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&acrossthestrahisresult，letmeillustrateitle.siderthatyou’reanastronaut，’oatingarouerspaewhere.Youseenothinganywherenearyou.Ifyouthenstartag，byholdingontoapassingspaceship，say，theyouthoughttyspaceburstsiicles.I’mexaggeratingalittlehere，ofcourse.Youwouldoaccelerateveryquideedtoseealargeievertheless，theprincipleissound.Wheeyoudetectparticleswherepreviouslytherewereroducegravityaionbeesevenmoreplicated.Gravityiscausedbyac，sobysittingatmydesk，iioheEarth，IambeioasmalliclesthatwouldhereifIwerefalliheoosmalltomeasure，butifImovedmydesksothatitwasnearablackhravityismuger）thenitwouldbeairelydiferentstory.Iwouldbebombardedwithhigh-eidradiation.
Allofthishasiingcesforblackholes，whioeratureassociatedwiththembasedoureofthepartidradiatio.Butitalsohascesforravitationalphysiology.IhegravitationalfieldofaodeloftheUniverseissimilartothegravitationalfieldofablackhole，andindeedGaryGibbonsandStephenHawkiheradiationthatblackholesproduceshouldalsobeproducedbytheexpansionoftheUhefastertheexpansion，themoreradiationthereshouldbe，andthehigheritstemperature.ThisradiatioedfromanythingwithintheUisaby-productoftheexpaisadiresequensideringquantumparticlesexistinginagravitationalfield.
iflation
Todate，themostsuccessfulapplitheravityhasprobablybeenintheveryearlystagesoftheUory.Thephysicistsusetodescribeenedduringthisperiodisi.InChapter5，wesideredtheBigBaheUsvarioussuccesses.Aslainingalotofastronomicaldata，hBasusroblems.OoftheseisthatsomeoftheripplesweseeintheCMBappeartobesethatlightadeitfromoheiredgestotheotherwithiheUhisisaveryseriousproblem，beothingtravelfasterthanlight.Soossiblyhavecausedtheseripples？
Theaobvious，butonepossibleexplanatioetheUhaveexpandedveryquiitsveryearlyhistory.Ifthishappehenverysmallrippleswouldhavebeeogrowinteoheproblemwouldbesolved.Thehypothesizedperiodofrapidexpansioniswhatisi.Now，asstists，thewaytotestahypothesisofthistypeistotryahercesthatitmighthave，aourtelescopestoseeifweverifythosepredis.Thisisdigin’atioklywhatcausedit.Italsohappeimeago.hereareanumberofgeionsonemake，andthatbeverifiedbythenightsky.Ohatthegeometryofspaceshouldbecloseto’at.Thismatchesobservations，aswehaveseen.Probablythemostimpressivepredi，however，iheapplii-classicalphysicsjustdescribed.
YouwillrecallthatGibbonsandHawkiedthatanexpandiesaseaofradiation.Itturnsoutthattheradiatioperfeateverypointiatistiatureofquantummeisthatrandom’usareihatatsomepoitlemoreradiation，asthereisalittleless.It’simpossibletopredictwhereahesequantummeical’uswilloccur，butthetheorydoesletuspredicthowoftenweshouldexpelyselettobeover-denseorualsotellsushowweshouldexpecttheover-denseandunder-deobedistributed，e.Theseareallpredii-classicalphysidwetestthetheorybylookingfortheirparticular，wetesttheideaofibylookingforthecesofthequantum’usthatitwouldproduce.
inChapter5wediscussedtheripplesthatexistiheseripplesareaveryimportantsouratiists，butsofareltoutwherethey.Thatis，wehaven’tsaidwhatcausedthesmallseedsfromwhichtheygrew.Theseseedsohaveaveryspeioexplaiicalpropertiesoftheripplesthatastronomersmeasureinthed，beforein’ationwasiherewasnoclearideaaboutwheretheyshouldhave.Ifin’ationreallydidhappenintheveryearlyUhenonewayofsowingtheseseedscouldhavebeenthroughthesmallquantummeical’usthatGibbonsandHawkied.ItturnsouttheripplesthatweremeasuredbyAP，andthePlanckSurveyorarejustwhatweshouldexpesuario.
Thiswasaremarkabledisotonlyhasthemostgeioionaryepoverified，butitalsoappearsthatwehaveverifiedthepeculiarcalsthatresultfromgquantummeicalproagravitatioypeofradiationthatHaredi1974stillhasn’tbeely，butitscesappearquiteplaiheevidehere，inthemapsoftheCMBthathavebeero，again，thisishestory;thereisverylikelymoreevidehere，waitied.ThesamequantummeicalprocessesthatgeeripplesintheCMBshouldalsobeexpectedtravitatioheseareexactlythegravitatiohetheBICEP2experimehoughtthattheyhaddeteMarch2014（seeChapter5）.Atthetime，thegravitationalwavesfromihavestillbeenedobservationally，butiftheybefoundbyfutureexperimeheywillopenupaontheearlyUniverse.
Theologistant
&edearlierthattheaisionissaidtobecausedbydarkewedidn’tgoiailaboutwhatdarkebe.Thetruthisthatwedoknowwhatdarkenergyis，butwedohaveafavouritedidate:theologistant.Iionwewillsidertheologistaail.
TheologistantwasfirstintroducedbyAlbertEinsteinin1917.Atthattime，itwasnotknownthattheUniverseandieinintroduologistaoproduodelthatwasstatieitherexpandingn）.Ofcourse，thatthereisverygoodevideosupporttheideathattheUniverseisexpaeinbecameawareofthishewithdrewhisologistant，tlybrusheduassomethingofastificembarrassmeheologistantremailytmodifithatoneaketothefieldequationsofhistheravity.It’sjustthattherewas.Not，thatis，untilitwasheexpansionoftheUniversewasag.
AologistaofasauniversalgravitationalforgallpartitheUheratthesamerate.Thisisexactlythesortofthingthat’srequiredtomaketheUniverseaccelerate.Allwehavetodoismakesuretheologistaopushthingsapart，giveittherightmagwillmaketheexpansionoftheUniverseaccelerate.Infact，it’sbyfarthesimplestwaytomaketheUniverseaccelerate.
Theologistaohavetheagsallofthetobservations.Ofcourse，weexpectthequalityofthisdatatoimprovesiderablyovertheihishappenswewillbeabletoseewhethertheologistantremainsagoodfit’ornot.Ifitis，thenthiswillbegoodevidesexiste’snot，thenwewillhavetinative.FornoewhatitwouldmeaniftherereallywasaologistantinourUhisisiiheologistant，althoughsimple，bringswithitanumberofproblems.
&aproblemassociatedwiththeologistantisthat，ifitistocausetheexpansionoftheUoaccelerateatthepresemusthavebeenveryfiheearlyUunihebugbearsoftheoreticalphysics.It’sooeuplanationforaphysicalefect，butifyourexplahingstediremelyspecialwaythenitstartstolooklessandlesspelliuningassociatedwiththeologistahefactthatitsvaluedoesime（it’sat）.ThismeansthatifwewantittohavetheaghenintheveryearlyUopickavaluethatisvery，very，verysmallparedtysatteratthetime，butnotquitezero.Iftheologistae，itwouldhavecausedtheexpansionoftheUoaccelerateatmuchearliertimes.Ifthishappearsandgalaxieswouldneverhaveformed，andtherewouldhavebeennolifeawereawouldherequiredamouion，andwewouldicedit.Tofitintothissweetspotweopickthemagheologistanttohaveaveryparticularvalue，withveryhighpre.Thispreiswidelythoughttobeatthelevelofaboutoin10120（that’sonefollowedbyowentyzeroes）.
Theologistadescribed，isexacerbatedbythefactthatquantummeicalefectsalsotributetoitsmagnitude.Givenourderstandingofquantummeics，wewouldexpectthesetributionstothrowthevaluewayoffromtheveryspecialvaluethatisneededobservatioterthisargumentbysayiyetfullyuhequawouldcausetheseefedhowtheyworkinthepresenceofgravity.Onecouldspeculatethattheremightbesomereasonwhythevariousquantumtributionsshouldceleachotherout，adon’tknowaboutityet.Thismightbepossible，butthere’safurtherprobleThepartitumtributioheologistanttoreceivearenotalwaysthesame’theygeduriepochsoftheUniverse’sexpansionhistory.Tosiderthepossibilitythatasetofquaionsmightallceleachotheroutisooassumethisshouldhappenoverandaiher.Thefactthattheologistantissoveryfihereforelooksevenmwheummeicalefetoat.Thisiswhytheologistantproblemhasbeencalled，bysome，theworstfiunihaseverophysics.
&iverse
Theproblemoftheologistantissideredsogreat，ahatmanyphysicistshavestartedtoeaiedrasticproposalsirya.Ofantastidmostwidelysidered，ideasisthepossibilitythatthereismorethanohiswerethediftheologistantweresomehowtotakeadiferentvalueineaiverse，thenitmightbepossibleforustofindourselvesihanygiveheologistahevalueweobservelooksfihismightjustmeanthatweareiivelyrareuhattherearemanyotheruniversesinwhiologistanttakesamorenatural-lookingvalue.
Thisideaofmanyuniverses，oramultiverse，doesn’tbyitselfalleviatetheproblemsassociatedwiththeimprobabilitytheologistahevaluethatweobserve.Insteadoffihevalueofthist，weareinsteadforcedtocarefullyselelikelyuniverseforourselvestolivein.However，ifwecoupletheideaofamultiversewithwhatisknowhropiciple，thenthingsbeeverydiferehropiciple，roughlystated，saysthatwe（aslifeforms）lyeverobserveauiscapablelife.Thissoundsobvious，butitprovidesameismtoselectwhichofthepossibleuniverseswemightbeabletofihin.Ifapartiiversesaologistantthatissethatstarsaseverform，thenitisuwewouldfindourselveslivihisautomaticallyde-selectsalargepartofthemultiverse，andmakesouruniverselookalotmorelikely.
Thisidearaisesalotofquestioheseotheruniverses？Howaretheyectedtoours？Howdoesthevalueoftheologistaweenthem？Andhowlikelyarewetofindourselvesihem？Theseareveryfualquestions，andalthoughtherearemeismsfmanyuniversesfromsometheoriesofi，itispushingtheboundariesofscetosaythatwevestigatethemasiftheywerephysicalrealities.Forsome，theideaofamultiverseisagloriousoedbyobservationsofhowgravityworksonastronomicalsd’eshedoutbyourtheoriesofeheBigBang.Forothersitisroblemitwasiosolve.ManyinthislattergroupsideritwrongtoinvokeunionsofspadtimeiosolveaprobleminourownobservableUniverse.Whileitmightbeself-t，aivated，todoso，theexisteherubetesteddireepeoplewithinthisgruethatsuapproachisessentiallyuifidbelongstotherealmofmetaphysics.
&herorsofsbestretcludeamultiverseisatopicoflivelydebate，withdiferentgroupspassitheirvariouscases.Futureastronomicalmissionswilladvahisargumehepropertiesofwhateveritisthat’sgtheUoaccelerateinitsexpauredevelopmeicalphysicsmayalsoshedlightouralheologistaomeasure.Fornoemust
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