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Chapter 6 Frontiers of gravitational physics（第1页）

Chapter6Frravitationalphysics

ThroughoutmostofthisbookwehavebeehegedgeravitationalphysicsreseargfromtestsonthesetrestothoseoheentireobservableUhischapterwewillsidersomeoftheissuesihetheoreticaldesofgravity。

&hasbeeheorythathasshapedofthegravitatioion。Thistheorytreatsspadtimeasasi，ahepropertiesofthatobjectbedetermihatexistswithinit。Ihopethatthereaderhas，bynow，beehephenomenalsuccessofEiheory。Itistrulyextraordinarythatasiheoryshouldbeabletoexplainsuchahysicalefestein’stheory，however，isuobethefinalwordiandingofgravity。Alothashappeheworldoftheoreticalphysice1915，andmuchofitsuggeststhatweshouldexpecttheretobeanevenmorefuheory。

Quantummedgravity

Notloeiheravity，atthebeginnihtury，theworldoftheoreticalphysicswasfed（on）bystistssuchasBohr，Heisenberg，Sger，andothers。Sion，andupuntilthispoint，physicshadbeenthoughttobedeterministic。Thatis，ifyouknowenoughinformationaboutthepositionandmotionsofallobjetheUhenyoushouldbeabletopredictthefuturewitharbitrarilyhighpre。Physicaltheoriesthatworkinthiswayareoasclassicaltheories。Einsteiytheoryisanexampleofaclassicaltheory。TherevolutionledbyBohr，Heisenberg，andSgerothertypeoftheory，quantummeiewquantumtheoryrobability，aediionofnatureinwhichitwasonlyeverpossibletocalculatetheoddsthateventseure，andwithinwhieverwhatthefutureholds。

Quantummeiastonishidescribedthe，andthebuildingblocksofallkraordinarypre。Later，PaulDirac，aprofessorattheUye，shothesenewideascouldbeusedtocreatequantumtheoriesofelectridmaghisledtowhatisaiclephysitummeicaldesofalloftheknowure，andtheforcesbetweenthem。Windingforwardtothepresentdaythereoseriousdoubtthatummeical。ThepredisoftheStandardModelhaveallbeehegjewelbeialverifi2012oftheexisteheHiggsboson（atheorizedparticlethatisanimportahemodernStandardModel，andwhichgivestheotherparticlestheirmass）。Itisquahatunderpinsmuistryandmaterialsgusthesemi-ductorsthatourputersaremadefrom;andthelasersaareusedtostructourDVDplayersandtelevisions。Quantummeicsisafactoflife。Itishownatureworks，anditbeesinglyimportaodescribethemicroscopicworld。

&eitsubiquityinallotherareasofphysics，theappliofthisapproachtogravityisstillood。WhiletheeleagicforbequantizediivelystraightforhilethematterintheUniversehaslongbeendescribedusingquantummeics，thequantumtheravityremaihisisprobablythesiunsolvedprobleminphysidhasbeefiftyyears。Itisextraordinarilydiiculttoapplythelogicthathasbeensosucotherareasofphysicstogravity，andsoourtbestdesofgravityisstillEinstein’sclassicaltheory。

Partoftheproblemwiththisstateofafairsisthattherearesituationsinwhiewouldexpectbothquantummedgravitytoberequired。Ohisisthetreofablackhole。Aswe’vediscussedalready，blawhearsuastrophiccollapse。Thematterthatwasinallypartofthestargetspressedbygravitytherdensities。Infact，agtoEiheory，thecollapseshouldueuntilallthematterissquashedtoasi。NtoquantumtheoryweshouldexpeewquaobeeapparentwhenwesiderverysmalldistancesdveryhigheI’vedescribedistherefimewherebothquantummedgravityareexpectedtoberequirediadesofthephysicsatplay。Butthereisheoryofquantumgravity，soitis（atpresent）impossibletoknowwhatshouldhappereofastarafterithascollapsed。Thisisobviouslyanunsatisfactorystateofafairs。Ifwewaodescribeeverythisiheumtheravity。

Thereasoinpatibilityofquantumtheravityaremany，aplicated。Firstthereisaceptualdifereein’sapproachtogravity，andhowforcesaretreatedinquantummeistein’stheravityisaresultofthecurvatureofspace-time。Thereiserpullsthiheapparentwayinwhichmassivebodiesmovetowardseachotherissimplyaresultofthecurvatureofspace-time。TheEarth，forexample，isowardstheSun，itissimplyinfreefall，followipathavailabletoitiime。Thisisnotthecaseforotherforces。Theelectricforple，isaresultoftheelectricfieldthatgetsgeedbychargedparticles。Theelectricfieldexistswithiime，butisnotthesameasspae，inanyseimearesimplythearenawithinwhichtheelectricforceplaysout。Thisideaofspadtimehavingtheirowexistendbeisoftheproblem，isbuiltintomostapproameics。TotrytousetheseapproachestravitytherefainsteretaughtbyEinstein。

Therearealsoodmathematiswhygravityandquantummeipatible。Theforemostoftheseisaprravityalizability。Whenquantummeicsisusedtodescribeaforce，theresultofthecalsthatweperformresultifinities。Ahisowesidertarticles。Toworkouttheforthem，quantummeically，wehavetoaddthetributionsfromallofthepossiblepositioieofthesefiguratiohepartievery，verywhichcasetheforesvery，verylarge。Summingoverallpossiblepositionsthengivesaresultthatisihisunreasonableanswerbecorrectedfor，iheelectricforce，byusingaprocessormalization。Thisprocessremovesthepartsoftheequationsthattributedtheiis，theinfiiallysubtratheiiohenasensibleanswer，whibetestedwithexperiments。Renormalization，however，doesn’tworkwithEinstein’sgravity。Theibesubtracted，asthereisnothingintheiionsthatlooksanythiermsthatbeeiumgravitycalsthereforeseemtogiveinfihingisobvi。

Therehavebeenmaofixtheseproblems。TheyrangefromgEiions（sothattheyappeartobemorerenormalizable）;togquantummeics（sothatitisnoloicles）;throughtogthewethinkofasspadtime（sothattheyarenotuous）。Itisnotpossibletogiveafairrepresentationofalloftheseapproacheshere，ointoahemiheyareveryplicatedideasandtheyareallwress。ThereareacoupleoftheoriesthatIfeelIshouldmentionthough’theseareStringTheoryandLoopQuantumGravity。Thesearebothextremelyboldandambitiousattemptstostrutumtheravity。Ifcorrect，thehopeisthatphysicistscouldusethesetheoriestodescribeereofblackholes。Theyare，however，quitedifereher。Theyprioritizedifereheproblem，andapproachthetedceptualdicultiesjustdesquitediferent>

StringTheorywasbornoutofparticlephysics。Itisbasedohebasistituentsofmatterarenotpoiicles，butareiinyone-dimensis。Thisisaradicalidea，andithasledtoalotmathsandphysideed，manyphysisiderittobeourbesthopeoffindingatheoryofquantumgravity。Thehypothesizedstringsareverysmall，sothatforthemostparttheyappeartousefectivelyaspoiiwetryandquahem，however，theirstriodifereheequationsthatgoverringsalsoaspectsthatlookalotliketheequationsthatgoverheravity。Ittherefravityis，tosomeextent，builtintTheory。Therearedrawbacks，however，asthecyoftheequationsthatgoverringsrequireustoaddbetweensixawoextradimensionsofspacetoourdesoftheUniverse。

&radimetobeact，sothatwedoheminoureverydaylives。heyhavetobethereforthetheorytobeself-t。Iiehesesmallextradimehepossibilitythatgravitycouldworkdiferentlyonverysmallscales。

LoopQuantumGravityisoftehemaiTheory。Thestartingpointforthistheoryistheideathatonverysmallscalesspace-timehasagranularstructure。Thatis，spadtimearehuousvariablesthatweusuallysiderthemtobe。Instead，space-timeisatomized。Quaheheloopsthatmakeupthisure。Thisisalsoaradicalidea，aobefavouredbyafiadosofGeybephasisoimeasthefualobjeterest’insteadofasabad。LoopQuantumGravityis，however，verymuchawress。ItremainstobeseeheapproachitemploysistheeTheory，orsomeotherasyetuheory，isapreferabledesofherworkisofuswouldbepreparedtomakeaheouteofthisdebate。

Partigravitationalfields

Quantizinggravityisfraughtwithdiculties。Solet’sturntoaioummeiagravitationalfield？HereweathesamewaythatEinsteindidinhisclassicaltheravity。Oncewehaveaclassicalspace-timeatourdisposal，hoillsiderereatthemattertasbeinggovernedbyquahismixedapproach，withmatterbeiummeidgravitybeiedclassically，isusuallyreferredtoassemi-classicalphysics。Itisalessambitiousprojefullquantumgravitybuthelessgivesusiinginsightsintohowquantumsystemsworkinthepresenceofgravity。

&hepiohissubjecthenHawking，whoin1974showedthatquantummeicsshouldleadtoblaittingradiation。Thisdiscoveryshockedthestifiunity，asagtoEiheeveresablackhole。Hawking’scalwasasemi-classie。Hetooktheclassicaldesforthespace-timearoundabladallowedquantummeicalparticlestoexistwithinit。Heshowed，usingafairlysimplequantummeicalcal，thatiftherewasnoradiatiopast，theexistradiatioure。Theonlypossibleexplanationforthiswasthattheradiationroducedbytheblackhole。Ofcourse，radiatiy，andinthissituationtheonlysourergyisthemasswithintheblaberthatmassisatypeyiheory）。SoHawkinghadshownthatblaaturallyshrink，byradiatingawaytheirmass，andthattheymusteveobe。

Hawkiwasnovel，andsparkedseveralnewfieldsravitationalphysiediatelyafterHawking’sdiscoveryitwasshown，byBillUnruh，thattheveryexisteibequestionedwhenwesiderthemwithiytheory。Particlephysicsisverymuainofquantummeiruhshowedthatifobserversareiion，withoiotheother，theirelypossibleforoecttheexisteumparticleswhiletheotherdetegatall。Thatis，whetherornotparticlesexistdepeionofthepersonwhtomeasurethem。

&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’safurtherproblem。Thepartitumtributioheologistanttoreceivearenotalwaysthesame’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