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Chapter5ology
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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.
BytheexpansionoftheUhereforehaveaoobservethecesofthegravitatioiohiswaywedaionsaboutgravitythatotbeeasilyprobedbyexperimentsintheSolarSysteTheseiionssuchas: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.
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