千米小说网

Chapter 5 Cosmology（第2页）

Theexpansionhistory

&heUniversebecameoughth，thereeriodthatastrooasthedarkages。Thiseriodaftertheinitialfireball，butbeforethefirststarsandgalaxiesformed。ThereisverylittleforastroolthispartoftheUory，asmostofthematterwasincloudsofgas。Afteraboutafewhundredmillionyears，however，thefirststarsandgalaxieshadbeguntoform。Siureshavegrownuously，anderscales，astheUniversehasevolved。ravitythathascausedthistohappen，andmuatioaigravitybylookironomicalstructuresaroundus。Forthemomentthough，let’sthinkabouthohysicalbodiesbeusedtoprobetheexpansionhistoryoftheUniverse。

Hubblestartedthisfield，withhisfamouspaperin1929。Aswithmostgreatstificresults，hisworkwasbuiltupohegehatfollowed。Theaimofallthisworkhasbeeermiadistaismovingawayfromus，aswellasexactlyhowfarawayfromusthatobjectis。ThisinformationbeusedtodetermitheUniverseisexpanding。Theformerofthesetwoproblemsisactuallyrelativelystraightfhtfromstars，andmostotherbodies，isemittedinspecificiesthatdtothechemitsthatit’smadefrom。Now，whenabodyisinmotion，asmostastrophysicalobjedtobe，thenthefrequehatreachesusisshiftedbytheDopplerEfect。Thisphehesameastheshiftinfrequencyyouhearwhenanambulayou;thesousseemshigherpitit’stravellingtowardsyoutha’smovingaway。Inbothcases，thefrequeforwardlyrelatedtothevelocityofthebodyinmotioifweknowthechemitsinanastrophysicalbody（whichweofte’srelativelyeasytoworkouthowfastthatbodyismovingawayfromus。

Accuratelydetermiarophysicalbodiesis，however，amoregtask。Thegehodologythat’susedforthisjobistolookatobjectsthatarereasonablynearby。Ifwedetermiahesenearbyobjects，whichisusuallyabiteasier，thenweusethemtocalibratethedistaosimilarobjectsthatarefurtheraleofthismethod，let’ssidertheCepheidsthatHubbleusedinhisfamouspaper。ACepheidisastarwhhtnessvariesinaperiodicway。Itwasalreadyknownthatthereisarelatioheperiodofaditsluminosity（itsactualbrightness，asopposedtoitsapparentbrightness，whidsondistanus）。Thiswasdeterminedfromarswhosedistanceswereknowhisinformationtoworkoutthedistaofaraheids。Thelogicisquitestraightforward:youlookatthedmeasureitsperiod;youusethisinformationtoworkouthowmuchlightit’semitting;andyouparethisthttheobjectappearsraphicplate。There’sasimplelawthattellsyhtaadistanceshouldbewheiainamountoflight，soyouhavealltheinformatioermiance。

Uhereareahingsthatgwiththismethod。Therulesusedtodetermiaoa（suchastherelatioweenperihtnessinCepheids）mightonlybeapproximatelytrue。Youalsohavetoassumethatthedistasobservedarethesameasthenearbyobjectsthatwereusedtodetermihisisn’talwaystrue，asitbehardtoideswhentheyareverydistamightbethatsomerulese（recallthatwhenyoulookfaraway，yatobjectsastheywerelongago）。Theseproblems，aobecarefullysidered，astheyesleadtoiinferenhis1929paper，forexample，HubbleinferredanexpaheU’sapproximatelytentimeslargerthanallmoderhiserrorwasduetoilyihedistaogalaxiesusingtheCepheids。

&stateoftheartinthisfieldisachievedusiionsofsupernovae（explodingstars），buttheunderlyihodologyisstillquitesimilartotheoneusedbyHubble。Anindividualsupernovabeasbrightasairegalaxy，sotheyarereasoospot，ifyouknowwhattolookfor。Theyalsobeseenfromveryfaraway。Now，therearediferenternova，and，ofcourse，astrivehemall。ThetypeofsuperaremostusefulftheexpansionoftheUniverseareknoeIa。Theseexplosionsarecausedbytheaatterontoawhitedwarffromaar。Wheterhasaccumulated，thewhitedwarfgerholditselfupagainstthepressureofgravity，anditcollapsesahegoodthingaboutTypeIasupertheytendtohappeninaverysimilarway，whereverorwheheyoccur。Thismeansthat，iftheybeproperlyideheirbrightnessbeusedtogetquiteagoodestimateoftheirdistance。

Itwasn’tue1990sthatthefirstresultsontheexpansionhistoryoftheUypeIasupere。BoththeSupernovaologyProjedtheHigh-ZSupereamwerewoheypublishedtheirfirstresultsatarouime。Usiiohatwereatvastdistandhehadexplodedseveralbillionso，theyfouhingverysurprisiermiheexpansionoftheUslowingdown，asoforobjectsthatfallawayfromeachotheruualgravitationalattra，buteedingup。Thiswasentirelyushockedthephysiunity。Intermsofofgravity，however，it’sespeciallyfasg。We’llgointoitscesfurtheriforurnte-scalestructureoftheUniverse。

&euniverse

Justasstarsgrouptogethertalaxies，galaxiesgrouptogetherintostructurescalledclustersaers。Thesearewhatologistsarereferrialkabe-scalestructure。Thestudye-scalestructureoftheUniverse，on，wasstartedbyHubble。ItwasHubblewhorealizedthatthespiral-shapedobjectsthatastronomersobservedthroughtheirtelescopeswere，infatgalaxies。UpuntilthispointithadbeeionastowhetheralaxywastheoexistedintheUniverse，likeanislaeos。Usihatwediscussedearlier，Hubbleshoiralsweremuchmoredistantthaarsweseearoundus。Theoionwasthattheywerelargerbodies，themselvesmadeupfromverymanystars。Thisstartedthequesttomapthestructuresthatexistaroundus。

Aswithmostbranchesofobservationalressinthisnewfieldwasinitiallyratherslow，aopickuppacetowardstheehtury。OhelandmarkmissionsiheHarvard-SmithsonianCfAsurvey，whiin1977andranuntil1995。TheCfAsurveymeasuredtherealveloost20，000galaxies，ahepositiohemonthesky。UsiheythehesevelotodistaartedtomapoutthestructurethatexistedintheUniverseescales。TheydiscalaxiesclumptogethertoformstructuresthatspanenormousdistanceseofthemostimpressiveoftheseiswhatisknownastheCfA2GreatWall。Thisstructureisahugetrationofgalaxies，whichisseitwouldtakelightmorethanhalfabillietfromoheother。

&galaxysurveyshavediscoveredevenlargernumbersofgalaxies。The2dFsurvey，whichusedtheAnglo-AustraliaTelesewSouthWales，ranfrom1997to2002，ahan200，000galaxies。TheSloaalSkySurvey（SDSS），whichstartedintheyear2000andisplaiil2020，hassofarmeasuredmillions。Infact，therearenowsomanyimagesofgalaxies（arophysicalbodies）thatitisimpossibleforastrhallofthemindividually。puterprogramsbeusedforthis，buttheyteregimportahehumaneye（andbrain）。Acleverwayaroundthisproblemwasthereforetoputtheimagesothepublictakepartiifyingthem’aprojeas’GalaxyZoo’。

Maureswerefoundby2dFandSDSS，andonevenlargerstheCfAsurvey。ThebiggestofthesewastheSloaWall，whichisaroundtwiceasbigastheCfA2GreatWall。Infact，theSloaWallissobigthatifyoutooksimilarsizedstrudputthemendtoend，youlyfitafewdozeireobservableUrulyenormous，butoneshouldbearinmindthatthisisstillonlyafraofthedistanceprobedbysuperheCMB。Therearemanymalaxiestobediscovered，aobeseenifthereareanystructuresthatareeveheexpeisthatthereareioalwaysrealized）。

Thisisallveryimpressive，butlet’sreturntowhatitmeaudyofgravity。Thestructuresthatareobservedinthesesurveysaretheresultofgravitationalattra。AtveryearlytimestheUniverselookedsmooth，asverifiedbyobservationsoftheetfromthatstatetothepresentdaysituatiohereexistsvastworksofstructure，thematterintheUhaveclumpedtogether。Thewayinwhichthisshouldhappenisthoughttobewelluescales，butstartstogetabitmorepliallscales。Bothimesawealthofinformationforthosewhoareiedingravity，solet’ssiderthemseparately。

escalesthegrowthofstructurehappensiableway。Thisisessehelarge-scalebulkmotioerintheUniverseissmallonthesesparedtotheologisiohofstrulargescalesis，however，verysehepreciserateofologision。Iftheexpansionisdominatedbynormalmatter，thenstructuresgroensonsmallerscalesfirst，anderscaleslateron。Now，becausehattheseedsofstructurelooklike，fromtheCMB，wecalculateectthestrulargescalestolooklike，andarethistowhatastrouallysee。Theresultsareveryiing。

First，theresultsstrulargesdicatesquitestronglythatthereismatterintheUdoeshlight。Thereasohisisbecausethereismorestruleherewouldbeifthiswereis，ifallmatterihlight，thenthehighlevelsofradiationintheearlyUniverseshouldhavesuppressedtheseedsofstruapredictableesee，however，isthelevelofstructurethatoneshouldexpectifradiationhadhelogiisthatthereexistsmatterintheUdoeshradiation，andthatitwasthegravitatiohismatterthatservedastheseedsofthestructuresthatweseearoundustoday。Whatismore，bylookingathowmuchstructureexiststhscales，wevaluableinformationabouthowgravityworksedistances。

&helarge-scalestrutheUniversebeusedasakiomeasurethesizeoftheUniverseandhowmuchithasexpahisisbecausetheinitialrippleshaveacharacteristigth。Byparingthescaleoftheseripplesinthebadradiatiohatothelarge-scalestructurearouhereforeseeiwayhowmuiversehasexpaheformeristhesourceofthelatter）。Thisleadstoyetanresult。TheUohaveexpashouldhavedosexpaedbythegravitatiohematterwithinit。Inotherwords，therulersieUobetoobig。

&’snowsiderensoancescales，muchsmallerthawallsdiscussedearlier。Ohevelocitiesofastrophysicalbodies，likestarsandgalaxies，arenotnecessarilysmallparedtotheologision。Theanalysisisthereforemuplicated，asthebodiesmoveainmuplicatedways。Thetbestmeththissituationisteputersimulationsofverymahespacethatthebodiesexistwithinisexpandieishould，butthegravitatiohebodieswithinthespaceareusuallytreatedihisisavastextrapolatiooitiswidelythoughtthatit’savalidroceed。Let’snowsiderhowgravitybeexploredinthisregime。

&andmostobviousthingtodohereistotrackthemotionofthegalaxies，andtheformerstructuresthattheycreate。Thisisverytricky，asit’sdiiculttotakeintoatalltheplicatedefectsthatthemanyastrophysicalprocessesthattakeplatheUniverse。Asupernova，forexample，crowthofstructure，orcloudsofgashaheless，orytomodelallsuena’andtherehasbeenalressindoingthissiurury。Whatseemsclear，asbefore，isthatthereappearsteamountofmatterthatwe’tseedirectly，butwhosegravitationalfieldisrequiredtomakegalaxiesmoveahewaytheydo。

Asedapproachistolookathowgalaxies，andclustersofgalaxies，behoflight。YouwillrecallthattheSuhepathhtthatpassesclosebyit，andthatthiswashowEddingtoheworldthatEiheorywascorrect。Thesameewithgalaxies。Welookathoesofdistantgalaxiesaredistortedbythegravitatiohosethataremuchclosertous’inaproasgravitatiohisefectisoftenverysmall，andit’susuallyanenormousgetoseeitatall。Ifwelookattherightgalaxies，oroughdata，however，thenweuseittodetermiioexistinspaore，wefindthatthereismravitythaheretobe，fromtheastrophysicalbodiesthatweseedirectly。Thereappearstobealotofmass，whidslightviaitsgravitatiohatdoeshthelightiheexatofbendingthatoccursalsopotentiallyeofinformationaboutthewaythatgravitybehavesonthescalesofgalaxies，andclustersofgalaxies。

Movingtoevensmallerscales，welookathowindividualgalaxiesbehave。Ithasbeenkhe1970sthattherateatwhichgalaxiesrotateistoohigh。WhatImeanisthatiftheravityinagalaxywasthevisiblematterwithinit（mas），thenanygalaxythatrotatedasfastasthoseweseearounduswouldtearitselfapart。Itwouldbeliketakingaheadofdandelionseedsandrotatingitsstemquicklybetweenyourhands。Ifyourotateitfastenoughthenyouwouldexpecttheseedstoflyof，asthebondsthatholdthemtogetherarenhtoresisttheforcesthatresultfromtherotation。Thesameistruewiththestarsiheydonot’yapart，despitetheirrapidrotatigeststhatthegravitatiohihaiallysuspected。Again，thelogiisthatthereappearstobematteriweotseebutwhitributestothegravitationalfield。

Theodel

MaphysicalprocessesthatotheUhesamesurprising:thegravitationalfieldsweinfer，bylookingattheUniversearouheretobemorematterthanweseewithourtelescopes。Beyondthis，ihelargeststrutheUohaveevolvedintotheirtstate，aheseedsofthesestructurestolookthewaytheydoihisbeallowedtoihlightatall（or，atmost，ionlyveryweakly）。Thismeansthatnotoseethismatter，butthatitotbeseenatallusinglight，becauselightisrequiredthtthroughit。

Thisisobviouslyaverystraeofafairs。Thesubstagravitatesinthiswaybutotbeseeoasdarkmatter。Theamountofdarkmatterrequiredtoexplaiionsisnotsmall。Thereobeapproximatelyfivetimesasmuchdarkmatterasthereisordipeople，whehearthishihingmusthavegoerriblywrong。Thatbethisstraheevideheexistenatterasourcesthatitishardtuewithit。Iftheevidenefromoneplace，youcouldtryahatwhoevercollectedthedata，ormadetheobservations，mighthavemadeamistake。It’sverydiiakethisargumentforallofthediferenttypesofobservatioomakesomanymistakes，andforallthosemistakestestthesameresult，seemshighlyunlikely。SoweareledtotheostofthematterintheUofthetypeearemostfamiliar，butisinsteadsomeerthatreviouslyunknown。

Butthisishesurprises。Notonlydowerequireextramattertogiveextragravitationalfieldsiomakestruandlightbeit’sobservedtodo，butlainwhytheUniverseisexpandihaitshould。RecallthatwekoftheexpansionoftheUniverseasobjeatter（suchasgalaxies）’yingawayfromeachotheruualgravitatioion。Ifthisistrue，andifgravityisalwaysattractive，thenweshouldexpectthelarge-ssionoftheUobealwaysdeg。Thatis，theexpansionshouldbegettiime。Theresultsofthevariousastronomicalobservationswehavediscussed，however，haveshoansionisspeedingup。Theisthattheremustbeatypeofgravitatioisrepulsive’inotherwords，thereseemstobeatypeofanti-gravityatelookathowtheUhisanti-gravityisrequiredioforcematterapart，ratherthanpullittogether，sothattheexpansionoftheUniverseaccelerate。Thisistrulyastonishing。Thesourceofthisrepulsivegravityisreferredtobystistsasdarketobefusedwithdarkmatter）。Thereobearouimesasmuergyasthereisdarkmatter，iomaketheUniverseaccelerateinitsexpansiorate。