天才一秒记住【爱看小说】地址：https://www.akxss.com
ThebadradiationsfurtherinformatioI’vejustdeseofthisI’lldescribelateron，asitismoreaprospectforthefutureratherthahathasalreadybee’sworthmentioninghere，however，thatastheradiatiohroughtheUheprimordialfireballtoourtelescopes，itpicksupalotofinformationaboutthegravitationalfieldsofobjebetweehishappensisthroughthebendingoflight，asdisChapter2.Thebadradiatioiontothisphenomenon，andasitpassesbymassiveobjects，itstrajectoryisbeatiohisdistortsthepatternofripplesinacalculableway.Itgespleslooklike，awasobservedbyPlanotherefectthatbeseeninthebadradiatioheevolutionofgravitatioheUniverseexpands.Ifthishappeoersagravitatiohagivenamplitudeditselfleavihadiferentamplitude.Thedifereudesgives（ortakesaway）energyfromthephotobservationsofthisefecttothetheoreticalpredisgivesfurtherevideheexpansionoftheUniverseisspeedingup.
Theexpansionhistory
&heUniversebecameoughth，thereeriodthatastrooasthedarkages.Thiseriodaftertheinitialfireball，butbeforethefirststarsandgalaxiesformed.ThereisverylittleforastroolthispartoftheUory，asmostofthematterwasincloudsofgas.Afteraboutafewhundredmillionyears，however，thefirststarsandgalaxieshadbeguntoforSiureshavegrownuously，anderscales，astheUniversehasevolved.ravitythathascausedthistohappen，andmuatioaigravitybylookironomicalstructuresaroundus.Forthemomentthough，let’sthinkabouthohysicalbodiesbeusedtoprobetheexpansionhistoryoftheUniverse.
Hubblestartedthisfield，withhisfamouspaperin1929.Aswithmostgreatstificresults，hisworkwasbuiltupohegehatfollowed.Theaimofallthisworkhasbeeermiadistaismovingawayfromus，aswellasexactlyhowfarawayfromusthatobjectis.ThisinformationbeusedtodetermitheUniverseisexpanding.Theformerofthesetwoproblemsisactuallyrelativelystraightfhtfromstars，andmostotherbodies，isemittedinspecificiesthatdtothechemitsthatit’smadefroNow，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.
本章未完，请点击下一章继续阅读！若浏览器显示没有新章节了，请尝试点击右上角↗️或右下角↘️的菜单，退出阅读模式即可，谢谢！