f o l l o w i n g second v a r i a t i o n f o r m u l a f o r t h e a r e a of W ~ W ~ at s~0.

d: Area W dM 2 [ d~ - - de - - 1 .I ( - - S c V ~- 2 K ( W ) - - T r a c e A W

W

2 ~- M 2 ) d w ~

SIGN AND GEOMETRIC MEANING OF CURVATURE

93

Now, we rcall the Gauss-Bonnet theorem

f K(W) ~--- 2~z(W), W

w h e r e • denotes the Euler characteristic and W is assumed compact without boundary. Then, if Sc (V) >_0, w have the following inequality, d 2 Area W < 2n z(W) +

d~

f ~ ) ; dw. W

In particular, if x(W) < 0 and W is a saddle surface, i.e. if ~1 ~2 ~--- 0, then d 2 Area W (++) d~ < 0. Notice, t h a t as at the previous occasions, this conclusion only applies to small equidistant deformations which do not distroy the smoothness of W. Now, instead of extending the above computation to non-smooth W~ as we did earlier for K > 0 and Ricci > 0, we follow the idea of Schoen and Yau and apply ( + + ) to smooth minimal surfaces W in V. (Non-smooth extension of the above is questionable because of the saddle condition). The existence of such surfaces is insured by the following theorem known since long in the geometric m e a s u r e theory (see, e.g. [Law]).

Every 2-dimensioncd homology class in a closed Riemannian 3 dimensional V can be represented by a smooth absolutely minimizing embedded oriented surface W c V. Recall t h a t means t h a t every surface W' c V homologous to W has Area W' _>_ Area W. REMARK. A similar result remains valid for minimal hypersurfaces W in V ~ f o r n _> 3 b u t now these W m a y have singularities. One knows, t h a t the singularity is absent f o r n _< 7 and, in general, it has codimension _> 7 in W. Now, since a minimizing s u r f a c e W provides the minimum for the function W ~-> Area W on t h e space of surfaces in V, the f i r s t

94

M. GROMOV

variation of Area is clearly zero and the second is non-negative. In particular d2 A r e a W

d~

_>0.

Moreover, every connected component of W, say We, also has d ~ Area Wc >_0. d~ F u r t h e r m o r e , minimal surfaces have M ~ I ~ - ~ 2 ~ 0 and so are saddle. Hence, the above inequality is incompatible with Sc(V) >__ 0 and the issuing inequality ( + + ) unless 7~(V) _> 0. Thus we conclude to the following. SCHOF_~I-YAU THEOREM. L e t V be a closed 3-dimensional Riemaz~ian manifold w i t h Sc(V) >_ 0. Then every homology class in H2(V) can be realized by an embedded oriented surface W whose every connected componenti has Z >-- O. EXAMPLE. L e t V ~ Vo X S1 w h e r e V~ is an orientable surface of genus _> 2 (i.e. x(Vo) < 0). Then elementary topology tells us that 11o---~ 11o X S,o c V is not homologous to a surface whose all components have genus _< 1. Therefore this V admits no metric with Sc >__O. (By an obvious r e a d j u s t m e n t of the above discussion, one rules out Sc ~> 0 starting f r o m genus ( Vo) -----1). Schoen and Yau have generalized their method to manifolds V ~ with n _< 7 and they proved t h a t if Sc(V n) _> 0, then every class in Hn-I(V") can be realized by a hypersurface W which admi2s some metric w i t h Sc >_ O. In fact, they t a k e the valume minimizing hypers u r f a c e f o r W c V~ and then modify the induced metric in W by a conformal factor to m a k e Sc >_ O. This does not w o r k f o r n :> 7 due to t h e (possible) presence of singularities on minimal W b u t later on Schoen and Yau indicated a w a y out of this problem. (See [Sch] f o r a b r i e f account of these results). The above theorem of Schoen-Yau shows (by a simple induction on n) t h a t there are non-trivial topological restrictions on V~ with S c ( V D > O. F o r example, the Cartesian product of surfaces of genus

SIGN AND GEOMETRIC MEANING OF CURVATURE

95

>__ 2 a d m i t s no such metric. F u r t h e r m o r e , their method can be refined in order to provide non-trivial geometric restrictions on V as well. F o r example, let V~ be a complete non-compact oriented Riemannian manifold w i t h o u t b o u n d a r y with uniformely positive scalar curvature, i.e. S e ( V ~) >_ c > O. Then V ~ admits no proper distance decreasing map to ~ of non-zero degree. In other words, V" is no larger than ~ . EXAMPLE. Let V" ~ $2 X 1R"-2 with the product metric. Clearly this V" (which has Sc >_ c > 0) admits no above map to 1R". B u t if we m o d i f y the product metric gs ~ gs on V" ~ S 2 X 1R~-2 by introducing a so-called waxTping factor, t h a t is a positive function ~ : t~ "-2 --> t~ +, and by m a k i n g g ~ ~gs ~- g~, then f o r (V, g) we can easily produce a contracting proper m a p into 1R~ of degree one, provided the function ~(x) satisfies the asymptotic relation ~0(x)-~ ~ for x - ) ~ . It follows, t h a t such a w a r p i n g ~ necessarily makes inf Sc g 0 non-uniformly on V.

w 61/,2.

SPINORS

AND T H E

D I R A C OPERATOR.

Now we r e t u r n to Lichnerowicz' approach. F i r s t we recall t h a t the f u n d a m e n t a l group of the special orthogonal group is

(SO(n))~tZ

for n ~ 2

7T1

•.

f o r n_>_3.

Thus SO(n), f o r all n >_ 2, admits a unique double cover denoted Spin (n) --~ SO (n),

where Spin(n) carries a n a t u r a l s t r u c t u r e of a Lie group such t h a t the above covering m a p is a homomorphism. (This is quite obvious f o r S 0 ( 2 ) ~ S 1 and easy but not all obvious f o r n _> 3).

96

M. GROMOV

Next, f o r a Riemannian manifold V, we look at the orthonormal f r a m e bundle SO(V), that is the principal bundle with the fiber SO(n), ~ = dim V, associated to T(V), and we ask ourselves if there exists a double cover Spin(V) --> SO(V) which reduces over each point v E V to the above Spin(n)-+SO(n). This is clearly possible if t h e t a n g e n t bundle (and, hence, SO(V) as well) is trivial, T ( V ) = V X 1R~, as one can t a k e V X Spin(n) for Spin V. In general, there is a topological obstruction f o r t h e existence of Spin(V) which can be easily identified with t h e second St~eSelWhitney class w2(V). This is a certain cohomology class in H2(V, •2) which measures non-triviality of T(V) and which is also known to be a homotopy invariant of V. In any case, w2 = 0 i f / ~ (V, Z2) = 0 and then Spin ( V) --->SO ( V) does exist. The space Spin(V), whenever it exists, has a natural s t r u c t u r e of a principal Spin(n) bundle over V and then one may look for associated vector bundles. These come along with linear representations of the group Spin(n). There are, for even n = 2r, t w o distinguished faithful (spin) representations of Spin(n) of (lowest possible) dimension 2 ~-1, for which the corresponding vector bundles, denoted S+ --> V and S_ --> V, a r e called the positive and negative spin bundles, whose sections are called (positive and n e g a t i v e ) s p i n o r s on V. Atiyah and Singer have discovered a r e m a r k a b l e elliptic differential operator between the spinors, i.e.

D+ : C~ (S+)-->C ~ (S_), which t h e y call the Dirac operator. This operator is constructed with t h e connection V+ in S+ induced b y the Levi-Civitta connection in V, w h e r e the connection in S+ is thought of as an operator f r o m spinors to spinor valued 1-forms on V, i.e. V+ : C~ (S+)-*9I(S+) 9 Then D+ is obtained by composing V with a certain canonical vector bundle homomorphism ~21(S+)-->S_ coming from some algebraic manipulations with spin representations. (We somewhat abuse the

SIGN AND GEOMETRIC MF~NING OF CURVATURE

97

notations by using .QI(S+) for the bundle of spinor-forms as well as for the sections of this bundle). Notice that D+ is defined locally and needs no > w2 ~ 0, but if w2 r 0, then spinors are globally defined only up to _+ sign. The interested reader may look to the book [L-M] for an actual construction of the spinors and Dirac. Here we just assume the existence of certain bundles S+ and S_ and an operator D+ with the properties stated below. Besides the operator D+ we need its twin, called D_ : C a (S_)-->C ~ (S+)

which is constructed in th same way as D + and which can be defined as the ad]oint operator to D+ for the natural Euclidean structure on the spin-bundles. Then one looks at the index of D + , i.e. Ind D+ ~ dim ker D+ - - dim ker D _ , where the dimensions of the kernels of D+ and D are finite if V is a closed manifold, since the operators D+ and D_ are elliptic. The remarkeble (and easy to prove) property of the index is the invariance under the deformations of D+ in the class of elliptic operators between spinors. In particular, this index does not depend on the Riemannian metric used for the definition of D+ and so it represents a topological inv~riant of V. The famous theorem of Atiyah and Singer identifies Ind D+ with a certain characteristic number called ft.-genus of V, but for our present purpose we may define A-genus as I n d D + . The only serious property of D+ we need at the moment is non-vanishing of A(V) for certain manifolds V. (Otherwise, what follows will be vacuous). Now we need another operator associated with Bochner Laplacian,

V+, called

=v+*v+ where V~+ : ~ ) l ( S + ) - - - ) C Qr ( S + )

is the adjoint to V + . This Laplacian makes sense for an arbitrary

8ominario

Matematico

e Fisteo- 7

98

M. GROMOV

vector bundle with a Euclidean connection over V and an important property of A+ is positivity, i.e.

f (A+ s, s) >_ 0 w

for all spinovs s : V-->S+. (The Bochner Laplaeian for the trivial 1-dimensional bundle reduces to the classical Laplace Beltrami operator A ~ d* d on functions whose positivity follows by integration by parts, as ( f A r =

f (dr, df), and a similar consideration proves

positivity of A + on spinors). Now the scalar curvature enters the game via the following IACHNE~OWICZ FORMUt~. The operator D_ D+ q- D+ D_ on S+ ~ S_

is related to A + q- A_ by 1 D_D+ + D+ D ~ A + q-A_ q- --~ S c l d . Recall that S e n S e ( V ) denotes the scalar curvature which is a function on V and Id is the identity operator on C | (S+ (~ S_). The proof of the Lichnerowicz formula consists of a straightforward (infinitesimal) algebraic computation which is quite easy with the definitions (we have not given) of D+ and D_. Yet the geometric meaning of the formula remains obscure. COROLLARY. I f ~b ctosed Riemannian spin (i.e. with w 2 ~ O) manifold V has Sc(V) > 0 then every harmonic spinor on V vanishes and so

A(V) ~ Ind D~ ~ 0. Here, ((harmonic spinor, means a spinor s ~ such that

(s+, s_) E C | (S+ (]~S_),

Ds~D+s+ +D_s_~O. de/

The proof of the corollary is obvious.

f (ns, s) = f v

v

s> + sc <s, s),

SIGN AND GEOMB'rRIC MEANING OF CURVATURE

99

which implies b y positivity of LI t h a t

j" (Ds, s) >_ f Sc {s,s) V

and then for

V

Ds ~ 0 w e get

f Sc(s,s) 0. Notice again, t h a t this Corollary is non-vacuous since there exist spin manifolds with A - ~ 0, e.g. complex h y p e r s u r f a c e s in G / ~ + 1 memtioned earlier in w 6. This p r o p e r l y of non-vanishing of Ind D f o r some V and the Lichnerowicz formula is all which is needed f r o m spinors and Dirac in order to show t h a t some manifolds V admit no metric with Sc > 0. Although we do not quite understand the g e o m e t r y behind the Lichnerowicz formula, we can use this formula to reveal some geom e t r y of V with Sc V ~_ c > 0. Namely, we w a n t to show t h a t such a V cannot be r too large >>. F o r example, it cannot be much larger than the unit sphere S ~. Indeed imagine t h a t V is much larger than S ~ in t h e sense t h a t t h e r e exists a smooth m a p f : V--> S ~ of degree d ~-O, such t h a t the differential of f is e v e r y w h e r e small, I[Df[l~ < e, v e V. Then we pull-back to V some fixed vector bundle Eo with a Euclidean connection over S ~. The pull-backed bundle, say E over V, is V with the trivial connection then S+ |

E -----S+ + S+ + . . . + S + k

and the t w i s t e d Dirac is D+ ~- D+ ~ ... ~- D + . The definition of the twisting with a non-trivial connection is such that the e-flatness of E

100

M. G R O M O V

makes the twisted Dirac e-close to D+ ~- D+ ~- ... ~- D+). I t follows, t h a t is E is small compared to c - ~ - i n f Sc V > 0, then the twisted v Dirac operator D + | E has IndD+ |

E~0

by an E-perturbed version of the Lichnerowicz formula. Now, one can find in certain cases a bundle Eo such t h a t Ind D | Eo ~ 0. In fact, on can always produce such a (complex vector bundle) Eo over an even dimensional sphere, as follows f r o m the A t i y a h - S i n g e r index t h e o r e m applied to D+ | E. Therefore, no spin manifold V with S c ( V ) _> c > 0 can be E-1 times g r e a t e r t h a n S ~ f o r e 0 has no rational explanation at the present state of art. In general terms, the Schoen-Yau method appeals to the (non-linear) analysis in the space of submanifolds in V while t h e Dirac operator approach uses the linear analysis (of spinors) over V. One m a y hope f o r the existence of a unified general theory which would t r e a t simultaneously non-linear objects inside V as well as linear ones over V in a w a y similar to w h a t happens in algebraic geometry. Probably, such a unification m a y be possible only in an infinite dimensional f r a m e w o r k . SCALAR CURVATURE _< 0. This condition has no topological e f f c t on V by a theorem of Kazdan and W a r n e r which claims t h e existence of a metric Sc < 0 on every manifold of dimension n __ 3. Probably, the global geometry of V is also unsensitive to Sc < 0 (though the condition Sc >_ c f o r c < 0 does have non-trivial corollaries) (*). (*) The flexibility and the h-principle (in the sense of [Gro]) for Sc < 0 is proven by Lohkamp.

SIGN AND GEOMETRIC MEANING OF CURVATURE

w 7.

THE

101

C U R V A T U R E O P E R A T O R AND R E L A T E D I N V A R I A N T S .

We have mentioned in the end of w 2 t h a t the sectional c u r v a t u r e function on the space of 2-planes of V, i.e.

K : Gr~ V--> IR uniquely extends to a quadratic f o r m (function) Q on the bundle A 2 T(V), and the symmetric operator R : A ~ T ( V ) --> A ~ T ( V ) corresponding to Q is called the curvature aperator. The condition K >_ 0 can be expressed in terms of Q b y Q(rA~', rA~') >- 0 f o r all tangent vectors r, ~, in T,(V), v E V, while strict positivity K > 0 corresponds to Q(~A~, TA * ) > 0 f o r all pairs of linearly independent pairs (T, ~). F r o m the point of view of Q a more natural condition is Q >__ 0 which means Q(a, ~)>_ 0 f o r all ~ E A ~ T ( V ) (that m a y be sums k r~A~', for k > 1) which is called pos@ivity of the curvature

a~Z ~1

operator R. Then strict positivity of R refers to positive definiteness of Q. Similarly one introduces the (strict and non-strict) negativity of Q and R. The above positivity of Q and R is a significantly m o r e restrictive condition than K >__0. Yet, the basic examples of manifolds with K _> 0 also have Q >__ 0. Namely, convex h y p e r s u r f a c e s in 1R~-1 and compact symmetric spaces have Q >_ 0. Also Cartesian products of manifolds with Q _ 0 have Q _> 0. To see the point of d e p a r t u r e between K _> 0 and Q >__ 0 we look at the complex projective space (~ P" with a U(n -{- 1)-invariant Riemannian metric g f o r a natural actio~ of the u n i t a r y group on (~ P~. It is not h a r d to see that such a g (which exists because U(n ~- 1) is compact) is unique up to a scalar multiple and ((~P~, g) is a symmetric space af rank oqze which is equivalent f o r compact symmetric spaces to K > 0. In fact, w e already know (see w 33/~) 1 t h a t the sectional c u r v a t u r e s of g are pinched between ~ - a and a

102

M. G R O M O V

f o r some constant a > 0 depending on (normalization of) g. (An inquisitive r e a d e r would be h a p p y to learn that a ~ n -1 (Diam

(r P~, g))-~). On t h e other hand, the c u r v a t u r e operator R is only non-strictly positive, i.e. the form Q is only svm/-po~itive definite. Thus a small p e r t u r b a t i o n of g m a y easily b r e a k the condition R >_ 0 without destroying K >_ 0. The above example of (~ P~ is especially interesting in view of the following well known CONJECTURE. If a closed n-dimensional Riemannian manifold has R > 0 then its universal covering is diffeomorphic to the sphere S ~. The positive solution is classical f o r n ~ - 2 where R is t h e same as

K and I K > 0 implies g(V) ~ 0 b y t h e Gauss-Bonnet theorem. v

The cases n ~ 3, 4 a r e due to R. Hamilton whose p r o o f uses a deep analysis of a heat flow an the space of metrics. Namely, Hamilton considers the following differential equation f o r a onep a r a m e t e r family of metrics g~ on V,

dg, dt for a~2~

~ - a~ gt - - 2 Ricci (gt)

-1 f Sc(g~)/Vol(V, gt), and he provss t h e solvability of Y

this f o r a given initial metric g ~ go. Then he shows t h a t t h e resuiting h e a t flow preserves the subspace of metrics g with R(g) ~ O. (This is called 0 becomes crucial because it is invariant under the flow. (Notice that the t e r m a~ gt in Hamilton's equation is b r o u g h t in f o r the purpose of a normalization, while the c u r v a t u r e discussion applies to the equation

dg, _ - - 2 dt

Ricci(gt)).

There a r e other m o r e stringent c u r v a t u r e conditions which a r e also invariant under the heat flow and f o r some cases one is able to prove the eventual contractibility to constant c u r v a t u r e F o r example one has as a corollary the following result f o r metrics g with point-wise pinched sectional curvature. (RUH-HUISKEN-MARGARIN-NISHIKAVA) Let the sectional c~rva~re K : Gr2(V)--> ~ of a closed Riemannian n-dimensionsional manifold V be pinched (i.e. restricted) at each point v ~ V by

c~ a(v) _ 2 are significantly more complicated than Ricci*. Yet one has the following result of Bochner-Yano-BergerMeyer, (see [L-M]). If R > 0 then Rk > 0 (i.e. positive definiteI for all k V:0, n ~ dim V. Thus every closed Riemannian manifold with positive curvature operators has Hk(V, ~ ) ~ 0 for 1 _< k _< n - - 1 . This result shows that R > 0 implies that V is a rational homology spherg which is significantly weaker than being diffeomorphic to the sphere required by the conjecture stated above. Now, a recent theorem by Micallef and Moore claims that the universal covering of V is, in fact a homotopy sphere, and hence homeomorphic to the sphere by the Poincar~ conjecture (solved for n >__ 5 by S. Smale and for n ~ 4 by M. Freedman. The remaining case n ~ 3 for Ricci > 0 is taken care of by Hamilton's theorem cited earlier). The method of Micallef-Moore is similar to that employed by Siu and Yau in their study of K~hler manifolds with positive bisectional curvature (see the end of w 5). Both methods make an essential use of harmonic maps of the sphere S ~ into V and the curvature appears in the second va~iation formula for the energy of a harmonic map as we are going to expain next.

106

M. GROMOV

w 71/~.

H A R M O N I C MAPS AND T H E C O M P L E X I F I E D C U R V A T U R E

Kr

.

The energy of a smooth m a p between Riemannian manifolds, say

f:W.--->V is defined by

E ( f ) ~ ~1 .t'][Df(w)[I2dw w

w h e r e t h e squared norm of the differential

D~Df(w)

: T,,~(W).--.> Tv(V), v ~ f ( w ) ,

at each point w ~ W is

[IDfll 2 = Trace D* D w h e r e D* : Tv(V)..--> Tw(W) is the adjoint operator. A map f is called the energy thought of W--> V. The stationary plain w o r d s t h a t for

harmonic if it is stationary (or critical) for as a smooth function on the space of maps condition f o r E at f, i.e. d E ( f ) ~ O , says in every smooth one-parameter deformation

ft of f ~ fo the derivative dE(ft) vanishes at t ~ 0. Notice t h a t this dt derivative of E at t ~ 0 depends only on the 9 d i r e c t i o n , of the deformation f t at t ~ - 0 ,

t h a t is the vector fields 8 ~ - ~

of in V

along f(W). More precisely, 6 is a section of the induced bundle T* ~ f* (T(V)) ...-> W. Harmonic maps can also be defined as solutions of a certain system of non-linear partial differential equations, namely the EulerI ~ g r a n g e equations corresponding to E. This system can be w r i t t e n as A f ~ 0 w h e r e the operator z] generalizes the classical Laplace operator. In fact, if one takes geodesic coordinates xi .... , x~ at w E W and y l , .... y~ at v ~---~-f(w) and represents f b y n functions y,~--~-y,(f(x~,..., x~)), then the above A f ( w ) becomes equal to the ordinary Laplacian of the vector-function YI .... , y,, at zero, t h a t is

9

J

OxJ

.... '

J

Ox'-'

J

SIGN AND GEOMETRIC MEANING OF CURVATURI~

107

EXAMPLBS. (a) I f V is the circle S 1 then every map f : W--> S ~ is locally represented by a function ~ : W --> 1R defined up to an additive constant and

E(f)-~-~

f ]]grad~l

9

W

Then the equation Aft---0 is the same as A~ ~ - 0 for the o r d i n a r y Laplace-Btltrami operator on W. (b) Now, let W ~ S 1 and V be a r b i t r a r y . Then

A f ~ V~ T, where at.:d~ ae: a

f o r the s t a n d a r d (cyclic) p a r a m e t e r s on S 1, w h e r e - ~ denotes the correspondence (coordinate) vector field on S 1, and w h e r e V is the covariant derivative in V. H a r m o n i c maps f : S 1 --> V are those where V r r ~ 0. These are exactly geodevic maps: the image of f is a geodesic in V and the p a r a m e t e r is a multiple of the length. The c u r v a t u r e of the ambient manifold V enters the picture once we look at the second variation

d~E(ft) at t ~ 0 . 62 E(f) a-~f ~ dt In general, the second derivative in t along f o ( W ) c

V at t ~ 0 clUE(f t) Of i.e. at t~---0). depends not only on the field ~ : ~ dt 2 at

af

t , ~ 0 but also on the derivative A~ ~ - . However, if f ~ fo is harmonic, then this derivative depends only on ~ which justifies the notation 62 E in this case, in fact, if a function E h a s zero (first) differential a t some point f, then t h e r e is a well defined second differential (or Hessian) H of e at f which is a quadratic f o r m on the vector fields ~ at f, such t h a t

6(6E(f)) = H(6, 6).

108

M. GROMOV

T H E SECOND VARIATION F O R M U L A

FOR H A R M O N I C MAPS. If f is a smooth

harmonic map, then (*)

O'~E(f)~-~ l" (IIVSI] ~ 4- K(~2))dw,

where V is the covariant derivative of ~ in V along W and K(5 "~) is au algebraic quadratic expression in J involving the curvature of V. (Notice that in our earlier formulas for the second variations for areas and volumes the field ~ w a s unit and normal to a hypersurface W and the ~7 ~-term w a s zero). Let us m a k e the above precise. F i r s t w e rcall that ~ is, in fact, a section of the induced bundle T*--> W and denote by ~7 the connection in T* induced f r o m the Levi-Civita connection in T(V). Then 11~75112 makes sense as V is a differential operator with values in the bundle ~(~1T~k ~ H o m ( T ( W ) , T*) which def

has a natural Euclidean s t r u c t u r e coming from those in T* and T(W). N o w we t a k e care of the c u r v a t u r e term. F i r s t we extend the sectional c u r v a t u r e K by bilinearity to all pairs (~, v) of vectors in V. In t e r m s of the form Q on A:T(V) this reads K(~A,)--

Q(vA,', ~-A,')-

Then the c u r v a t u r e K(J:) is expressed at every point w ~ W with an orthonormal f r a m e rl,..., T,, in T~(W), m ~ - d i m W, b y

-~(&~) ~ - - - - Z

K((Dz~)A~), for D ~ D f ,

i~l

w h e r e the result does not depend on the choice of the frame. (See [E-L]1 and [E-L]2 for an extensive discussion of these matters). Now, w e see that K(V) _ 0 and so one m a y expect t h a t every harmonic maps provides a local minimum f o r the energy. In fact. every harmonic m a p of a compact manifold W into a complete V with K < 0 gives the absolute minimum f o r the energy function (see [E-L]1). If K ( V ) >_ 0 one m a y expect (~2 E to be negative for those fields ($ whose (covariant) derivatives along W a r e small. F o r example, if ~ is V-parallel along W, i.e. V b ~ - 0 , then ~2(E) _< 0.

SIGN AND GEOMETRIC MEANING OF CURVATURE

109

EXAMPLE. Let W - - S 1 , then f ( S 1) is a geodesic in V and every vector x E T~(V), v - ~ f(w) admits an extension to a V - p a r a l l e l field along this geodesic. When we go around the circle, the vector x does not go, in general, into itself, but into a n o t h e r vector, say x' E T,(V). The resulting map say L:T~(V)-->T~(V) f o r L ( x ) ~ x ' , is called the holanomy transformation (or p~rallel transport) along the loop f ( S 1) and is known (by the basic property of the Levi-Civita connection) to be an orthogonal linear map. Since the curve f ( S ~) is geodesic (and has ~7~ v ~ 0 as we have seen above) the t a n g e n t vector

df

(Dr) ~

E Tv(V) is i n v a r i a n t under L.

then we look at

the orthogonal complement N, c T~(V) of m and observe t h a t the poerator LIN, : N , --> N~ fixes a unit vector ~,, i.e. L(~,)~----v,, in the following two cases (i) n ~ dim V ~ dim Nv ~- 1 is even and L is an orientation

preserving map, i.e. Det L ~ ~ 1 ; (ii) n is odd and L is o~ientation reversing, i.e. Det L ~ -

1.

Notice, t h a t L is orientation preserving f o r all loops in V if the manifold V is orientable. But if V is non-orientable then t h e r e exists a homotopy class of loops in V, such t h a t L is orientation reversing for all loops in this class. I f L(~,,,)= ~,~, then the vector ~,~ extends to a global (periodic) parallel field ~ in V along W for which the second variation of E is

~E(f)---~-- f K(TA~) ds. I f K(V) > 0 this variation is strictly negative and so f is not a local m i n i m u m of the energy. On the other h a n d it is not h a r d to show t h a t if V is a closed manifold, then every homotopy class of maps S 1 --> V contains a smooth h a r m o n i c (i.e. geodesic) m a p f giving t h e absolute m i n i m u m to the energy on this class of maps. Thus we obtain the classical

SYNG~.THF_~REM (See [Mil]). Let V be a closed Riemannian manifold with K(V) > 0. If n ~ dim V / s odd, then V is orientable and if n

II0

M. GROMOV

is even, then the cananical oriented double cover of V is simply connected (e.g. if V is arientable then it is simrpIy connected). Notice that the proof of this theorem uses the positivity of K along some (non-specified) closed geodesic in V and no geometric information is needed (nor revealed) a w a y from this geodesic. This sharply contrasts with our study of K _> 0 b y means of t h e tube formula (though the second variation formula f o r ~ follows f r o m the t u b e f o r m u l a applied to (germs of) h y p e r s u r f a c e s normal to the geodesic in question). THE SECOND VARIATION OF THE ENERGY FOR dim W ~ 2. If dim W >_ 2 then, generically, there is no V-parallel field ~ along W as t h e system V 5 ~ 0 on W is overdetermined. In fact, the operator V applies to sections J of the induced bundle T*~-f*(T(V))--->W (where f : W - - > V is our harmonic map) which are locally given by n ~ r a n k T* functions on W while the t a r g e t bundle for ~21 T* ~ Horn (T(W), T*)), has r a n k ~ n dim W which is ~ n for dim W >_ 1. N o w let W be an oriented s u r f a c e and let S--> W be a complex vector bundle with a complex linear connection ~7. The Riemannian metric in W together with the orientation defines a complex strructure in the bundle T(W). Namely, the multiplication b y i ~ - - - I / - 1 is given b y rotating t a n g e n t vectors by 90 ~ counter clockwise. Each fiber ~1 Sw of the bundle $21 S, which consists of -linear maps Tw(W)--~ Sw, splits into the sum of t w o subspaces, ~Q' and ~2" in ~21 S~, w h e r e .(2" consists of ~-linee~r maps ~" : T~(W)--> S~ i.~ commuting with multiplication by l / - - 1 , which means e ' ( } / ~ 1 x ) : V- - 1 e'(x), while the maps e" E ~ " anti-commute with 1/~-l, i.e. e"( ~ / ~ 1 x ) : - - J / ~ 1 d"(x). This gives us a splitring of V into the sum of t w o operators, V : ~ 7 ' - 1 - V " for V ' : C~ (S) --> C ~ ( i f ) and p " : C ~ (S) - ~ C | (~"). Notice t h a t the bundles ~2' and ~Q" have the same rank over ~ as S and so the systems ~7' ~ ~ 0 and V" ~ : 0 are determined. N o w t h e r e is a good chance they are solvable. In fact, there is an i m p o r t a n t case w h e r e solutions a r e known to exist; Namely, let S be the camplexification of a real vector bundle T with a Euclidean connection over W. Then one has the following PROPOSITION (See [M-M]). I f W is homeomorphic to the sphere S 2 then the equation V " ~ ~ 0 has (at least) n linearly independent

SIGN AND GEOMETRIC MF,ANING OF CURVATURE

111

over ~ so,lutions ~, ..... ~o, : W - ~ S ~ - T ~ ~ / m l T for n ~ r a n k T, which, mareover span in each fiber Sw a (complex) subspace of dimension >_ n/2. IDEA OF THB PROOF. There is a natural complex analytic s t r u c t u r e on the total space of S f o r which holomorphic sections a r e exactly those which satisfy V " ~ - ~ O. F u r t h e r m o r e , the vector bundle S - ~ W is self-dual as a complex analytic bundle, because the Euclidean s t r u c t u r e on T (which is a quadratic f o r m on T) extnds, by ~;-linearity to a no~-singuIox quadratic f o r m on S which is parallel and, hence, hotomarphic on S. Then the existence of the required n holomorphic sections follows f r o m RiemannoRoch combined with the B i r k h o f f - G r o t h e n d i e c k theorem on splitting holomorphic vector bundles over S 2 into line bundles. N e x t one shows that the Laplace operator associated to V " , that is A " ~ - ( V")* V " , is related to A ~ formula

V *V A :

by the following Bochner-Weitzenbock 4A" =

-

I/~

-

1 K"

w h e r e K'" is the (skew-Hermitian) endomorphism of S associated with the c u r v a t u r e of V and the unit bivector field (codensity) on W associated with the metric in W. (We recall t h a t the curvature of V is a 2-form on W with values in E n d S and K " equals the value of this f o r m on ~i A r2 f o r orthonormal tangent vectors at every point in W). In particular, one relates the integrals

W

W

and

i'ltv" ll d :--

f
dw

by

f llv ll dw=4 I'IlZX" II:dw-I-]I--1 f w

iv

i~

< K" ~,~>dw.

I 12

M. G R O M O V

(The above integral formulae are obtained b y integration zJ ~ V * V and A " : ( V " ) * ~7 * and ( ~7")* b y postulating sections 9- Also notice t h a t

f o r ~ < A 9, 9 > and ~ < A" 9, 9 > b y parts. In f a c t one could define V " as well as the adjoint operators these integral formulae f o r all smooth < , > denotes the H e r m i t i a n inner

product in S-~-~ T ~ l/ ~ 1 T associated to the Euclidean structure in T). W e w a n t to r e w r i t e the second variation f o r m u l a (*) for H(J,J)~--j2E(f) (for harmonic maps f ) w i t h [[ V"91[ 2 instead of I[ ~7 91 ]s. F i r s t of all we extend the formula to the complexified bundle S* ,-- T* ~]~ ~/-- 1 T*, w h e r e it expresses t h e Hermitian extension of the Hessian of E at f. On every complex field 9 which is a formal combination of t w o real fields, 9 ~ ~ + I/ ~ 1 ~2, this Hessian is

H(9, "~)~ H(O~, 6j) + H(Je, d e ) ~ 6~E(f) + ~ E(f). de r

Then by the second variation formula, HQ0,~) ~ - / l [ V 9]] ~ + ~(~2))

dw,

where

Ilvgll ~ = IIv~ll ~ + IIv6~ll ~ and / ~ ' ( 9 2 ) ~ K ( 6 ~ ) + K(O~) which is expressible in term of the sectional c u r v a t u r e K of V and orthonormal vectors r l , ~ at each point w E W

by

K.(9)~---X

K(Dr~ASi) f o r t h e differential

D--Dr(w), as w e have seen earlier. ( H e r e as always w e identify 61 and 62 in T* with their images in T(V) under the tautological map T* =/*(T(V)).-., T(V)). N o w we substitute

] Ilvgl? in H(9,

by 4

(ll v" 9ll ~ + W

r

fl < K"9, 9 > W

according to the previous formula and obtain the following expression f o r H,

H ( 9 , ~ ) = 4 fIIv"911 dw + .l K"(9~) dw W

W

SIGN A N D GEOMI~,TRIC

MEANING

OF CURVATURE

113

for

(Notice that/~"(~0 e) is real since < K" ~0, ~0 > is purely imaginary). We are going to apply this formula to fields ~ satisfying V" ~ ~ 0, which makes H(~, ~ :

[ ~:"Qo~)) dw, W

and we want to know when /~"Qo 2) is negative (compare the Synge theorem). The answer is obtained with the following notion of the eomplexified sectional curvature K r of V. Extend the form Q by complex multilinearity to the complexified tangent bundle ~ T ( V ) ~ T ( V ) G t; - - 1 T ( V ) and let Kr (,~A~) = Q ('aA~, aA~) for a and ~ in (~ T ( V ) (and the obvious conjugation z ~->z in the complexified bundle). If we write ~ - t l -F } / m l t2 and ~ t 3 -t- } / 5 t4 for vectors t~ E T(V), then a trivial computation expresses Ke in real terms as follows, Kr ( ~ A f l ) ~ Q ( t l A t 3 - - t 2

At4, f l A t 3 - - t2At4) -~-

Q(~IA t4 - - t2A t,~ , t~A t4 - - t2At3).

With this one sees that the condition Kr _> 0 interpolates between K >__0 and Q _> 0 (i.e. the positivity of the curvature operator). Also notice that the inequality Kr > 0, by definition, says that K r 0 for all pairs of r vectors ~ and /? in C T(V). A useful sufficient condition for positivity of Kr is the 1~4-pinching of K. That is Kr > 0 at every point v where the sectional curvatures K(o) satisfy 1 / 4 a < K(o) < a for some a = a ( v ) > 0 and all 2-planes o E Tv(V). Similarly, the negative 1~4-pinching of K(o) (between - - a and ~ 1 / 4 a) insures Kr < 0. (This is due to Hernandez. Earlier Micallef and Moore proved

8tminario

Matematico

e Fisico-

114

M. GROMOV

a slightly w e a k e r result needed f o r the present application. Also notice t h a t the pinching criterion is s h a r p : The complex projective space has 1/4 ___K _< 1 and Kr is non-strictly positive). Now we r e t u r n to our m a p f : W ~-> V, we t a k e two orthonormal vectors T~ and ~2 at some point w E W and let D' ~ for D ~ - D /

-~1 (DT1 - - f ~

1 D-c~) E ~, T ( W )

: T(W).-> T(V).

LEMMA. The above curvature term K" satisfies at each point f(w)EV, K " (g~) ~ - - 4~:r (D% A ~)). This is proven in [M-M] by a s t r a i g h f o r w a r d computation based on the definitions of the curvatures in question. COKOLLARY. I f K r ~_ 0 then the (complexified) second variation the enery is non-positive on the solutions q) of the equation ~7 " ~) ~ Furthermore, if Kr > 0 and ~o is non-tangent to D ' v E C T(V) some pains f(wo)EV, then H ( ~ , ~ ) > 0. I t follows that f is not local m i n i m u m of the energy function f ~-> E(f).

of O. at a

PROOF. The only point which m a y need explanation is the relation of (the sign of) the complex Hessian with the real variation of the energy. B u t for ~ ~ 51 + 1/ - - 1 52 the complex Hessian is the sum of the two real ones

H(~, ~) ~ H(51, J1) -F H(52,62) :

62 E(f) + ~".2 E f t )

and so t h e negativity of H(~, ~) implies t h a t for some of the two real variations 5~ E ( f ) or 5~ E(f).

Q.E.D.

Now we assume t h a t W is homeomorphic to S 2 and the map f : S 2 --> V is non-constant. Then we have with our earlier proposition n ~ dim V linearly independent solutions ~01, ..., ~ of the equations V " ~ ~ 0 which span a subspace of dimension _ n / 2 at some point f(wo) E V where D " r ~ 0. Thus f o r n ___ 4 we obtain at least one field ~ for which H(~, ~) < 0 and in general, for n >_ 4

SIGN AND GEOMETRIC MEANING OF CURVATURE

115

n

we have at l e a s t - ~ - - 1 fields, such that H is negative definite on their span. Recall, t h a t all this neds K a > 0. In fact, Micallef and Moore prove the above u n d e r the (weaker) condition of positivity of K a (only) on those complex 2-planes in E T(V) on whch the complexified Riemannian metric (which is a E - q u a d r a t i c f o r m on E T(V)) vanishes. The above Corollary shows, in particular, t h a t no smooth nonconstant harmonic map f : S 2 -> V is energy minimizing in its homotopy class f o r n >_ 4. On the other hand, a f u n d a m e n t a l theorem of Sacks and Uhlenbeck claims the existence of such f whenever the second homotopy group ~2(V) does not vanish (where V is otherwise an a r b i t r a r i l y closed Riemannian manifold). Thus Kr ( V ) > 0 implies ~ ( V ) ~ - ~ 0 f o r d i m V _ > 4 and this suffices for n ~ 4 (by elementary topology) to insure t h a t the universal covering of V (which is compact as K(V) > 0) is a homotopy sphere. REMARK. The subtlety of the Sacks-Uhlenbeck theorem is due to the f a c t t h a t the space of maps f : S 2 -~ V with E(f) < const is nancanvpact. Moreover, a simple computation (using the f a c t t h a t dim S 2 ~ 2 ) reveals t h a t the energy is i n v a r i a n t under the (noncompact !) group of conformal transformations of ~-' and so even the space of harmonic maps with bounded energy is non-compact. Thus one can h a r d l y expect convergence of a n y kind of a minimization process for obtaining a harmonic m a p with minimal energy in a given homotopy class of maps. In fact, one does have divergences were a map S" --) V > into several pieces, see fig. 20 below

9 Fig. 20.

116

M. G R O M O V

Such bubbling t r a n s f o r m s a single m a p f : S ~ --> V to a finite CO1k

lection

of

maps f~ : S: --> V, i ~ 1, .... k, such t h a t ~

E ( f O V and showing for n _>_4 t h a t either a closed n-dimensional simply connected Riemannian manifold V is a homo@oy sphere or there exists a non~cansta~t harmonic map f : $2--> V, n w h i c h admits at m o s t k 0 implies (by the above existence discussion for ~i w i t h negative H) V is a homo topy sphere. We have mentioned earlier t h a t the strict 1/4-pinching condition on the sectional c u r v a t u r e K implies strict positivity of K a . Thus the above theorem of Micallef-Moore implies the Sphere Theorem (see w 33/~). Notice, t h a t Micallef and Moore need only local pinching, i.e. 1 --~ ~(v) 0 } corresponding to the strict positivity of the c u r v a t u r e operator. The closure of this cone (given by Q >_ 0) can be defined as the minimal closed convex O(n)-invariant cone which contains the c u r v a t u r e Q of the product metric on S 2 X 1R~-2. This suggests other definitions of interesting (?) classes of metrics defined with natural cones C. (Compare c u r v a t u r e positivity conditions in [ G r o ] : aimed at bounding the size of V). In the search of interesting cones C one m a y be guided b y how C interacts with natural differential operators on V. (Compare the invariance of { Q > 0 } under R. Hamilton's heat flow on the space of metrics and various Bochner formulas we have seen in w 7). More geometrically one m a y look at ~ as a subset in the space 6 + of Riemannian metrics g on a given manifold V which a r e considered as sections of the symmetric square S ~ T*(V). Then the above mentioned global analytic features of the underlying C can often be interpreted in t e r m s of infinitesimal g e o m e t r y of ~:. It is w o r t h noticing at this point t h a t % c @+ is a cone f o r every C and that ~; is invariant under the natural action of D i f f V on @+. B u t ~: is not a convex cone unless C is empty or equals t h e space of all forms Q. In fact, ~ + (which itself is a convex Diff-invariant cone in the linear space of sections V--->S~T*(V)) contains no non-trivial Diff-invariant convex subcones at all, if the underlying manifold V is compact connected w i t h o u t boundary (see p. 231 in [Gro]) and also pp. 24 and 111 in this book).

SUMMARY. This is an expanded version of my 9 Lezione Leonardesca~ given in Milano in June 199.0. I t r y to reveal to non-initiates the inner working of the Riemannian geometry by following the tracks of relatively few ideas from the very bottom to the top of the edifice. m

REFERENCES Besides the references given in the main body of the paper we list below some books and expository articles relevant for our purpose. [Bet] 1 [Ber]~_

B E P ~ t M., RAUCH H. E., Gdom~tre di/[drentiel, in Differential Geometry and Complex Analysis, p.p. 1-14, Springer Verlag, 19~5.

B~n~ER M., La gdomdtrie mgtrique des varidtds Riemannienn~s, in Ast~risque, hors serie, p.p. 9-66, Soc. Math. de France, 1985.

[Bes]

BBSS~. A., Einsteim manifolds, Springer Verlag, 1987.

122

M. GROMOV

[B-G-P]

BURAGO Yu., GROMOV M. and PERELMAN G., A. D. Alezandrov's spaces with curvatures bounded from below I. Preprint.

[B-G-S]

B~MANN W., GROMOV M. and SCHROEDF_~ V., Manifolds of nonpositive curvature, Progress in Math. Vol. 61, Birkhiiuser, Boston. 1985.

[Bou]

BOURGUmNON J.-P., L'dqu~tion de la chaleur associde ~ la courbure de Ricci (d'apr~s R. S. Hamilton) in Ast6risque 145-146, p.p. 45-63, Soc. Math. de France, 1987.

[Bu-Ka]

B u s ~ P., K~CHER H., Gromov's almost flat manifolds, Ast4risque 81, Soc. Math. France, 1981.

[Bu-Za]

BURAGO YU., ZA~ALLER V., Geometric inequalities, Springer Verlag, 1988.

[Chef

CHEEGER J., C~itical points of distaawe functions and applications to geometry, C.I.M.E., June 1990 session at Montecatini, to a p p e a r in Lecture Notes in Math., Springer.

[Ch-Eb]

CH~;ZR J., ESI~N D., Comparison theorems in Riemannian geometry, North Holland, 1975.

[Cor]

CoR~rrE K., Flat G-bundles with canonical metrics, J. Diff. Geom. 28 (1988), p.p. 361-382.

[E-L]~

E~mLS J. and LEMAIRE L., A report on harmonic maps, Bull Lond. Math. Soc. 10 (1978), p.p. 1-68.

[E-L]~2

E~s J. and LEMAIR~ L., Another report on harmonic ma/ps, Bull. Lond. Math. Soc. 20 (1988), p.p. 385-524.

[Esch]

ESCHE~IBURG J., Local convexity and non-negative curvature - Grwntov's proof of the sphere theorem, Inv. Math. 84 (1986), n. 3, p.p. 507-$22.

[F-K-M]

FF~RUS D., KAItCHE~ H., MiJNZN]~ H., Cliffordalgebren und neuo isopava/metrische Hyperflgehen, Math. Z. 177 (1981) p.p. 479-502.

[Gal]

GALLOT S., Isop~rimetric inequalities based on integral norms of Ricvi curvature in Astgrisque 157-158, p.p. 191-217, Soc. Math. de France, 1988.

[Gh-Ha]

GHYS E., DE LA H~.P~., eds., Sur les Groupes Hyperboliqucs d'apr$s

Mikhael Gromov, Birkhiiuser, 1990. [Gro]

GROMOV M., Partial differertttial relations, Springer Verlag, 1986.

[Gro] 1

GROMOV M., Diman~ion, nan-linear spectra and uridth, Geometric aspects of Functional Analysis, J. Lindenstrauss and V. Milman eds., in Lecture Notes in Math. 1317, p.p. 132-185, Springer Verlag, 1988.

[Gro] 2

GROMOV M., Large Riema~nlan manifolds, in Curvature and Topology, Shiohama et al eds., Lect. Notes in Math. 1201 (1986), p.p. 108-122, Springer Verlag.

[Gro] 3

GROMOV M., Synthetic geometry of Riemaamian manifolds, Proc. ICM1978 in Helsinki, Vol. 1, p.p. 415-419.

[G-Pa]

GROMOV M., PANSU P., Rigidity of discrete groups, an introduction, C.I.M.E. session in June 1990 a t Montecatini, to a p p e a r in Lecture notes in Math., Springer-Verlag.

SIGN AND GEOMETRIC MEANING OF CURVATURE [G-L-P]

[Kaz]

[Law] [La-Mi] [Mill [Miln] [M-M]

[Mi-S] [Seh] [Str]

123

GROMOV M., LAFONTAINEJ. and PANSU P., Structures mdtriques pour les varidtds Riemanniennes, Texte Math. n. 1, CEDIC-Nathan, Paris, 1981. KAZDAN J., Gaussian and scalar curvature, an update in Seqninar on Differential Geometry, Yau ed., p.p. 185-193, Ann. of Math. Studies 102, Princeton, 1982. LAWSON B., Lectures on minimal submanifolds, Publish or Perish Inc., 1980. LAWSON B., MICH~.LSOHN M.-L., Spin Geometry, Princeton University Press, 1989. MILNOR J., Morse theory, Ann. Math. St. 51, Princeton, 1963. MILNOR T., Efimov's theorem about complete immersed surfaces o/ negative curvature, Adv. Math. 8 (1972), p.p. 474,543. MICAL~F M., MOOP~ J., Minimal two.spheres wad the topology of mamifolds with positive curvature on totally isotropic two-planes, Ann. of Math. 1 2 7 : 1 (1988) p.p. 199-227. MILMAN V., SCHECHTMAN G., Asymptotic theory of finite dimensional normed spaces, Lecture Notes in Math. 1200, Springer Verlag, 1986. SCHOEN K., Minimal surfaces and positive scalar curvature, Proc. ICM-1983, Warszawa, p.p. 575~ (1984) North-Holland. S ~ C H ~ ' Z It., Sub-Riemannian geomet~'y, Journ. of Diff. Geom. 2 4 : 2 (1986) p.p. 221-263.