Towards the Edge of the Universe: A Review of Modern Cosmology

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.WIIEY.PRAXIS SERIES IN ASTRONOMYAND ASTROPHYSICS Series Editor: John Moson, 8.5c., Ph.D. F c w s t t b j c c t sh a vc b ccn a t th c ccn tr c o f' su ch in r p o r ta ntrl cvcl opntcnl sor sccn such a rvcal thol 'ncw a n d c x c i t i n g , i l ' sttn tctilr tcsctln tr o vcr sia l,cla ( aa s n r o t l crnasl rononty.astrophysi csl urd crl snr,rl og-v. ' l ' h l s s c r i c s r c l l c cls tltc vcr y r a p id a n d sig n ifica n t p roqrcssbci ng nrarl c i n currcrrtl cscarch.as a c o n s c q t l c n c c o 1' n cw in slr u tttcltla lio n r r n tl o llscr vi ng tcchni (l ucs, appl i ctl ri ght acr()ss l h(c l c c t r o n r a g n c { i cs p cclr u n t.co n lp u tcr n t< td cllin _agn r l n torl crnl l rcrl rcl i cl l rrrcthocl s. T h c c r u c i a l l i nks b ctwccn o b scr va tio na n r l th co r y arc cl rphasi scd.putti ng i nto pcrspc.cti vcthc l a t c s t r c . s u l l sl l , on l th c n cw g cn cr a tio n s o l' a str u n o nri cal tl cl ccl rrrs,l cl cscrrpcsunrl spl cc-bornc i n s i r u n r c n t sC . o r r tp lcxlo p ics a r c lo - e ica lly< lcvclo p ccl a ncll l l l y cxpl ai nctlancl ,u'hcrc rnathcnrati c-s is u s c c i .t h c p h y s i c a lco n ccp tslr ch in r lth c ctlu llio n s a r c clcarl v sunrnrari sccl . ' l - h c s cb o o k s l r c u ,r ittcn p r in cip a lly lo r p r o lcssio n all strononl crs,asl rophl ,si ci sts. cosnrol ogi sts, p h y s i c i s l s : t t t t l s p a cc scicn lists.to Scth cr r vill p o sl- g ri rrhntL'an(lun(l cr!r:l (i uul cstu(l cntsi rr tl rc-sc I ' i c l d s . C c r t l i n btto ks in th c scr ics r vill a p p ca l to arl r.l tcurastrononrcrs.hi gh-11r,i n,t'A '-l cvcl s t u d c n t s ,a n < tn o n - scicn tists$ ,i( h a kccn ir r tcr cstin a stronol n),l rrdastropl rysi cs. ( ) RItis R O i l 0 1 ' t CO I l S l . R VA1

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thatraisinsin a doughmixturearemovedawayfromoneanotherwhenthebreadrises(see Fig. 1.8).lnterestinglyenough,whenEinsteinhadderivedthe equationswhich definethe spicetime continuum,the equationsdid not allow the Universeto be static; it had to be eitherexpandingor contracting.Since,at thetime,Hubblehadyet to provethe expansion so that theyincludeda for presentation ofthe Universe,Einsteinhadrecasthis equations presented his results, When Hubble static. the Universe would hold new constantwhich mistake constantandcalledit the greatest removedhis cosmological Einsteinimmediately ofhis life! Generalrelativityprovidesus with a way to visualisehow gravity is created,by exThe theory statesthat massiveobjects plainingthe way massinteractswith spacetime. distort the spacetimecontinuuminto curvesin muchthe sameway as a heary ball might deforma rubbertabletop (seeFig. 1.9).Anyttringwhich getstoo closerolls downthe curveas if it werebeing attractedto the massiveobject.Thus,an intimaterelationship continuumcaused betweenspaceitselfandmatterexists.The curvatureof the spacetime by massin turn createsgravlty, which tells othermasseshow to movethroughit. havehad to dealwith is that, alOne of the universalpaladoxeswhich cosmologists thoughgravity is by far the weakestof the four fundamentalforcesof nature,it dominates the Universeon its largestscale.The stong nuclearforce andthe weaknuclearforce only operateover*re distanceof an atomicnucleus,whilst the elecfomagneticforcecancels out over largedistances.With nothingleft in competition,gavity andthe motion it causes cansculpttheUniverseon all but the smallestof scales(seeFig. l. l0). Indeed,it is graviin this tationalforceslvhichareresponsiblefor everystructurewe haveso far discussed fate of gavity sealed the has also is concerned, chapter.As far asthe future of the cosmos is! what fate work exactly that to out cosmologists is for the remains All that the Universe. all scalesof the Universe.The Cosmoloryis a rich field of studywhich encompasses only by thinking in termsof quantumtheory,the method Big Bangitself is understandable the Universeon its smallestscale.The long-termfutureof the we havefor understanding the only ifwe usegeneralrelativity:a theoryfor understanding Universeis understandable Universeon its largestscale.Cosmologyis, literally,universalin contentandscope.It is to it by takingourmindson thesciencefromwhichall othersarebom, andwe gainaccess joumey of the Universe. the towards edge from here a

The tools of the trade The astronomical censuspresentedin chapterI hasbeenmadepossibleby the collection of electromagnetic radiationfrom space.Astonomy, by its very nature,is an observational science,not an experimental one.As muchas theywould like to, astronomers are not freetojourney to thesefar distantobjectsin the exoticdepthsofthe Universeandset up experiments to learntheir secrets.Instead,they mustrely on collectingthe radiation whichhasbeenreleasedby thesecelestialbodies. The detectionofelectromagnetic radiationis, by far, themostadvanced ofour methods for examiningthe Universe.Sincethe dawnof humanexistence, mankindhasdonethis quite naturally,sinceour eyesarereally rathersophisticated detectorsof visibleelectromagneticradiation.Technologically, thestudyof visiblelight beganin 1609whenGalileo useda telescopeto look at the night sky. Today,a plethoraof collectingand detecting devicesis capableof samplingtheradiationfrom manyareasof theelectromagnetic specrrum. 2.1 ELECTROMAGNETIC RADIATION In the sameway that thevery fact of theUniverse'sexistence hascausedmanyto wonder aboutit, so t}renatureof light hasalsopuzzledmankindthroughouthistory.plato, Aristotle andPythagoras all wonderedaboutit but nothingreally cameof their pontifications. Betweenthe 1300sand i600s,European researchers concentrated on thedevelopment of lensesandmirrorswithoutreallywonderingwhatconstituted the light theywerestudying. Whilst doingthis, however,manybecameawareof theway in which raysof light moved throughthe air andinteractedwith otherraysoflight. Gradually,curiositywasraisedand thephysicalnatureoflight waspondered. In 1665,whilst attemptingto understand a seriesof observations which generated the phenomenon of diffraction,RobertHookeproposedthat light is a rapid vibrationofthe mediumthroughwhich it is passing.Hooke'spresentation marksthe beginningof the wavetheoryof lightwhichis still in useroday. Manyexcellent contributions weremadeto theburgeoning science of optics,butpossibly the nexttwo greatestmilestones werethe proofthat light traveiledwith a finite velocity and that it was a type of electromagnetic wave.Both conclusionswere finally and irrefutablyreachedin themiddleof thenineteenth cenhry.

20

lch. 2

The tools ofthe trade

From the daysof Galiieo,scientistshad beentrying to measurethe speedof light. Thoseearlyattemptsnow seemridiculouslycrudeandtheir failureto showanytime delay in light's propagationfrom one locationto anotherled someto believethat it navelled Ole Christensen R.smerobservedan infinitelyfast.In 1675,however,Danishastronomer phenomenon whichhecouldonly explainif thespeedof light werefinite.He astronomical wastrying to measure the orbitalperiodof Jupiter'smoonIo, andkeptobtainingdiffering results.lt wasthe fust directevidencethat light did not propagateinstantaneously and it evenyielded a crudeestimatefor its velocity.Just over fifty yearslater, anotherphenomenon,knownas aberration,wasdiscoveredby the EnglishmanlamesBradley.This, too, was explainableonly in termsof light travellingwith a finite speed.Like Rsmerbeforehim, Bradleyalsocalculateda speedfor light. of the speedof light wasmadeby Frenchman The first relativelyaccuratemeasurement ArmandHippolye LouisFizeauin thesuburbsof Parisduring 1849.His calculatedfigure was 315,000km/s which,comparedwith today'sacceptedfigureof 299,792km/s,is prettygoodfor the technologyofthe day. in optics were taking place, another At about the sametime as thesemeasurements unrelatedfield of physicalstudywasalsomakingprogressin leapsandbounds.Michael Faradayhadappliedhis greatmind to the studyof electricity andmagnetism.Within Faraa link berweeneleckicityandmagday'slifetime,HansChristianOerstedhadestablished netismwhen,in 1820,he noticedthatanelectriccurrentin a wire affectsa magneticcompassneedle.F4ladayhimselfnoticeda link betweenlight andelectomagnetism whenhe thatrastong magneticfield couldalterthe propertiesof a light ray. discovered Buildinguponthis informationandthe collectedresultsof manyotherexperimentalists anotherphysicist,JamesClerk Maxwell,expertlyextendedthe work and eventuallydistilled the behaviourof elecfomagletism into a seriesof four theoreticalequations.His work standstodayasoneofthe greatestpiecesoftheoreticalphysicsever,secondonly to thegeneraltheoryof relativity.In thecourseof his investigations, Maxwellbecameaware predictedthatelectromagnetism couldtheoreticallytravelin theform of thathis equations waves.Solvinghis equationsto give the speedof the wave,Maxwell discovtransverse

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eredthat it ravelled at Fizeau'smeasured speedof light. This wasso big a coincidence thatMaxwelrfelt compelled to drawtheconclusion th;t light*u, *.r..iorugnetic dis_ turbancepropagatedby a wave (seeFig. 2.1). Not only tiat, but light *ur-"oi..nour.o into sucha restrictedsetof wavelengths thatothertypesof electromagnetic radiation(with greaterand smallerwavelengths) could be predictid to exist. r" rsag, .-prii*.nulir, HeinrichRudolfHertzdramaticallyprovedMaxwell'stheoryuuou, .r..uo*'ugnetic radi_ ationby producingthe fust radiowaves.

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electromagneric specEum.All rypesof electromagneuc wave are similar to one flq-,22 Th: anotheri all that separates them is their wavelength,which dictatis how the wave interacts with = matter. I nanometre(nm; 194. Man-madedivisionshave been drawn to reflect these differences,arthoughth€ transitionfrom onetype-of€lectrorugn"ti. wave to anotheris l;r;;ii; smoothlycontinuous. (AdaptedfromKaufmann, w.J., Untverie,w.H.Freeman. 19g7.)

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Electromagneticspectra 23

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divisions:radiowaves, radiationinto sevenman-made Todaywe split electromagnetic there microwaves,infrared,visible,ultraviolet,X-raysandganrmarays.Fundamentally' freand wavelengths have different they simply radiations; these is no differencebetween (seeFig.2.2). quencies The definingpropertiesof the wave,namelythe wavelength,1",andthe frequency,f, areinterlinkedby thespeedofthe wave'spropagation, C= IA

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specradiationis knowntodayastheelectromagnetic The entiregamutof electromagnetic trum. The word spectrumwas first coinedby IsaacNewton,who wasthe first personto produceandstudytheopticalspechum.Theword itselfis Latinfor'ghost' or'apparition" patternofcolourswhichdancedon the andwasusedby Newtonto describetheephemeral wall of a darkenedroomwhenhe placeda prismin thepathof a light bearn. Initially, the spectraobservedweresimplycontinuousbandsof colour.tn 1802w.H. Wollaston observeda few dark lines in the solar specffumand assumedthat they were magnifieda spectrumofsunlight sapsbetweenthecolours,but in l8l4 JosephFraunhofer trat it was full of the dark lines of the absorptionspectrum.Later that Ld discouered century,emissionspectrawere observedby fwo chemists,RobertBunsenand Gustav Kirchhoff.Theypassedthe light from chemicalflameteststhrougha prism anddiscoveredthat their specta were simplepaftemsof colouredlines ratherthan dark lines superimposedupon continuouscolours.Furtherinvestigationsshowedthat eachelementhad a patternof theseemissionlines.It wasalsorealisedthateveryemissionspeccharacteristic absorptionspectrum(seeFig. 2.3). trum hada corresponding It appearedthat, in someway, atomscould only absorband emit at specificwavethat a truly continuousspectrum lengthsof radiation.By now, physicistsalsorecognised wai only producedby heatingan objectandlettingit radiatethat heatas electromagtetic radiationbecameknown as radiation.Thus,this methodof producingelectromagnetic it impossibleto understand made however, a wave, as light thermalemission.Thinkingof why atomsandradiationshouldbehavein this way.The productionof thermalradiation by threeempiricallawswhich assumethat the objectbeing heatedis a is characterised perfectabsorber(and perfectemitter)of radiation.Sincea perfectabsorberwould not reflectanyradiation,its colourwould be black.Thus,an objectofthis typehasbecome knownas a blackbody.All radiationabsorbedby the objectwill be convertedinto heat As well as a solid object at a specific energyandthenre-radiatedat lower frequencies. as a blackbody if its constituentatomsand radiate gas canalso tempirature,a denseideal andcosmologicalcases,it is astronomical most In equilibrium. are in thermal molecules emissionby a denseidealgasin thermalequitibriumwhichproducesblackbodyradiation. as beingblack bodiesand,as we shallsee,the For instance,starsarewell approximated by a black body curve. is alsocharacterised radiation background cosmicmicrowave Hence.frorn now on, this book will assumethat Planck(black body) curvesare being by denseidealgasesin thermalequilibrium. produced

Absorbtion Spectrum

3;::i?xil Fig. 2.3. The productionof emission,absorption,and continuousspectra.The contlnuous sp-ctrumis producedby a hot, solid bodyor by a hot, densegas.Emissionspectraareproduced by a hot, tenuousgas,andabsorptionspectraareproducedby cold cloudsoftenuousgas. The first law which describes the process of black body thermal emission is known as law. It gives the total radiated power per unit area, R, for a black the Stefan-Boltzmam body at a given temperature, T.

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The proportionality is maintainedby the Stefan-Boltanarm constant,o:5.67 x 10rWm2K4. The second law is known as the spectral radiancy and describeshow the intensity of radiation changeswith wavelength and temperature. Graphs constructed for &e spectral radiancy at specific temperaturesare known as Planck orblack body curves (seeFig. 2.4). When the spectral radiancy is integrated ovel all wavelengths it equals the Stefan-Boltzmann law.

t 24

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The tools ofthe trade

iantly and presentedhis theoretical equationwhich perfectly matched the spectralradiancy curves.

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is equalto 2898pm K' The Wien constant,w, which At thetum of this century,physicswasfacingthe challengeof findinga formula The best basis' theoretical a from could fit the Planckcurvei and be totally derivable Lord by made the attempt was offer could physics classical attemptat fitting thecurvethat fit thecurvesat very longwaveRayleighandlaiermodifiedby JamesJeans.It could_only but lengths.A secondattempt,madeby WilhelmWien' fittedbetterat shorterwavelengths however, theory, classical from somewhat Wiendeparted faitia to fit at tongwavelengths. and the by assumingthat there *ui * analogybetweenthe spectralradiancycurves gas' ideal of an molecules the for curves distribution Maxwellspeed presentMax Plancksolvedthe problemby interpolatingbetweenthe two modelsand his derive to then set out He wavelengths. at all ing a formulawhich fittedthe radiation brillHe succeeded assumptions. theoretical of set a simple from time this .q"uutionagain,

The key to quantumtheory is wave-particleduality. It statesthat light, traditionally be a particleandthatelectrons,traditionallythought thoughiof asa wave,cansometimes we arefree behaveaswaves.As physicistsandastronomers, of aJparticles,cansometimes at hand' problem the to solve to regardlight andelectronsas particlesor wavesin order andthe part the scientists the of on to take liberty This seemslike quite a fundamental The answer particle? wave or a it a Is really? an electron, is what arises: naturally question is that,just like light, it is bothandyetneither. particlenatureand a wavenature' Quantumtheorystatesthat everythinghasboth a formationaswell. The key to why wave is a just but object solid a This very book is not very muchmoremassivethanan it is is because however, object, a solid like it behaves that whenthe quantumtheoryequationsfor electron.Electronscontainsuchtiny masses their behaviourare solved,neitherwavenaturenor particle natureshowsdominance' Hence,it displaysthe qualitiesof both. If the quantumequationswere solvedfor this book,however,it's solidparticle-likenaturewouldoverwhelmthe wavenature. The conceptof quantizationand wave-particledualityhas shownthat electronscan only exist aroundaiomicnuclei in certainstatesdefinedby the quantumnatureof the atom.Whenradiationcomesinto contactwith electronsaroundatoms,thephotonwill be absorbedonly if it containsenoughenergyto allow the electronto jump to a newenergy state.Later,whenthe electronjumpsbackdownto its original level,the energyis given back out as a photonwith a specificwavelength.Thus,the processresponsiblefor the andthe reason productionof absorptionand emissionspectrais finally understandable, why thereareline spectrafollowsnaturallyfrom the explanation' quanalejumpingbetween theelectrons occursbecause ofline spectra Theproduction with their association their losing electrons the stage are no at but tized energylevels, (seeFig'2'5)' transitions areknownasbound-bound parentnuclei.Thus,theseffansitions An idealgascan radiateeithera continuousblackbody spectrumor a line spectnrm, beuf,onthe densityof thegascloud.A tenuousgashasnegligiblereactions depending

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way in which radiationis eitherabtweenits constituentatomsand so the predominant sorbedor emitteddependsonly upon the way in which the electronsinteractwith the surroundingphotons.This leadsto emissionand absorptionspectra.In a densegasthe energylevelsarealteredby theproximityof the atomsto oneanother,andthis smearsthe radiationout into a continuousband,producinga continuum. profile.This is because shapeknownasa Gaussian Spectrallineshavea characteristic there is a naturaltendencyfor the lines to spreadout over a small rangeof wavelengths. For the atomsin an ideal gas,the temperatureof the cloud will ffanslatedirectly into a velocitydistributionfor its constituentatoms.Whenan electronabsorbsandthenre-emits a photonof radiation,the wavelengthof the photonwill be slightlyalteredby the individual motion of the emittingatom.When the cloud is studiedas a whole,the statistical profiles.Theprodistributionof velocities(describedby Maxwell)createstheseGaussian ln the caseof turbulentmotionwithin the cessis knownasthermalDopplerbroadening. will alsotakeplace.Theanalysisof the cloudbeingstudied,turbulentDopplerbroadening by the shapeof spectrallinescanthereforetell us whatphysicalconditionsarepossessed cloud. emitting(or absorbing) the atomsbecomemorecrowdedandthe amount As the densityof the cloudincreases, of interactionbetweenthembecomessignificant.Our frst clue can be foundby rememberingthat oneof our conditionsfor blackbody radiationwasthat the cloud mustbe in thermalequilibrium.This impliesthat theremustbe a largenumberof atomic interactions in order to spre4dthe thermal energyevenly betweenatomsin the gas.In the courseof thesecollisions,the atomswill interactwith eachother.This occursbecausethe wave natureof theelectronsallowsit to actasif i* chargeis distributedevenlyabouttheatomic nucleus.Thustheregionin whichthe particulateelectronmustbe locatedis knownasthe electroncloud.The atomsin a denseidealgaswhich is in thermalequilibriuminteractby feelinga repulsiveforcebetweentheirsunoundingelecton clouds.This altersthemotion changeto the energyof the system. of the interactingelectronscawing a corresponding we canthink ofthe energies Thatenergyis lost in the form ofa photon.For our purposes involvedin theseprocesses asbeingunquantized andso theradiationis emittedin a congivesa Planckcurve. tinuumwhich,because ofthe distributionofatomic energies, So,the carefulanalysisofthe spectrumfrom any celestialobjectcantell us a a great dealaboutthe mattercontainedwithin the objectunderscrutiny.The shapeofthe spectrum, the presence or absenceof spectrallinesandthe intensityof radiationall help in by whichtheradiationwasemitted. understanding thephysicalprocesses

2.3 ELECTROMAGNETIC RADIATION EMISSION MECHANISMS Having lookedat thermalemission,it is now importantto considerthe other types of for electromagnetic radiation.Considering emissionmechanism bound-boundtransitions,it becomes naturalto wonderif it is possiblefor electronsto completelyescapethe influcnceof their atomicnucleisimplyby absorbingphotonsof suffrcientlyhigh energy. 'l'his is a processknownasphotoionisation andis definedasa bound-freefansition. The energyrequiredto ionisea hydrogenatom,if its electronis in the lowestenergyor groundstate,is l3.6eV.(An electronvolt is a measure of theenergygainedby an electron il'it is accelerated througha potentiaidifferenceof I volt.) Photonscontainingthis amount

Sec.2.3l

Electromagnetic radiation emission mechanisms 27

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Fig 2.5. Energylevelsandtransitionswithin the hydrogenatom.ln the simpleBohr picture ofthe atom, energylevelsaresaid to be quantized,i.e. the elecbonsmay take ui only certainspecific energies-For the hydrogenatom, an energylevel diagramcan bi drawn'showingall available energy levels and the s€riesof spectral lines (e.g. Lyman, Balmer, paschenInd Brackett) producedby transitionsbetween.them.. Electronsjumping from higher to lower energylevels produce emission lines, white thosejumping from iower to hig-herenergy levels"produce absorptionlin€s.

ofenergy or morecanbe absorbedandusedto ejectthe electronfrom the atom,leaving an atomicnucleuswith a net positivechargeknownas a positiveion. Energyover and abovethe 13.6eVneededfor ionisationwill be convertedinto kineticenerg/ty the free electron.As thephotonenerryincreases pastthe ionisationlimit, however,th! probability that the photonwill be absorbeddecreases and so absorptionbandsare createdin the spectrumwhich havea sharpdiscontinuitycorresponding to the energyof ionisation(see Fig.2.6).Several suchbandsarepossible withinthesamespectrum because ionisation can takeplacefromelectrons in anyenergylevel.A gascloudwhichis largelycomposed of positiveionsandelectrons is knownasa plasma.

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freeelectronscanbe captuledby positiveionsandtheir energygiven out Conversely, asphotons.This is the processofrecombinationandis knownasa free-boundtransition. is a rangeof mechanisms which to be considered Thefinal typeof emissionmechanism the elecfronsare uncould be classedas free-freetransitions.In all ofthese interactions boundto atomicnucleiandremainthatway,evenafterthe interactionwhichproducesthe radiation. The most obvious of the free-free transitions is known as thermal by an interactionwith This occurswhen a free electronis decelerated bremsstrahlung. anotherchargedparticle.The otherparticlemay be a positiveion (i.e. an ionisedatomic nucleus)or anotherfree electron.Whateverthe precisesituation,the energylost by the electronin the interactionis releasedas a single photon of radiation.A thermal spectrumis continuousbut of a very differentshapefrom a black body bremsstrahlung emissionof a tenuousplasma' curve.It is the characteristic is Magnetobremsstrahlungalsopossiblebut usuallygoesby the nameof synchotlon involvesparticlestravellingat relativisticvelocities radiation.This emissionmechanism theelectronto loseenergyby in circulartrajectoriesthrougha magneticfield. This causes mechanisms theradiationproemission other Uniike radiation. givingout electromagnetic of being that, instead randomlyorienmeans This polarised. is highly way ducedin this placedin the samedirection. tated,the electricvectorsofthe photonsareconsistently The compton effectoccurswhena high energyphotoninteractswith a lower energy andknockedinto a lower electron.lt is convenientto imaginethatthe photonis scattered Theinverseprocess occurs to higherenergies. energystate.whilsttheelechonis boosted when'-ahigh Energyelectroninteractswith a low energyphotonand boostsits energy' Describedin this way it soundsrathersimilarto synchrofionradiation,with a photonfield replacingthe magneticfield. Althoughwe havereferredto theseeffectstakingplacebetweenelectronsandphotons,whichwill applyin the majorityof asfophysicalcases,the electronscouldbe replacedby otherparticles. The wavelengthshift, Al., sufferedby a photonin the inverseComptoneffectis given bv theequation: Al" = tr"(1 - cosg)

(2.6)

whereg is the scatteringangleof the photonand l" is the Comptonwavelengthof the the photoninteraction.The Comptonwavelengthis the wavelengtha particleundertaking if it caniedtherestenergyof theparticle;henceit is defmed: photonwouldpossess ^h I

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wheremois the massof theparticleinvolvedin the scattering. 2.4 WINDOWSON THE UNIVERSE As the previoussectionhasshown,differenttypesof emissionmechanismareproduced mechof eachemission Withintheboundaries to differentphysicalconditions. in response can alsobe produced.Obsewingthe Unianism,a wide rangeof radiationwavelengths andscrutinisingspectracanthereforegive us an insightinto verseat differentwavelengths presentthroughoutthe cosmos.ln the differentphenomenaand physicalenvironments

S ec.2.4l

W i ndow s on the U ni v ers e 29

some ways, we have becomeso usedto seeingthe night sky at optical wavelengths,that spacecraft images of it in anything other than visible light often take us by surprise. It is important to remember at all times, however, that visible light is such a tiny part of the electromagneticspectrum that to ignore the rest would be folly. If we start at the lowest energies of electromagnetic radiation, the Universe becomes the realm of the radio galaxies. The entire sky is dotted with their tremendouslypowerful radio emitting lobes. Also visible at radio wavelengths are objects from our own Galaxy known as supernova remnants. These are the glowing remains of exploded high mass stars. In the direction of Sagittariuswill be the centre of the Milky way. At certain radio wavelengths, vast clouds of molecuiar gas can be seen. These also exist within our own Galaxy and are the clouds out of which stars eventually form. The different wavelengths of radio trace out different molecular species.By mapping the strength of eachmolecule's radio emission, contour maps of the clouds can be obtained, which show the number of each type of molecule at each point throughout the cloud. The nearestmolecuiar clouds would appear to be enormous to us. For instance, the molecular cloud with which the Orion nebula is associatedencompasses the whole of the constellationat radio wavelengths. At the next man-madedivision - microwave wavelengths- the view is totally different, since the cosmic microwave background dominates.Insteadof a dark slcypunctuatedwith both point and extendedsources,the entire sky is bright and glowing with energy. Superficially, the brightnesswill look the samein all directions. There are variations in the radiation however, and the first, most obvious is known as the dipolar anisotropy. It is produced by the motion of the Earth relative to the cosmic microwave background radiation. When all the componentsof the Earth's motions are summed, such as the component due to the Solar system's motion aroundthe centreof our Galaxy and the Galaxy's motion within the Local Group, the resultant velocity increasesthe temperature of the cMBR in the direction ofthe Earth's motion. It also decreasesits temperatureby a corresponding arnount in the antithetical direction. Underlying this is the microwave contribution &om the material and objects in the Milky way. If all of these are removed and the remaining microwaves subjectedto very careful analysis,fluctuations, which coincide with the emergenceof Galacticstructure,would becomevisible. Changing our observationsto the next, more energetic bands ofradiation, we arrive at the infrared. The sky is once again dark and starshave reappearedon the scene.In general these are stars cooler than can be easily seen at optical wavelengths. They are the red dwarfs and red giant stars.Also presentare the nascentstars in stellar nurseriessuch asthe orion nebula. These protostarsand other young stellar objects are obscured from view at optical wavelengthsby their dusty birth clouds but, at infrared wavelengths,shine through. The Milky way continuesto stretchacrossthe sky again, illuminated not so much by stars but by the infraredglow ofwarm dust clouds.A secondbandalso stretchesacrossthe sky. This one intersectsthe Milky way at an angleofjust under 80oand marks the planeof our Solar System.lt is glowing becausethere is warm dust in interplanetaryspace. Beyond infrared, the familiar optical spectrumoccurs. our view of the Universe at thesewavelengthswas describedin chapterl. At higher energies,the ultraviolet,the very hottest stars in the Galaxy dominatethe view. They are the o and B-type supergiantstars which havesurfacetemperatures of 20,000K or more.Using wien's law l,.o can be calcu-

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asdo otherregionsof of galaxiesglow faintlyin X-raysthanksto thermalbremsstrahlung, hot, tenuousplasmaspresentin our owr Galaxy.The generalbackgroundglow is also punctuatedby point sourcesof intenseX-rays.Someof thesearethe nucleiof powerful activegalaxies.Othersmarkthe positionsin our Galaxywherestarsarebeingrippedto areexplainedtheoretipiecesby incrediblystronggravitationalfields.Both phenomena cally asbeingdueto the actionof blackholes. spectrum,the If we wereto continueto thehighestenergyrangeof theelectromagnetic night sky would look perfectlyblack againexceptfor the occasionalflashof an elrant garnmaray. As yet, thesebursts,which occuronceeveryfew daysfrom totally random directions,are unexplainedand makeup one of the mostfascinatingaspectsof modem highenergyastronomy. weredesignedto illustratejust how biasedour view of the The precedingparagraphs wouldbe if wewereto persistin myopicallypeeringat it with opticalwavelengths cosmos would be ableto tunehis telescopeto pick up alone.ln a perfectworld, an astronomer certainfactorsmitiwhateverwavelengthof radiationhe wantedto detect.Unfortunately, gateagainstthis. Oneis thatdifferentstylesoffocusingdevicesanddetectorsareneeded in orderto receivedifferenttypesof radiation.Anotherfar moreseriousproblemis that, evenif you wereto build a detectorfor eachkind of radiationandmountthemin a field, stops the Earth'satmosphere only someof them would receivesignals.This is because certaintypesof radiationfromreachingthesurfaceof our world (seeFig. 2.7). Photon endrgy {eV) 'to3

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regionof the spectrum'Thus' latedto be 70 nm; a wavelengthin the extremeultaviolet in the Galaxywould bestars brightest the obscurity, to whereasmoststarswould faje the spectrum' comeevenbrighterif we couldseethe ultravioletregionof around.This is knownas As well asthe individualstars,thereis alsoa glow from all rhe SolarSystemand ,bubble' which within 'cavity' andis a rougtiiy spherical the local explosionlong supernova a by produced been have thoulghito is ,urro*oing starssit. It a bubbleshape into atoms ugo.ff,e siock wavefrom thatixplosionhassweptinterstellar *hi.h no* glowsat ultravioletwavelengths' the sky continuesto glow faintly at passrngon to evenmore energeticwavelengths, the X-raybackgroundcomesfrom ultraviolet, the locally*produced X_rayenJgies.Unlike fromdistantgalaxies'Clusters andis thoughtto 6. tn. combinedemission vastdistances

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Sec.2.5l

Telescopes,detectors and spaceprobes 33

Abovewavelengths of 20 metres,radiowavesarereflectedby the Earth'sionosphere. Most,but not all of the infraredradiationis absorbed by molecules in the atmosphere. Opticalwavelengths, obviously,passthroughwithouthindrance.Apart from theultraviolet radiationwhich causessuntans,all the high energyphotonsare blockedby various interactions Thesetake placebetweenaltitudesof a few with atomsin the atmosphere. tensto a few hundredsof kilometres.The typeof radiationwe wishto observedetermines the style of detectorwhich hasto be built and whetheror not it hasto be madeinto a satelliteandsentinto space. 2.5 TELESCOPES.DETECTORSAND SPACEPROBES

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FromEarth-based astronomers can detectpredominantlyopticaland radio observatories waves.Usingtelescopes for opticalashonomybeganwith Galileoin 1609.He appliedthe designof HansLippershey,a Dutchspectaclemaker,andimprovedit. His telescopeallowedhim to makethe mostremarkable discoveriesin his dayandage,includingobservathe Copemicanview of a heliocentric tions of the moonsof Jupiterwhich strengthened thosewho followedhim to build biggerandbetterteleSolarSystem.It alsoencouraged scopesin an effort to discovermoreandmore.Newtonalsodesigreda styleof telescope which is still widely usedtoday by amateurastronomers and, indeed,is known as the Newtonian. areenormousin sizecomparedwith those Today'splofessionalastronomytelescopes original ones.Many existingtelescopeshavemirrors which are 4 metresin diameter, whilst a new generationoftelescopesis cunently underconstructionwhich possess 8-metremirrors.The largesttelescopein the world is the Keck Telescopeon the extinct volcano,MaunaKea,Hawaii,whichpossesses a primaryminor l0 metes across.It is so largethat it wasunrealisticto build a single,continuousmirror of the size.Instead,it is constructedlike an insect'scompoundeye,with 36 hexagonalmirror segments held in place by electronicsupportarmsto createthe giant mirror. The Keck Telescopehas provedto be so successful thata second,identicaltelescopeis currentlyundergoingconstructionon the summitof MaunaKea. This siting of the telescopeis not unusualfor modemobservatories diffrcult conditionswhichworkingat altitude despitethesometimes cancreate.Mountaintopsarean idealsituationbecause thetelescopes areelevatedabove msstofthe weatherandturbulentair, whichobstructobservations from sealevel. Atmospheric turbulencecanalsobe combatedin otherways.For instance, a technology knownasactiveandadaptiveopticsis just provingitselfviable.Active andadaptiveoptics form a systemwhichallowsa reflectingsurfaceto be controlledby mechanical actuators.The qualityof the imagebeingproducedby the telescopeis continuously monitored andanydeviationfrom perfectionis registeredso that stepscanbe takento makeconections.A computerdetermines howto movethemirrorto maketheimageperfectagain(see Fig.2.8).Thissystemhastwo advantages. Thefirst is thata largemirrorcanbe manufacturedandsupportedin manyplaces.Formerly,the rigid construction of a miror to a level which preventedflexuremadeit prohibitivelyheavy.The secondadvantage is that,providingthesystemcanreactfastenough,it candetectimperfections in the imagecausedby atmospheric turbulenceandcompensate for themby manipulatingtheminor. This typeof technologylookscertainto be installedin moreandmoreobservatories overthe coming decades.

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Fi g 2.8 S c hemati c di agramofanadapti v eopti c s s y s tem.Thek ey c omponenti s thew av efront detecror.which sensesthe distortionin the incomingwuu. rront,ito*ini,r,. the mirror to comDensate ""rp"i"ri"'.q*,

All a telescopeprimarilydoesis collectlight. It is up to the astronomer . what is done with that light onceit hasbeencollected.originatly, viiual observations weresufficient, butcontemporary sciencedemands hardfactsanadatato backup anyassertions whichare made.A plethoraoftechniquesanddetectorshassprungup to help astronomers record theirfurdings. Two basictechniques ofanarysing the light colrected by telescopes arethoseofspec_ troscopyandphotometry.A spectroscope splitsthe incominglight into a spectrumso that its spectrallines can be studied.As discussedearlier,the pattem of spectrallines can indicatethe chemicalspresentin the objectunderinvestigation, andthe iize andshapeof thespectrallinescanleadto conclusions aboutthemotionofthat object.Aboveandbelow the spectrum,the spectroscope superimposes referenceemissioniin"., .o that accurate wavelengths for the observedspectrallinescanbe measured. Devicesknownas micro_ densitometers canbe usedrostudythe intensityof radiationalongthe spectrum, andit is fromthesetracesthattheshapes ofthe spectrai linescanbe founj(seenig.2.9i. It is fair to say that the vast majorityof astronomers usespectroscopy to studytheir chosenobjectsbecause the level of informationreturnfrom a singleobservation canbe veryhigh.However,spectracanbe time-consuming to recordbecalse,in traditionarspec_ troscopes, the light froman objectis passedthrougha slit aperfureandthenexpanded into its constituentwavelengths. This can dramaticallydecreiseits brightnessano thus in_ crease thetimenecessary for theexposure. Photometry is a technique wherebythe brightness ofa celestialobjectis takenat variousdifferentwavelengths. Thosewavelengths areknownasphotometric bandsandhave

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FrequencY by a specificshapeknown as a Gaussian Fig. 2.9.Absorptionandemissionlines arecharacterised profile, which can be obtained by tracing with a microdensitometeracrossa spectral line. (Adaptedfrom Kitchin, C., Stars,Nebulaemd the InterstellarMedium'Adam Hilger' 1987.)

They havealsobeengiven letternames:for to specificwavelengths. beenstandardised of 2.22micronswhich is in the the K photometricbandrefersto the wavelength instance, practised by placingfilters betweenthe is The technique infraredregiol of the spectrum. telescopeandiome form ofphotoncountingdevice.The filters only allow specificwavelengthsoflight throughto the counter,which recordsthe intensityoflight at that wavelength(seeFig.2.10). it canrevealtheirblackbodytemperaPhotometrycanbe usedto classi$ starsbecause tures.It is very usefulat trackingvariationsin theamountof radiationwhich is outputby celestialobjectsover a periodof time. A goodexampleof wherephotometryis particularty usefulis in the classificationof variablestars.Photometryshowsjust how they vary (seeFig. their light outputoverthecowseof time andallowslight curvesto be constructed analysisof photometricdatacoveringa wide rangeof wavelengths 2.11). Sophisticated can yield many ofthe sameconclusionswhich can be derivedfrom spectoscopybut, than the the collectionof photometricdatais evenmoretime-consuming unfortunately, data.Thus, the consfuctionof highly complicatedspectrocollectionof spectroscopic scopesis still favouredoverthe useof the simplerphotometers. A third, lesspopularthoughno lessvalid,techaiqueis thatof polarimetry.Thismethod of observingcelestialobjectscollectsinformationaboutthe orientationofthe electric It canbe a powerradiationbeingcollectedby thetelescope. vectorin theelectromagnetic ful tool in probingregionswhereradiationis beingscatteredby dustor electrons.It can alsohelpto showup regionsof spacecontainingmagneticfieldswhichhavealigneddust grains.Thus,the techniqueis excellentfor probingthe interstellarmediumin this and othergalaxies. astronomer. The light caparenow insufficientfor theprofessional Visualobservations the above techniques must now be processed via of one telescopes and large tured by recordedfor later analysisand publicationto scepticalcolleagues.Photographyis still usedby some,but by far the mostrapidly developingtechnologyis in a type of detector

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WaVelength{nm} Fig.2.10.Thespectral response ofthedark-adapted eyeandoftheU (ultraviolet), centred on365 nm,B (blue),centred on440nm,andV (visual,i.e.yellow-green), centred on550nm,pass-band filtersusedfor photometry in theUBV system. known as a ccD, or 'charge coupled device', an instrument which is so sensitive that it records almost every photon which strikes it. It is a computer chip which usesa detection mechanism,similar to the photoelectric effect, to record the number of photons striking it. These are instantly tumed into an elecfrical signal which can be read out into a computer memory for later analysis and display. Another way of saying that a ccD has the ability to count almost every photon which strikes it is to say that it has a high quantum efficiency. This is a ratio which can be easily understood as the number of detected photons divided by the actual number of incident photons. In other words it gives the percentageof photons which are actually detected when they strike the detector. A ccD will typically have a quantum effrciency in excess of 75 per cent, whereasa photographic systemwill rarely be more than I per ceht. ccD chips are made of semiconductors,the choice of which determineswhich part of the electromagnetic spectrum will be detected.ulfiaviolet, optical and infrared can now all be observedwith the correct CCD. The collection ofradio wavelengths,however, takes place with totally different t,?es oftelescopes and detectors. A radio dish works in exactly the same way as an optical telescope.The reflector still needsto be parabolic but, becausethe wavelengthsofradio radiation are so much longer, it no longer needsto be silvered. The detector is placed in the prime focus position on the radio dish, where a secondarymirror is usually, but not always, found on an optical telescope. Radio astronomy has pioneereda techniqueknown as interferometry, which effectively increasesthe resolving power of a telescope by linking it in tandem with another. The

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Otherwaysofobserving theuniverse37

The resolvingpowerof a telescopeis proportionalto the wavelengthof radiationdividedby the apertureof thetelescope. Thuswith an increasein the wavelength, theresolving powerof the telescopecanbe maintainedonly if a commensurate rise in the apertwe ofthe telescopeis alsoachieved.The radiowavesgivenout by neutralhydrogenaresix ordersof magnitudelongerthanvisiblelight so, in orderto get the sameresolutionfrom the aperhrewould needto be one million timesthe sizeof opticalteleradiotelescopes Obviously, anotherwayhadtobe found. scopes! Interferometrywas that otherway. It was pioneeredat CambridgeUniversityin the 1940s.It allowsan objectto be observedsimultaneously by two or moreradiotelescopes. The signalscollectedby the dishesarethen combinedand interferencepatternsare obpatternscanbe usedto constructdetailed tained.UsingFourieranalysisthoseinterference imageswith muchgreaterresolutionthana singledishon its own couldachieve.Lnterferometryin theradioregionof the spectrumhasbeenso successful thatthemethodhasnow too. A pioneeringteamof astonomersat Cambridge beenappliedto optical telescopes hasappliedthe technologyto producean imageof the starCapellausingfour smalltelescopestogether.Now, a numberof much larger observatoriesare under construction aroundtheworld which all hopeto exploitopticalinterferometry. The impenetrability ofthe Earth'satrnosphere makesit necessary to sendspaceprobes andsatellitesinto orbit in orderto collectthosewavelengths from which we areshielded. High energyradiationis almosttotally blockedout by the atmosphere. Apart from the smallamountofultravioletradiationwhichpenetrates theatnosphereandcausessuntans, the vastmajorityofultraviolet radiationhasto be collectedby spaceprobessuchasthe IntemationalUltravioletExplorer(IUE). At evenshorterwavelengths, the EinsteinX-ray Observatoryhas surveyedthat region of the electromagneticspectrum.Thesespace probesrequirea slightly differenttype of mirror systemknown as a grazng incidence mirror.This type of mirror is an annulustakenfrom the cylindricalwall of a paraboloid ratherthanfromthebowl asin opticaltelescopes. In orderto maximisetheamountof high energyphotonscollected,annuliof successively smallerradii are nestedwithin one another. Molecularabsorptionblock a lot of infraredradiation,but this too hasnow beensampled from spaceby the Infra-RedAstronomicalSatellite(IRAS) and the InfraredSpace ObservatoryQSO).Evenwavelengths which can be studiedfrom Earthhavebeencollectedin space.High abovethe Earth'satrnosphere, the starsremainsteadyandunwaver(HST) is currentlyperformingmagingly sharp.The celebrated HubbleSpaceTelescope nificentlyby collectingunprecedented imagesof thecosmos.

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resolving power of a telescope is simply a measureof how apparently close two celestial objects can be to each other before the telescope is incapable of showing them as two distinct objects.For optical telescopes,the resolvingpower can often be so good that the distortion of the images through atmospheric trubulence is the biggest source of image degradation. For radio telescopesthis is not the casebecausethe wavelength ofradiation is so much larger.

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2.6 OTHER WAYS OF OBSERVINGTHE UNIVERSE Whilstit is trueto saythat90 per centofobservations areundertaken by detectingelectromagneticradiationfrom space,thereare otheremissionswhich can be studied.One methodis the detectionof tiny particlesknownas neutrinos.Theseparticlesarea fundamentalconstituent of the cosmosandcany excessenergyawayfrom nuclearreactions. Neutrinodetectors havebeenverysuccessful in detectingthesefleetingparticlesafterthey havebeenproducedin the heartofthe Sun.Additionally,andperhapstheir greatestcollectiveclaimto fame,is thatthe detectorsidentifieda burstof neutrinosfrom thenearby supernova of I 987,exactlyaspredictedby theory.

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detectors. Undeniably,they are still very crudewhencomparedwith electromagnetic they sky the ofthe neuffinos area which tell from to For instance,they cannotbe used powerprovide will another devices ofthese refinement The continual come. have detect detail' ful resourcewith whichto probethe Universein everincreasing which is in its infancy at the moment,is the instrumentation, of tecbnology Another gravitationalradiationdetector.Gravitationalradiation is predicted,by the generaltheory of relativity,to be releasedfrom massivebodieswhich are changingtheir gravitational continuumin the throughthe spacetime relationshipwith oneanother.It wouldpropagate andrarefacpattern compressions of (i.e. a repeating wave motion form of a longitudinal tions) and distortany objectsin its path.The distortionswould manifestthemselvesas minusculevariationsin the lengthof objects.As yet,however,technologyis insuffrciently to be ableto detectthesewaves,sincethe sizeofthe variationsthey induceare advanced is of the diameterof a hydrogenatom.Meanwhile,research vanishinglysmallpelcentages foreveradvancingand the sensitivityis graduallybeingpushedcloserand closerto the detectionlimit. Gravitationalradiationis the final testofthe theoryofgeneralrelativity.Thereis even radiationsimia hopethat it maybe possibleto detecta cosmicgravitationalbackground lar ti ttre cosmicmicrowavebackgroundradiation.If this wereto prove possiblethen, a view of the Universeas it existedonemiltheoretically,it would afford cosmologists Bang!. Big the after a second of lionthof a millionth

2.7 PARTICLE ACCELERATORS AND THE ARCHITECTI]RE OF MATTER Oneof the mostinnovativewaysin whichphysicistsstudythe beginningof the Universe This technologyis is to usegiganticpiecesof equipmentknownas particleaccelerators. the perfectsynthesisof the very small,namelysub-atomicparticles,with the very big' namelythe entireUniverse.Insidethesedevices,streamsof sub-atomicparticlesareacceleratedto velocitiescloseto the speedof light, which suppliesthemwith similarquantiduringthe earlyUniverse.To simulatethe ties of energythatthey wouldhavepossessed beamsof particlesarethen densityof matterfrom thoseearlytimes,highly accelerated a liberation of energy cause result which collided with eachother. The annihilations populated the earlyUniparticles must have which of creation to the leads which,in turn, versein largenumbersbut no longerexistnaturallyin ourpresentdaylow energycosmos' to testtheirtheories,particlephysicistshavedevThroughtheuseoftheseaccelerators almostall particlesandtheir interactions.The explains elopeda standardmodelwhich go to makeup matterfall naturally into two famiwhich particles nature of fundamental lies. Theseareknown as the leptonsand the quarks.Both familiescontainsix particles of pairs.The mostfamiliar leptonsarethe first genwhich aresplit into threegenerations neutrino(usuallyreferredto simplyastheneuelectron the pair: and the electron eration arepopulatedby heavierversionsof both the electron trino). The othertwo generations (themuonandthetau) andthe electronneutrino(themuonneutrinoandthetauneutrino). heavierversionsof the up anddown of successively arecomposed The quarkgenerations quarks;the charmandthestrange,thetruth andthe beauty(seeFig.2.12). particlescancombinein manywaysto becomethe mattercontent Thesefundamental chargeof -1. The up an electronhasan elechomagretic of the Universe.For example,

Sec.2.7l

Particle acceleratorsand the architecture of matter

LEPTONS z o

E

4

g

{ g a = o

e -a

39

HADRONS vso Eslron lleufim

El€atm Rosponsiblo tor slectricity wilhnoelectric and Particle charge redctions. chemical lhstrar€lyinteracts withoth6r matter. your It hasa charge of-1. Billions tlythrough bodyov€rywond,

lla Up Oulrk li€sanelectric charge ol +28, Protonrhavetwoolthem,while neutrons naveone.

O. DowtrO[ork Hasanelectric charge of -1/3, Protons haveon6of thom,while nsutrons havstwo.

vpa lluor ilautritro iluon rslative A heavi€r ofth6el*tron Crested alongwithmuonswhen $m6 particles dtray.

co

so

E g 2 E

Cham Ourrl A heaviflrelative ol theup.

Strrnge Ouail( A h€avier rslative of thsdown.

z o

rf

I

E c a 2

E u 2 g

o a e

p'a

Tru H6avierstill.

a v1 Tru Nautrino yst Not dismveredbutbeli€vsd to exist.

o n:ii'I,

Top tne

quarkol all.

bo Eottom Ourrk fleavigrstill.

Fig.2.l2. Leptonsand hadrons,the fundamentalparticlesof the Universe.(Adaptedftom gig Bang Science,ParticlePhysicsandAstronomyResearch Council.)

quarkhasa chargeof +213whilst the down quarkhasa chargeof -ll3. In orderfor the quarksto combineinto themorefamiliarsub-atomicparticles,neufionsandprotons,they mustcometogetherin the right combinations. Two up quarksandonedownquarkmake up a protonwhich possesses a chargeof+I. Oneup quarkand two downquarkscome togetherto form a neutronwhich caniesno charge.Thesehybrid particlesmadefrom quarksare lnown as hadrons.Hadronscan be subdividedinto quark fiiplets known as baryons,suchas protonsandneutronsandmesonswhich consistof a quarkand its antimattercounterpart.Electrons,protonsandneutronsthen combineto form atoms.The exactnumberof electrons,protonsandneutronsdefinesthe atom'schemicalidentity. Thereare very few stablepaniclesin the Universetoday.Only the electronand the electronneutrinoare truly stable(photonsof radiationcan also be consideredstable). Neuffonswill decayinto an electronanda protonin about896 seconds, if removedfrom an atomicnucleus.Protonsmay or maynot be stable.If they arenot stabletheydecayin lifetimesof aroundl03ryearsandbecomea positronanda pi-mesonwhich,itself,decays into trvophotons.Althoughtheirvastlifespansmakethemstablefor all currentproblems, the very long-termfutureofthe Universecould be affectedifprotons do decay.All the otherhadronshavelifetimesof a merefractionof a second. All the particles,be theyleptonsor quarksandtheir composites, interactwith oneanothervia the exchangeof a third groupof particlesknown as gaugebosons.It is these particleswhich carry the forcesof nature.Throughouthumanhistory,what were once thoughtto be totallydifferentforceshavebeenshownto be differentmanifestations ofthe sameforce. For instance,Newtonshowedthat the motion of the planetscould be explainedby the sameforcethatcausedtheproverbialappleto fall to theEarth:gravity.On thefaceof it, two totallydifferentphenomena which,in realiqv,weremanifestations of the sameunderlyingprinciple.Earlierin this chapterwe sawhow electricity,magnetismand light wereunified.Later,Einsteinshowedan interconnection befweenspaceandtime in specialrelativityandthenlinkedthis work to gravityvia his generaltheoryofrelativity.

40

The tools ofthe trade

lch.2

forces.Theyare Todaywe recogniseonly four forces,which we tenn the fundamental the weaknuclearforceandthe strongnuclearforce.Eachone gravity,elecffomagnetism, ofthem actsdifferentlyfrom the others.Theyarecarriedby gaugebosonsin the standard is carriedby the photon,which we havealreadymet,andthe model.Electromagretism weaknuclearforce is canied by threedifferentparticles,the neuftalZ" andthe charged to thephoton W" andW-. Theseparticlescanbe thoughtof asbeingsomewhatanalogous mass.The stong nuclearforceis a chargeandtheyall possess exceptthatthe Ws possess also mediatedby a gaugeboson,known as a gluon,which transmitsthe force between quarks.MesonsthenFansmitthe forcebetweennucleons.As yet,thereis no convincingquantumtheoryofgravity, althoughthehypotheticalgaugebosonhasbeennamedthegraviton. forces The massof thegaugebosonsaffectsthe distanceoverwhichthesefundamental uncertainlyprinciplewhichstates: ofthe Heisenberg canact.This is a consequence

Sec.2.7l

Particle acceleratorsand the architecture ofmatter

4l

i

E

E

-P E a

aEAt=I

(2.8)

2n

Sincethe energyAE to createthe gaugebosonsis bonowedfrom the energyfield ofthe spacestrrounding the interactingparticles,that energyhas to be paid back before equation(2.8)is violated.Equation(1.2)tellsushow muchenergyneedsto be borrowedand so the timethe particlecan'live' for, At, is givenby a combinationof thetwo equations. A t= -\ 2{nd

Q.e)

Therangeofthe force,r, canthereforebe shownto be inverselyproportionalto themass ofits gaugebosonbecause,accordingto specialrelativity,nothingcantavel fasterthan the speedof light in a vacuum. r-< c At=

1 h * 2nmc m

(2.10)

The weaknuclearforce is canied by massiveparticleswhich limit its rangeto approximatelylO-r?m.Themesonswhich carrythe strongnuclearforcearelessmassiveandcan canexistforeverandsotherange reactovera distanceof l0-r5m.Photons,beingmassless, forceis theoreticallyinfinite.Overlargeregionsofspace,however, ofthe electromagnetic forceis, in fact,confined.Gravitons electricalchargeis neutralandsotheelectromagretic which canbe positiveor negaand,unlikeelechomagnetism, aretheorisedto be massless cannot be cancelled out. This is the reasonit thus it gravity athactive, is only ever tive, shapesthe Universeon its largestscales. canexaminethe behaviouroftheseparticlesandforcesin condPanicleaccelerators itionswhich mimic the early Universe.By doing so, a remarkabletheoryhasbeenconandthe weaknuclear firmed.A groupofphysicistshad proposedthat electromagnetism in the Confidence forcewouldactin exactlythe samefashionat sufficientlyhigh energies. by theparticleaccelerator Salem,WeinbergandGlashowtheoqywasboostedenormously andthe weaknuclear confrrmations oftheir predictions.Fromnow on, electromagnetism force.This success hasIedphysicistsand forcewouldbe foreverlinkedastheelecffoweak cosmologists to believethat the strongnuclearforcecould alsobe unified with the electroweakforce.It is postulatedto takeplaceat evenhigherenergiesandis explainedby a

t t t

1015

10rs (Planckscalo)

(GeV) Energy Fig. 2.13. Unification offorces. Centralto most cosmologicalideasis the notion that, as the temperatureof the Universewas higherat earlierald earliertimes in history, so the distinction betweenthe separateforcesofnature waslessand lessapparent.(Adaptedfrom Silk, J..A Short HBloryof the Universe,W.H. Freeman,1994.)

setof ideasknownasthe grandunifiedtheories(GUTs).This is a set of theorieswhich setsout to uni! all the fundamentalforcesexceptgravity. A numberof GUTs exist, but asyet nonehavebeenproven.GUTspredictthatprotonsshoulddecay. Somephysicistsand cosmologists believethat, following a successfulGUT, gravity wouldbe unified andeveryinteractionin the Universewould be describedin termsof a singlefundamental forceof nature.Thislevelof unification,if it evenexists,is a longway intothe future.A quantumtheoryof gravityis requiredfust (seeFig. 2. I 3).

t t t t t t

L L

F

Observationalcosmology Cosmologyis, by its very nature,an observational science.It is passivein that we must wait for eventsto happenin the Universewhich we canthenwatchandlearnfrom. Not for the practisingcosmologistis the relative easeof making a universein the laboratory,although simulationsof the early Universeare partially possible in particle accelerators. havephysics,an experimental Luckily, as pointedout in chapterl, asfronomers science, on their side. Every piece of cosmologicalinsightgainedis achievedby painstakingly usingthe physicsgleanedfrom laboratory modellingobservedcelestialcharacteristics, experiments. The first fundamentalobservationwhich canbe madeaboutthe Universe,as a whole, is that it containsmatter.The radiationcontentis not so surprising,aswe shallsee,but the fact that solid lumps of matter aboundis rather a shock! The reasonfor the surpriseis currentlybeingconducted because ofthe resultsfrom experiments in particleaccelerators, mentionedin chapter2. In theseexperiments, the interchangeability of massandenergyis explored.Energy,caniedby photons,canbe changedinto massundercertahconditions: for example,if the photongetscloseto a heavyatomicnucleus.Whenthis happens,two particlesare produced- one of matter,the other of its anti-mattercounterpart.This is necessary to conservechargeamongotherquantities.For the purposesofthis example, imaginethat the two particlescreatedarean electronandits counterpart,a positron.Eventually and usually soonerratherthan later,the positron comesinto contactwith.the electron (or anotherwhich is just like it) andthey annihilateeachother,returningtheir energy backinto photons(seeFig. 3.1).Ifthis processholdstue univefsally,thenfor everypadicle of createdmatter,thereshouldbe an antimatterequivalent.Eventually,mutualannihilationeventswill returnall themassenergybackinto photons,leavingnothing&om which to makestars,planets,you or me.Couldit be thatthe antimatterhassomehowbeensegregatedfrom the matter?This is a ratherunsatisfactoryexplanation,sincethereis no obserfrom the point ofview ofthe cosvationalevidenceto supportit. It is alsounsatisfactory mologicalprinciple,aswill be explainedshortly.Somewayhasto be devised,within the knownlawsof physics,which enablesthe ratio of matterparticlesto antimatterparticles to be greaterthan l. Currentapproximations, baseduponthe ratio ofphotonsto matter particles,suggestthat the asymmebyof matterto antimatteris probablyonly onepart in a billionl Very smallindeedbut, asinsignificantasonein a billion may sound,it hasled to a profoundlydifferentUniversethanonefilled with radiationalone.

lch. 3

44 ObservationalcosmologY

/ \ Two photons collide

a -

€./._A

/ \

-i=-*^

v

/ \ / \

)

Positronand electroncreated

/ \

Sec.3.ll

Look-back time

45

aneously,over somethingaslargeasthe universe,however,eventscantakea longtime to propagate.so long in fact,thatthethe moredistantthe object,the morein thepastwe areviewingit. To quantif,thischaracteristic ofthe universe,astronomers havedeveloped a distanceme4surement systemknownasthe light year.This is the distancefavelled by light in a singleyearandis equivalentto 9.4607x l0r2kilometres.If two starsareseparatedby a distanceofone light yeartheneventswhichtakeplaceon onestellarsurfaceare only apparentto the other after a period of .oneyear,during which time the light released by the eventhastravelledthe9.4607x l0r2 kilomehesbetweenthe two objicts. closer objectswill be ableto observetheeventbeforemoredistantobjects.Thus,a galaxywhich is onebillion light yearsawaywill appearto us as it did onebillion yearsago.This phenomenonis knownaslook-backtime (seeFig. 3.2). The informationthat an eventhastakenplace propagatesoutwardsin a sphericalvolumearoundthe eventwith a radiuswhich increases with the speedof light. The ,surface' of the sphereis knownas the particle'shorizon.over smallvolumesof space,an event whichtook placeat time,t,, will havea presentday(i.e.time = to)particlehorizon,r", of: r.= c( h*t r ) (3.1) Over largedistancesof space,relativisticeffectswill destoy the simplicityof this relationship.Diagrammatically, this behaviouris represented by a light cone(seeFig. 3.3).

{

i, t,

i

I

Positronand electroncollide

\

n/ @-1.-c

This image is only one quarterof the way to Earth.

lI.,tr

\

ir

/

lt i

\

i,.

I

i

li

i',

This is how the galaxiesappearnow.lt will takeone billionyearsfor this imageto reachEanh.

)

Thi s i mage i s one hal f of the way to Earth.

Two protons

Produced

Fig. 3. L Pair production:the creationof a matterparticloand its antimatt€rcounterpart,resulting fiom photon interaction.

3.1 LOOK-BACKTIME At fust, thevery ideaof cosmology,i.e. observingtheUniverseasit existsnowsothatwe It would be like taking a snapshotof a can tell how it all began,seemspreposterous. personand telling his or her entire life story from that one singlephotograph.Yet the aim feeljustified in doingthis. A key element, of this chapteris to explainwhy cosmologists is that light and all ofour consideration, the beginning right at introducing which needs radiationcanonly travelat a finite speed. otherformsof electromagnetic with a value in chapter2, the speedof light in a vacuumis uhsurpassable, As discussed almostinstant' of 3 x l0Em/s.On terrestrialscales,this meanseventsare communicated

AQ) One billionlightyears

6

(t)

This image is three quartersof the way to Earth. Thi s i s how thes e galaxies appear to us on E arth. The i mage i s one bi l l i on y ears out of date!

Fig. 3.2. The finite speedof light causesthe phenomenonof look-back time The deeper astronomers look into space,the moreanciontthe objectstheysee.

t t t t t i

t L

; ; ;' l ll

,l

lt bl

rt,i

I

LJ

I

,i

_l l

46

ObservationalcosmologY

lch. 3

namedafterthe physicistwho Light conesare drawnon Minkowski spacetimediagrams, exists in the universe can be which first proposedthe conceptof spacetime.Anything as 'world lines'.For unacknown paths will follow they diagram. spacetime ;;;;" themto becomecurves' causes " acceleration but straigbt are lines world ieleratedtravel, is travelling' For exartobject fast the rrr" gaoie* of a world line is an indicationof how or infinity depending zero either gradient of line world objectwillhave a ;;, ;;;r;"rry thg* in the Universe' fastest the being rays, Light axis. time the of orieniation ;;ilr; .geodesics,through6e spacetimecontinuumandhavea set gradient iJrr"* p"a, called i'e. arecausallyconnected, ro. trr.i,.world lines.objectswithin iach other'slight cones their during to occur effects ian cause hence and photons ,h.y i";. beenableto exchange Objects a timelike seParation' ,"rp".iiu. lifetimes.The two |Uitttt *. thensaidto have separation. space-like possess . noi wittrinoneanother'slight conesaresaidto obviousthat Universe,thenit becomes the ligfi coneofthe observable Ifone considers becauselight from themhas theie could very well be objectswhich we do not know exist

Sec.3.2l

Olbers'paradox 47

not reachedus yet. Sincethe light coneofthe Universeis defuredby its age,any object from us cannotpossiblyhaveinfluencedour evolution. whichhasa space-likeseparation Theseunknowableregionsaretermeddomainsand could,conceivably,havetotally differentlawsof physics.The limit of our observable Universeis knownasaneventhorizon. we cannotobserveit because thecosmicmicrowavebackground Unforhrnately, blindsour sight,aswe shallsoondiscover. 3.2 OLBERS'PARADOX observation which canbe made,andis possible Olbers'Paradoxis anotherfundamental with nothing more than the naked eye. Ask anyoneto describethe night sky and the chancesarethat oneof the first thingstheywill tell you is that it is dark. Kepler,whoselaws of planetarymotionwe encountered in chapterl, pointedout in 1610that if the Universeis infinite, with starsscatteredrandomlythroughoutit, why is it dark at night?After all, in whateverdirectionone caresto look, our line of sightwill, sooneror later,cometo reston the surfaceofa star.Although light suffersfrom an inverse squarereductionin its intensitybasedupon its distance,the further one looks into space, themorestarswill appearin our line of sight,compensatingfor the dimming.This curious phenomenonwaspopularisedover a centurylater by Heinrich Olbers,andnow bearshis name(seeFig.3.4).

I

I

I

Porticte hor'zon If oneeventis situatedwithin Fig. 3.3. Light conesshowthe propagationof light from anevent' event is situated outside the it E'rigiti .o"ni of *t" other,.then the/are causall-yconnected.If one other's existence (Adapted each yet of awate nbt are events two the then o*er, of tfre iiJfrllone 1994') to Cosmologt,Wiley, lztoduction M., frim Roos,

Fig. 3.4. Olbers'Paradoxaskswhy the Universeis dark at night.If it were infinite in extent,then our line of sight would eventuallymeetwith the surfaceofa bright star.The reasonthe night sky is dark, therefore,is that the Universeis not infinitely largeand that the redshiftmakesdistant objects more and more faint.

t 48 Observationalcosmology

lch. 3

The most obvioussolutionto the problemis that starsare not distributedrandomly throughspace:insteadthey form galaxies.This providesonly temporaryrespiteandthe aswe shallsoonsee,on thelargestscale,galaxproblemreappears with galaxiesbecause, iesandclustersofgalaxiescanbe thoughtofas beingspreadrandomlyt},roughoutspace. One solution to the problem,which gives us our fust fundamentalinsight into the naof look-backtime.Imagineour Galaxy ture of the Universe,comesfrom a consideration and anotler more distantone to be plotted on a spacetimediagram.We only become awareof the other'sexistence(andvice versa)whentheir light conecrossesour world line. If the Universeis not infinitely old, thentheremust be distantgalaxieswith light conesnot yet in contactwith us. ln orderto resolveOlbers'Paradoxwe havebeenforcedto assumethat the Universe maynot be infinitein age.In doingthatwe havetakenour first steptowardstheBig Bang, becauseit impliesthat at sometime in the pastthe Universemust havebeencreated. resolutionalsoexists,aswill be explainedin thenextsection. Anothercomplementary

Sec.3,3l

lineswhich havebeenshiftedfrom their measuredlaboratorywavelengths. This can be explainedastheopticalequivalentof the Dopplereffect.we areall familiarwith theway in whichbellsandsirenschangepitchastheypassus.Insteadofthinking ofcarsandtrains which emit sound,let us transpose the ideato starshipsandlight rays. Imaginetwo starshipsheadingout into deepspace.Initially, they are both travelling with the samevelocity,althoughstarshipA is in front of starshipB. Atop eachof the spacecraft is a flashinglight.Bothregularlypulseon andoffwith a frequency,f. someone on starshipB canseeboththe flashinglight on his ship andthe oneon starshipA. Ifthis personmeasures the frequencyof both lights it will be found that they arethe same.If starshipA suddenlyincreases to anothervelocityandthe frequencies ofthe two flashing

#

time = tnr

-+

3.3 THE DOPPLER EFFECT The galaxiesin the night sky appearstatic until they are studiedspectoscopically.AstronomerVestoSlipherdiscoveredthatthe spectaof galaxiesdisplayspectal absorption

c

An observer heresees a redshift

le l +€

Ad

#

I J

_->

a J

ti me = tB2

IB I@

c



i i

f L t6mper6tu6: the Curie curi6temperature:Magnet belowth€ Magn€tbelow

r I I

,, .

later,at about l0-roscconds. Wlren tlre grand unifictl syllrnetry spontancou:;ty breaksbecar.rse the ternperaturehas

i{lN x\rr rr :;:T*lt"ltrt:*fi'"T::,Tx"?:i:',"f#,';""";r*jj#lilTr"il: i /irlil>9i*SN--Z:.tl

- .

{:::'i:'I?

. :'ii;.i1'.i..1

.' i '.'...t.: j.. ;. '.--.^.]:j. .:'. 3 x 10-25c m

t'. .i:t.';'t t

.:

a

(4.31) a

lf this is substitutedinto the Friedmannequation(4.29)it can be shown that the rate of universalexpansionis proportionalto the scalefactor rnultipliedby the squareroot ofthe dcnsity,sincethe densityitselfis proportionalto the inversecube ofthe scalefactor

a

Today

n (t)* n %

(4.32)

ln thc inllationaryepochthe densityremainsconstantbecausethe energygiven out by the phasetransitioncan be convertcdinto nrafterby (1.2). This affectsthe expansionrate of the Univcrseand it becornes n (t)ccR

(4.33)

Ifthese two equationsare plottedon a graph it can be easilyseenthat the constantdensity ofthe inflationaryepochlcadsto an exponentialincreasein the sizeofthe Universe,i.e. it undergocsa period in which it continuallydoublesits size in a constanttime (seeFig.

-iegion Fig-4'g.Non.inflationaryandinflationaryexpansion.(a)lnnon.inflationarycosmology'today's about lmm acrossat l0-r5 seconds.Even.though observabteUniverse.,.p;;;1';;;;; (b) trorirondistanceof the universeat that time. i"rg.ittr"-nirl" this is very small it lr rtil ,ui to becomemuch period inflation of th€ during In an inflarionaryun,".rr.,-#..ir'.*pr"o.o larger the Universe-is-much i.tg"t iftt" tttl n6riron aisii#e ofthe Universe As a consequence' oJ the IJniverse,w.H. Freeman' rhan we can ousene. lniap-iJ i.. sitt , !., A ShortHistory t994.)

4.9),A tt hest ar t of t heinf lat ionar yepoch't hechar act er ist ict im einwhicht heUniver s Eventhoughinflationendedwhenthe Universe doublesits sizewasaboutl0-!aseconds. it hadtime to inflateby a reachedan approximateageof l0-32secondsit still meantthat

[' tltt

'l'hc llig llang

O u r t i n y r e g io n h a s a r a d i u s o f cu r va tu r e wh ich i s s o l a r g e it is vir tu a lly i n d i s t i ng u ish a b le fr o m t h e f l a t ca se .

T h e U n ive r se after inflation ------+-

Scc.4.9l

Inflationarycosmology89

r4 secondsthe Universe'shorizon was l0-34light seconds.If we were to extrapoAt l0 latebackwardsfrom our observedlrorizonusing (4.30) we would find that althoughsmall, horizon the size of the Universeat l0-14secondswould still be about l5 centimetres.The in thermal be to the Universe expect we can which upon scale gives the maximum distance these equilibririmand possessthe samephysicalconditions.Beyond the horizon distances, very ditf' be could and to communicate time no had disparateregioni of the cosmoshave grew exponenthe Universe within point every inflation, During each other. from erent the tially larger by a factor of 105n.Inflation then stoppedat l0-32seconds,by which time l016 to seconds light l0r4 from in size increased bubble ofthermal equilibrium had been with an light seconds(3x106 light years).This enorrnousexpansionhlled our Universe energy which was all at the sametemperature,and it solves our horizon problem. ln aaaitionto theseproblems,we mentionedearlierthat the GUT phasetransitioncould and lead to topological deiects in the spacetimecontinuum such as magneticmonopoles despite yet found, to be have defects these by theory, predicted Although cosmic st;ings. the fact that they have well-definedobservationalcharacteristics.Inflation provides a reason for theseobjectsnot to be found, in that, as the Universeinflates,most of thesenewly createdobjectswill be pushedfar beyond our cosmichorizon' Finally, the energy releasedby the inflation signifiesa fundamentalchangein the vacuum of spaceand allows matterto form. In the coming chapterswe shall refer.to inflation severaltimes and indicatehow it hashelpedshapeour ideas,especiallythoseabout galaxy formation and the cosmic microwave background.

t t t t t t t i

Fig.4 l 0.'lhellatrtess problcnr issolvcdbvinUation.rvhichpostulates thatourUnivcrse isa tiny regiorr onthcsurlacc ol'arnuchlargcr curvcd surl'ace. f'actorof some 1050 tinrcs!This rvouldsolveour lla{ncssproblenlbecausel'orall but thc mostextl'emegeometriesof spacetimc,the stretchingol'inflation would flattenthe surface (seeFig. . I 0). By vastlyinllatingtiny areasof the tJniversethe horizonproblemcan also be solved.

t t t t L

l'll

lt

'lI 'll til ']l

lI ll

;rl ,,1

The cosmologicaldistanceladder In previouschapters we havediscussed theway in whichthe Universehasbeenobservcrl to be expanding. This meansthatthescaleof the Universemustbe an importantthingt1. Fronrthe sirnplcfact thatrvc cxisl in an cxPandittgUnivcrscrvhiclrcorrtains gravitationally-built constructions, we cattcslinratctltatottr [.]ltivcrsoliessorrrervhr:rc in the region0. l0 andk=0. We havechosento featureit because thesearethc p:' because but it is alsohighlyinstructive simplestcasemathematically;

l l l l l l l l l l l l l l

''x

l{t4

I ' h e fa te o fth c lln ive r se

fch e

Cosmological models

S e c . 9.6 l

I tl5

etersof an inflationaryI Inivclsc.It is kno',vnas the Einstein-deSitterrnodeland, once again,sonresimplificationof tlrc slarrdard lrriedrnann equationis required.-l'histime, becausewe are dealingwith a non-zerOdensityof nratteran actualvalue must be substituted. We statcdseveraltintcs in chapter4 that the densityof matteralterswith the inversecube of the scale factor. l'his allows us to constructan equationfor the density p, of the Universeat any time, frorn its value at the presentepoch,po,and the ratio ofthe presentscale factor Rnto the scalefactor at any time. R:

(n"\' 0 = oo[-*J

(e.| 4)

When this and the energyconstantis substitutedinto the Friedmannequationwe obtain

oz 8nGpoRus 3R

(e.l 5) +-zlgT

From this we can seewhere equation(4.32) camefiom becausewe can define a constant: ^

SnG poRs l

present day

(e.16)

J

and then statethe proportionality:

!)'* 1 R,=rg \ dt l R

T (e.r7)

which is equivalentto (4.32). (9. I 7) can then be arrangedinto the integral:

()f period of time for our Universe to have been in existence, based upon the age estimates

Rt

lR'dRc

(e.| 8)

ldt

JJ

00

which evaluatesto (4.30) and providesus with the time dependencyof the scalefactor:

(e.re)

R = at'l'

l'his time the scale factor of the Universeis agnin dependenton a consfant,a, but this is not the speedof light, and the scale factor altersrvith time becauseof the term involving t. Differentiationgives the rateof clrangeof the scalefactor with time:

"'l' n = 3u1

(9.20)

J

The agc ofan Einsteintlc Sittcrtlnivcrseis lhclefioregivenby 3 itt

t/'

3

1,n1,i, z,

modelshowshow a 'flat' Universeexpandsbecausc Fig.9.3,'l'heEinstein-desittercosmological it containsboth matterand radiation.ln this casegravity graduallyslowsthe expansion.and so of the ageofthe Universe. the Hubbletime is an overestimate

iome stars. The members of some globular clusters, for instance, are approximately l0 bitlion years old. We shall return to this dilemma when we consider the cosmological constant. Open and closedUniverseswhich containmatterare more complicatedto handleusinl' the Friedmann equation, but they too can be reduced to diagrams showing the way irr which the scale factor changeswith time (see Fig. 9.4). The fact that the rate of changeof the scalefactor, given in (9.20), is a function of tirrrl meansthat it, too, changeswith time. In other words, the universal expansioncan eitltcr accelerateor decelerate. We know, from our consideration of the fundamental forccs ol nature in chapter l, that gravity is the shaping force of the Universe on the largest scalc'; and so we would expect a decelerationrather than an acceleration.If we diffcrcntiirt' equation(9. I 5), the decelerationequationis

*l(

(e . r) 2

which statcsthat thc llubblc tirrrcis an over-estirnate of the ageby a factorof two thirds (see Fig.9.3). T'ltisrncirnsllral, in nn llinstein--0.5.

fll-

Mi l ne model f)-n

K =-l

9.7 GENERAL RELATIVITY

Open Q0 p r e se n l oaY

t

Fig.9.4.open andcloscdmodclsofthe universe