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Chapter 5 Cosmology（第1页）

Chapter5ology

ologybeganasastificdiseatthebeginnihtury，withtheworkofAlbertEinsteinandEdwiwasEiprovidedthetheoryaspossibletosensiblysidertheentireUwasHubblethatprovidedsomeofthefirstobservationsthatshowedtheUniverseahe20theitherofthesethingshadbeenpossible，andologyhadremaiexclusivelywithintheprovinceionandphilosophy。Sihasflourishedasasditistlyintheproingapresce。

&ioionisfuhestudyofology，asgravitydomiherforlargedistauisnotpossibletosisteheUhheravityalohisisbeewtohisinversesquarelawofgravityisapplicabletintheUhatitistraahismeansthat，agtohegravitationalfieldweexperiehshouldbeasumofthegravitationalfieldsofeveryobjectthatexistsiireUhisisn’tnecessarilyaproblembyitself，butitdoesbeaticifyoutrytoaddupthegravitationalfieldsofinfinitelymahiscaseitturnsoutthatheorytellsusthatthetotalgravitatioaintheUniversedependsontheorderihegravitationalfieldsofalloftheseobjects。Thisisobviouslyisfactorysituation。

Ofcourse，thatheoryisonlyanapproximationofthemorepletetheorydevelopedbyEihankfully，theproblemdescribedabovedoesnotoEiheory。Iariodelsthatareself-t，andthatbeusedtomodeltheUniverseweseearoundus。Infact，becauseofEinstein’sfospadtime，wegetamuchdeeperuandingoftheUhroughhistheorythanweeverwouldhavegotfromhisisbecause，usiheory，wearenotoomodelhowobjetheUniversemove，withrespecttoeachother，wearealsoabletocreateamodelofhowthespadtimethatmakeuptheUusnowsiderthisiail。

Themodernhistoryofology

Modernology，aswe，beganinRussiaintheearly1920swiththeworkofAlexanderFriedmann。UsilypublishedGeheoryofRelativity，Friedmaauwasthesameatallpointsihatlookedthesameiions，shouldbeexpectedtoeitherexpandortract。Thisremarkableprediusthaveeasquiteasurprise，becauseatthistimenoastronomerhadbeeoreachsuusiioheless，Friedmanoproduceasetofequationsthatsuiversewouldhavetoobey，andhadevehatthegeometryofspathesemodelscouldeitherbeflatorhavepativecurvature。Thatis，hewasawarethatthereexistedsolutioeiionsinwhietryofspacecouldbecurved，likethesurfaceofagiantthree-dimensiohesurfaceofsaddle（seeimagesinFigure10）。

10。Examplesofspaceswithtpositivecurvature（k〉0），vanishingcurvature（k=0），aivecurvature（k〈0），respectively。

Friedmanniohisidelyreizedatfirst。HewasinitiallycriticizedbyEihoughthewasieirodualternativemodeloftheUniverse，whichheforcedtobestatitroduewtermiioheologistant。ThismodelwasshowablebytheBelgiaAbb’GeesLema’tre，ingsimilarideastoFriedmanntowardstheendofthe1920s。Ire，whohadbeenacolleagueofSirArthurEddington，hadwritteniifi1927thatobservatiotheUniversewasindeedexpanding，inwhatlaterbeasHubble’slaw。TheartitainingthismoaldiscoveryublishedinFrenianjournal。StrawastraoEnglish，in1931，theseHubble’slawwasmissiheless，Lema’treisrememberedtodayasoimpuresiofmodernology。

Friedmarewerebothmathematidalthoughthelatterhadagoodkronomy，itwasnoturonomerEdwinHubblepublishedhisfamousresultsin1929thatotstartedasaionalsce。HubbleshowedtheworldthattheUniverseahisbygthedistaweoasgalaxies，andbyusingknowninformationabouttheirmotiohattherealvelocityofagalaxyisproportionaltoitsdistanus（sothat，e。g。，ifgalaxyAistwiceasfarawayasgalaxyBthenitshouldrecedeattwicethevelocity）。ThisisjustwhatLema?trehadprediEiheory，anditprovedbeyondreasohattheUniversewasindeedexpaeiheideaofastatiiverse，aheologistablunder’ofhislife。

TheexpansionoftheUperhapsseemlikequiteadiferehoseweusuallythinkofasbeiogravity，butitishelarge-ssionoftheUimatelyectedwithgravity。InaveryrealsehinkoftheexpansionthatFriedmare，andHubblediscoveredasbeialaxiesfallingawayfromeachotheruualgravitatioion。Asamoredomesticexampleofthesamephenomenohrowingatennisballdirectlyupwardsintothesky。NormallythetennisballwillreachamaximumdistahesurfaceoftheEarth，beforeitstartsfallingba。Intheperiodbeforethis，however，whentheballistravellingupwards，itisstillbeingabygravity，anditisbyusiionsthatgravitatiowecalculatethepropertiesofitsmotion，suchashowfastitwillbemovingataimeiure。Twalaxiesareverysimilartothis。Thegalaxiesmaybemovingapart，buttherateatwhichtheymove，atheywillfallbacktowardseachother，isdictatedbythegravitatioweeein’stheorysimplyallowsustostrusistentpictureofaireUniversefilledwithobjectsthatare’yingawayfromeachother。

&ennisballanalogyraisesanobviousquestion。Ifgalaxiesare’yingawayfromeachother，likethetennisball’iesupwardsfromthesurfaceoftheEarthit，thehismeanthegalaxieswilleventuallystop’yingapart，andstartfallingbacktowardseachother？Orinotherwords，couldtheUuallystopexpandingandstarttore-collapse？Thisisaperfectlygoodquestion，andtheanswerbegivenbygthetennisball。Ifihrowiennisballupwardswelaunchitathighspeed，fromsomesuper-powered，thenit’spossibleitmightneverebatoEarth。Stistscallthespeedrequiredtomakethishappentheescapevelodit’sveryeasytocalculate。IfthetennisballislauaspeedgreaterthantheescapeveloitwilloEarth。Ifit’slauhaspeedlessthantheescapevelocity，itwilleventuallyfallba。Thesituationwithgalaxiesisverysimilar。Iftheyareregawayfromeachotherwithsuitvelotheyshouldbeexpectedto’yawayfromeachotherforever，andtheUheoexpaherateofreistoolow，thenthegalaxieswilleventuallyfallbacktowardseachother，andtheUarttocollapse。TherateofrebetweengalaxiesisknownastheHubblerate，ayrequiredtomakethem’yawayfromeachotherforeverisknowicalrate。Theorydoesn’ttellusifourUniverseisexpandingaboveorbelowthecriticalrate。Todiscoverthis，ointourtelescopesoutintospadobserve。

BytheexpansionoftheUhereforehaveaoobservethecesofthegravitatioiohiswaywedaionsaboutgravitythatotbeeasilyprobedbyexperimentsintheSolarSystem。Theseiionssuchas:HasgravityalwayshadthesamestrehaveitsowngravitatiohewaythatEiheorypredidenstogravitywheyofmatterbeesverylarge？Thereasonwhyweswerthesequestionsinologyisbecauseelengthsvolved;becauseofthefactthattheUniverseisexpanding;ahefinitespeedoflight。Let’sthinkabouthowthisworks。

Inmostsituationsinlifewe’reusedtotheideathatweseeening，asithappehisisjustappearsthatway。Becauselighthasafixedveloilliorespersed），ittakessometimefhtemittedorre’eobjecttoactuallyreachoureyes。Thespeedoflightisveryhigh，sowedon’tusuallyourselveswiththisdelay。Wheisveryfaraway，however，thedelayesignifit。IftheSunsuddenlyexploded，forexample，itwouldtakemorethaminutesforustoknowanythingaboutit，becausethat’showlohelightemittedfromtheSuntoreadnothingmht）。AnotherwayofthinkingaboutthisisthatwhetheSuwasalittlehtmihesamethinghappensinology，buttheobservableUniverseisverymuchlargerthaaheEarthtotheSubeeshuge。Forexample，ittakesmorethanfhtfromthearstoreadsometensofthousandshtfromthegalaxies。Ifwelookevenfurtherawayweseeobjectsastheywerebillionso。Inasense，weseebatimebylookingfaraway，andifwelhawayweseewhattheUniverselookedlikewhenitwasveryyoung。

Now，it’sawell-khermodynamicsthatwhenyoupressa（likeaballoonfulletshotter。Likewise，ifyoumakethesameobjedthescooler。TheUniverseisothisrule。IfwethinkoftheexpandingUniverseasplayingohehereelbackwardsweshouldexpecttoseetheUiter，untilatveryearlytimesitburstsinto’ames。wefactseetheearlystagesoftheUion，andyoumightexpectthatweshouldseeafireballifwelhaway（andhence，lhbatime）。ThispossibilityredictedbyRalphAlpheraHermae1940s，butitwasn’tuntil1965thatitwasatallyobservedbytheradioastronomersArnoPenziasaWilsoheydeteowknownastheCMB，ortheicrowaveBad。

ThedisBedtotheworldthatastronomycouldbeusedtoseetheveryearlystagesoftheUiowasiirelydiferentstateofbeiimeitopeotestinggravityinwildlynewes，wherethegravitationalfieldoflightcouldbestrohatofnormalmatter，andwherewesidertimeanddistaraireobservableUniverse。

TheearlyUniverse

&heearlydaysofthe1960s，ologyhasblossomediudiedfieldofbothobservationalaicalphysics。Thepositionsofhuhousandsofgalaxieshavebeenmapped，we’veseenastrophysitsthatocybillionso，aPenziasandWilsondiscoveredhasbeeoincredibleaccuracy。Theseobservations，andothers，havebeeogivepreswerstoquestionssuchas’HowoldistheUniverse？’，’WilltheUniverseexpandforever？’，and’esofmatterexistintheUheahesequestiopuzzling，buthaveprofoundcesforofgravity。Wewillsiderthemiion。

&atthebeginniheUniversewassmallerahepast，thenifwesiderearlieraimes，theyofmattershouldbeexpeelargerandlarger。Now，itturnsoutthatnotalltypesofmatterindensityatthesamerate，aswegobatimeinthisway。Thedensityoflight（orradiation，asit’softeobyphysicreasesatafasterratethayofmostothertypesofmatter。Thismeansthatatveryearlytimesthedensityofradiationbeevehedensityoftheeleeutrons，andprotonsthatmakeupnormalmatter。InthiscasethegravitationalfieldofradiatiohedomiheexpansionoftheUniverse。

&ioageoftheUioivelybrief。ItonlylastsforthefirstfewtensofthousaertheBigBaheless，itisaerestiime，particularlyforthestudyofgravity。Ohephysicalprocessesthattakesplagtheradiation-domihesyhelightelements（hydrogehium，etanyfactorsthatihisproeofthemostimportantistheexpaheUailedcals，aiosenandheliumweseeintheUniversearoundus，allowustoplacetighttsoionalfieldproducedbyradiationintheveryearlyUheresultsofsuchstudiesaretwiththepredisofEiheory，withuiesatthelevelofonlyafewpert。ThisislesspreobservationsofgravityintheSolarSystemorinbinarypulsarsystems，butit’snotbadgitiingenedbillionso。

&hesynthesisoflightelemehereareotheriingphysicalprocessesthattakeplagtheearlystagesoftheUory。OheprocessthateveheformatioastrophysicalstrutheU’sbeenkhefindingsofPenziasandWilsonthattheearlyUniverselooksveryclosetobeilysmooth。Veryclose，thatis，butlyso。TherearesmallripplesintheCMBradiatioronomersfound，andtheseripplesarethoughttobetheseedsofwhateventuallyturheplexwalaxiesandclustersofgalaxiesthatweseearoundustoday。Itisgravitythatohecollapseofthesesmallripplesintogalaxies，butbeforethisravitystillplayedacrucialrole。

IntheearlyUherewasabattlebetweengravity，ontheonehand，whidstomakematterclumptogether，andradiatioher，whiteractswiththematter，andcauseittospreadout。Anysmall’usthatexistiingsoupofmatterandradiatioarttooscillate，asthey’repulledtravity，abytheradiatioheseossdependontheirsizeiareeasytocalculate。This’uiyofmatteruesuntiltheUoasuitdegreethatitbeestraveryearlytimesit’sopaque，likeafireball，asdiscussedbefore）。Atthisstagetheradiatiohematter，aelescopesofdistantobserversbillioer，withverylittleiioPenziasandWilsondiscoveredismadeupofexactlythistypeofradiatiohanthirteenbillioerthefireballesofthewarbetweengravityandradiatioedastiheCMB。TheseripplesalotofinformationabouttherateoftheUniverse’sexpaofradiationintheUhewaysinwhichradiatioypesofmatteriheyalsoformationaboutthespacethroughwhichtheradiationhastravelled，beforeweobserveitoh。Inshort，theCMBisastifictreasurechest。

&ailedobservationsoftheCMBfirstbeganwiththelaunASA’sicBadExplorer（1989。Thissatelliteexperimehebadradiatioiresky，aheradiatioheformoifitwasemittedfromtheprimordialfireball。TheCOBEexperimentalsomadethefirstattemptatthesmallrippleswejustdistheehavetheresolutioraformationfromtheseripples，butitmadeapromisingstart。Sihen，aseriesofballooshavebeenperformed。AmoheBandMAXIMAexperiments，whichwerelauhelate1990s。Thedetetheseexperimentshadsuitresolutiohelargestoftheripples，andthiswasenoughinformatiohattheUremelyclosetothe’critical’valueofexpansihtattheborderliweenre-collapseaernalexpansion。ForthistobepatiblewiththerateofexpansionweseearoundusintheUoday，however，itlookedlikesomethiohavehappehinghadtohavespeduptheUniverse’sexpahetimeofthefireballandnow’andbysomesiderabledegree。

Badradiatioookanotherleapforwardatthebeginnitury。In2001，NASAlauheWilkinsonMiisotropyProbe（）iheerimentwasabletoseenotooftheripples，butsomeofthesmallerohiswastremendouslyimportant，asitallowedtheprovolvihoftheseripplesintheearlyUobemadethesubjectofobservationalsy。Thiswasfollowed，in2009，bythelaunchofthePlaheEuropeanSpackbeyond，andallowedmaheripplestobemeasured。TheresultsoflanckwereagloriousatioicalphysicsthathadbeeouhegrowthoftheripplesintheearlyUheyshowedthatthecollapseduetogravityexpeEiheorytookplacejustasexpedthattheamountofradiationintheearlyUniverseatiblewiththatrequiredbytheprimordialhesiscals。TheyalsofouhereappearedteamountofmatterintheUdidhradiationinanyway，otherthanthroughitsgravitatiohisisn’thownormalmatterbehaves。

ThebadradiationsfurtherinformatioI’vejustdeseofthisI’lldescribelateron，asitismoreaprospectforthefutureratherthahathasalreadybee’sworthmentioninghere，however，thatastheradiatiohroughtheUheprimordialfireballtoourtelescopes，itpicksupalotofinformationaboutthegravitationalfieldsofobjebetweehishappensisthroughthebendingoflight，asdisChapter2。Thebadradiatioiontothisphenomenon，andasitpassesbymassiveobjects，itstrajectoryisbeatiohisdistortsthepatternofripplesinacalculableway。Itgespleslooklike，awasobservedbyPlanotherefectthatbeseeninthebadradiatioheevolutionofgravitatioheUniverseexpands。Ifthishappeoersagravitatiohagivenamplitudeditselfleavihadiferentamplitude。Thedifereudesgives（ortakesaway）energyfromthephotobservationsofthisefecttothetheoreticalpredisgivesfurtherevideheexpansionoftheUniverseisspeedingup。