CMB Anisotropies A Decadal Survey

arXiv:astro-ph/0002520v129Feb2000CMBAnisotropies:ADecadalSurveyWayneHuInstituteforAdvancedStudy,Princeton,NJ,08540,USA,[e-mail],“AfterIheardyourwords,oneyearandIranwild,twoyearsandIwastame,threeyearsandpositionsinterchanged,fouryearsandthingssettleddown,fiveyearsandthingscametome,nineyearsandIhadthegreatsecret.”–Chuang-tzuAbstractWereviewthetheoreticalimplicationsofthepastdecadeofCMBanisotropymeasurements,whichculminatedintherecentdetectionofthefirstfeatureinthepowerspectrum,anddiscussthetestsavailabletothenextdecadeofexperiments.Thecurrentdataalreadysuggestthatdensityperturbationsoriginatedinaninflationaryepoch,theuniverseisspatiallyflat,andbaryonicdarkmatterisrequired.Wediscusstheunderlyingassumptionsoftheseclaimsandoutlinethetestsrequiredtoensuretheyarerobust.Themostcriticaltest-thepresenceofasecondfeatureatthepredictedlocation-shouldsoonbeavailable.Furtherinthefuture,secondaryanisotropiesandpolarizationshouldopennewwindowstotheearlyandlow(er)redshiftuniverse.1IntroductionThe1990’swillberememberedasadecadeofdiscoveryforcosmicmicrowavebackground(CMB)anisotropies.ThelaunchoftheCOBEsatelliteusheredinthedecadein1990andleadtothefirstdetectionofCMBanisotropiesat10◦scales[1].Throughthedecade,acombinationofhigherresolutionexperimentsmadethecaseforariseintheanisotropylevelondegreescalesandasubsequentfallatarcminutescales[2].Thefinalyearsawexperiments,notablyTocoandBoomerang,withsufficientangularresolutionandskycoveragetolocalizeasharppeakintheanisotropyspectrumatapproximately0◦.5[3].Inthisreview,wediscussthetheoreticalimplicationsoftheseresultsandprovidearoadmapforcriticaltestsandusesofCMBanisotropiesinthecomingdecade.2OnceandFuturePowerSpectrumThetiny10−5variationsinthetemperatureoftheCMBacrosstheskyareobservedtobeconsistentwithGaussianrandomfluctuations,atleastontheCOBEscales(10◦),asexpectedinthesimplesttheoriesoftheirinflationaryorigin.AssumingGaussianity,thefluctuationscanbefullycharacterizedbytheirangularpowerspectrum1T(ˆn)=XℓmaℓmYℓm(ˆn),ha∗ℓmaℓ′m′i=δℓℓ′δmm′Cℓ.(1)Wewilloftenusetheshorthand(ΔT)2=ℓ(ℓ+1)Cℓ/2πwhichrepresentsthepowerperlogarithmicintervalinℓ.1Conventionsforrelatingmultipolenumbertoangularscaleinclude:θℓ≈2π/ℓ,π/ℓor100◦/ℓ.Totheextentthattheseconventionsdiffer,noneofthemarecorrect;wehereafterrefertopowerspectrumfeaturesbymultipolenumber,whichhasaprecisemeaning.12FIRSBAMSP91IABPythMSAMWDOVROSuZIETeneSP94MSAMViperRINGBIMAATCAArgoQMAPMAXCOBEPythTOCOBoomCATSaskIACMSAM10100100020406080100l101001000l101001000lΔT(μK)W.Hu–Jan.2000CurrentMAPPlanckΩm=1,Ωmh2=0.25,Ωbh2=0.02Fig.1:PowerSpectrumFig.1(left,1σerrors×windowFWHM)showsthemeasurementsthepowerspectrumtodate(see[3]foracompletelistofreferences).Thedataindicatearathersharppeakinthespectrumatℓ∼200withasignificantdeclineatℓ∼1000.Thispeakhasprofoundimplicationsforcosmology.Theprimaryclaimsindecreasingorderofconfidenceandincreasingneedofverificationfromprecisionmeasurements(e.g.fromtheMAPandPlancksatelliteFig.1center,right)are•Earlyuniverse.Thesimplestinflationarycolddarkmatter(CDM)cosmologieshavecorrectlypredictedthelocationandmorphologyofthefirstpeakintheCMB;conversely,allcompetingabinitiotheorieshavefailed,essentiallyduetocausality.Confirmitsacousticnaturewiththesecondpeak.Usepolarizationasasharptestofcausality.•Geometry.Theuniverseisflat.Lowerlimitsonthetotaldensity(Ωtot≡PΩi∼0.6[3])arealreadyrobust,unlessrecombinationissubstantiallydelayedorh≫1.Calibratethe“standardrulers”(acousticscaleanddampingscale)inthisdistancemeasurethroughthehigherpeaks.•Baryons.Atleastasmuchbaryonicdarkmatterasindicatedbybigbangnucleosynthesis(BBN)isrequired(Ωbh2∼0.01[3]).Confirmwithrelativeheightsofthepeaks,especiallythethirdpeak.•Reionization.TheThomsonopticaldepthislow–howlowdependsontherangeofmodelsconsidered.Theopticaldepth,assumingitislow,willonlybeaccuratelymeasuredbyCMBpolarizationatlargeangles.•Darkenergy.Thematterdensityislowandcombinedwithflatness,thisindicatesamissingenergycomponent,possiblythecosmologicalconstant.Currentlythe95%CLincludesΩm=1butthemaximumlikelihoodmodelincludingBBNandhconstraintshasΩm≈0.3[3].MeasureΩmh2fromthefirstthreepeaks.Theearlyuniverseandgeometrytestsbasicallyrelyonthepositionofthefirstpeakandhencearemorerobustthanthelateroneswhichrelymainlyoninterpretingitsamplitude.Moreover,allclaimsarebasedoninterpretingthepeakatℓ∼200asthefirstinaseriesofacousticpeaks.Basedonthesharpnessofthefeature,thisinterpretationisnowreasonably,butnotcompletelysecure.Thedetectionofasecondpeakinthespectrumiscriticalsinceitwillprovideessentiallyincontrovertibleevidencethatthisinterpretationiscorrect(orwrong!).Oncethisisachievedandthepeakspassthemorphologicaltestsdescribedbelow,theCMBwillbecomethepremierlaboratoryforprecisioncosmology,asmanystudieshaveshown[4].TheseexpectationsalsorelyonthefactthatCℓcanultimatelybemeasuredtoΔCℓCℓ=s2(2ℓ+1)fsky,(2)31stExtremaΘ+Ψη2ndExtrema1st2ndPeaksFig.2:HarmonicAcousticPeaks.basedonGaussiansamplevarianceonthe(2ℓ+1)fskyindependentmodesofagivenℓ,fromafractionofskyfsky.Therestofthisreviewwillmakethetheoreticalcasefortheabovestatements.3SoundPhysi