0708.4231v2---TASI Lectures on the Cosmological Constant

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1、PreprinttypesetinJHEPstyle-HYPERVERSIONTASILecturesontheCosmologicalConstantRaphaelBoussoCenterforTheoreticalPhysics,DepartmentofPhysicsUniversityofCalifornia,Berkeley,CA94720-7300,U.S.A.andLawrenceBerkeleyNationalLaboratory,Berkeley,CA94720-8162,U.S.A.Abstract:Theenergydensityofthevacuum,Λ,isatle

2、ast60ordersofmagnitudesmallerthanseveralknowncontributionstoit.Approachestothisproblemaretightlyconstrainedbydatarangingfromelementaryobservationstoprecisionexperiments.Absentoverwhelmingevidencetothecontrary,darkenergycanonlybeinterpretedasvacuumenergy,sothevenerableassumptionthatΛ=0conflictswitho

3、bservation.ThepossibilityremainsthatΛisfundamentallyvariable,thoughconstantoverlargespacetimeregions.Thiscanexplaintheobservedvalue,butonlyinatheorysatisfyinganumberofrestrictivekinematicanddynamicalconditions.Stringtheoryoffersaconcreterealizationthroughitslandscapeofmetastablevacua.arXiv:0708.423

4、1v2[hep-th]11Sep2007Contents1.Introduction:Thecosmologicalconstantproblem22.Someideas,andwhytheydon’twork62.1Quantumgravity72.2Infraredorultravioletmodificationsofgravity72.3Violationsoftheequivalenceprincipleandde-gravitatingthevacuum82.4Initialconditions82.5Nongravitationaldynamicalattractormecha

5、nisms92.6Gravitationaldynamicalattractormechanisms92.7Summary103.Anoldideathatmighthaveworkedbutdidn’t103.1ρΛvanishesintheasymptoticfuture113.2Predictions114.Anoldideathatcouldhavebeenfalsifiedbutwasn’t124.1ρΛisanenvironmentalvariable124.2ExperimentalPredictions134.3TheoreticalPredictions145.Whydar

6、kenergyisacosmologicalconstant155.1Callingitaduck155.2Twoproblemsforthepriceofone165.3Therealsecondproblem176.Atheoryofthecosmologicalconstant186.1TheBrown-Teitelboimmechanism186.2Thediscretuum236.3Stringtheory277.Outlook:Thelandscapeofstringtheory29–1–1.Introduction:Thecosmologicalconstantproblem

7、WhenEinsteinwrotedownthefieldequationforgeneralrelativity,1Rµν−Rgµν+Λgµν=8πGTµν(1.1)2hehadachoice:ThecosmologicalconstantΛwasnotfixedbythestructureofthetheory.Therewasnoformalreasontosetittozero,andinfact,Einsteinf