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Chapter 6 Frontiers of gravitational physics(第2页)

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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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