Dark Matter and Dark Energy

arXiv:0706.2986v1[astro-ph]20Jun2007DARKMATTERANDDARKENERGY∗MARCKAMIONKOWSKI†CaliforniaInstituteofTechnology,MailCode130-33,Pasadena,CA91125,USAThisisashortreview,aimedatageneralaudience,ofseveralcurrentsubjectsofresearchincosmology.Thetopicsdiscussedincludethecosmicmicrowavebackground(CMB),withparticularemphasisonitsrelevancefortestinginflation;darkmatter,withabriefreviewofastrophysicalevidenceandmoreemphasisonparticlecandidates;andcosmicaccelerationandsomeoftheideasthathavebeenputforwardtoexplainit.Aglossaryoftechnicaltermsandacronymsisprovided.1.IntroductionNowisthetimetobeacosmologist.WehaveobtainedthroughremarkabletechnologicaladvancesandheroicandingeniousexperimentaleffortsadirectandextraordinarilydetailedpictureoftheearlyUniverseandmapsofthedistributionofmatteronthelargestscalesintheUniversetoday.Wehave,moreover,anelegantandpreciselyquantitativephysicalmodelfortheoriginandevolutionoftheUniverse.However,themodelinvokesnewphysics,beyondthestandardmodelplusgeneralrelativity,notjustonce,butatleastthrice:(1)Inflation,thephysicalmechanismformakingtheearlyUniverselookpreciselyasitdoes,positssomenewultra-high-energyphysics;wedon’tknow,however,whatitis.(2)Thegrowthoflarge-scale-structureandthedynamicsofgalaxiesandgalaxyclustersrequiresthatweinvoketheexistenceofcollisionlessparticlesorobjects;wedon’tknowwhatthisstuffis.(3)TheacceleratedexpansionoftheUniverserequirestheintroductionofanewterm,ofembarrassinglysmallvalue,inEinstein’sequation,amodificationofgeneralrelativity,and/ortheintroductionofsomenegative-pressure“darkenergy,”again,thenatureofwhichremainsamystery.Inscience,though,confusionanduncertaintyareopportunity.Therearewell-definedbutfundamentalquestionstobeansweredanddataarrivingtoguidetheory.Ongoingandforthcomingobservationsandexperimentswillinthenextfewyearsprovideempiricalinformationaboutthenewphysicsresponsibleforinflation,thenatureofthedarkmatter,andthepuzzleofacceleratedexpansion.Futurediscov-∗Submittedforpublicationin“VisionsofDiscovery”(inhonorofCharlesTownes),tobepublishedbyCambridgeUniversityPress.†[e-mail]1DarkMatterandDarkEnergy2eriesmayhelpusunderstandthenewphysicsthatunifiesthestrong,weak,andelectromagneticinteractions,aswellasgravity.Therearealsoalwaystheprospectsforamajorparadigmshiftinphysics,whichmayberequiredtounifygravitywithquantummechanics.InthisChapter,Ireviewthecurrentstatusofourcosmologicalmodelaswellasitsshortcomingsandthequestionsitleavesunanswered,andIdiscusspossibleanswerstothesequestionsandpossibleavenuestowardstestingtheseanswers.Inparticular,Ifocusonthreesubjects.InthenextSection,Idiscussthecosmicmicrowavebackgroundandinflation.Althoughthemainsubjectofthisreviewisdarkmatteranddarkenergy,theparadigmuponwhichmanyofourobservations—includingthosethatsuggestdarkmatteranddarkenergy—areinterpretedisaUniversewithprimordialperturbationsremarkablylikethosepredictedbyinflation.Moreover,themostpreciseinformationwehavenowabouttheUniverseanditscontentsisthecosmicmicrowavebackground,andsoitbehoovesustoreviewthissubjectbeforeconsideringdarkmatteranddarkenergy.IthenmoveoninSection3todarkmatter.Ifocusprimarilyonparticledarkmatteranddiscusstheprospectsfordetectionofsuchdarkmatter,aswellassomevariationsonthesimplestparticlemodelsfordarkmatter.Section4reviewsthecosmic-accelerationpuzzle.Ireviewtheevidenceandthendiscussseveralpossiblesolutions.Section5providessomeclosingremarks,andSection6containsaglossary(preparedincollaborationwithAdrianLee)oftechnicaltermsandacronymsusedinthisreviewandintheChapterinthisvolumebyAdrianLee.2.TheCosmicMicrowaveBackgroundandInflationAconfluenceoftheoreticaldevelopmentsandtechnologicalbreakthroughsdur-ingthepastdecadehavetransformedthecosmicmicrowavebackground(CMB)intoaprecisetoolfordeterminingthecontents,largest-scalestructure,andoriginoftheUniverse.Tiny(fewpartsin105)angularvariationsinthetemperatureoftheCMBwerediscoveredintheearly1990sbytheDifferentialMicrowaveRadiometer(DMR)aboardNASA’sCosmicBackgroundExplorer(COBE)[1],andduringthepastfewyears,high–signal-to-noisehigh–angular-resolution(∼0.2◦)CMBtemper-aturemapshavebeenobtained[2].TheseprovidetheveryfirstsnapshotsoftheUniverseasitwasroughly380,000yearsafterthebigbang,nearly14billionyearsago,whenelectronsandlightnucleifirstcombinedtoformneutralhydrogenandhelium.Thesenewmapshaveprovidedseveralextraordinarybreakthroughs.ThemoststrikingamongtheseisfairlyrobustevidencethattheUniverseisflatandthatlarge-scalestructure(galaxies,clustersofgalaxies,andevenlargerstructures)grewviagravitationalinfallfromanearlyscale-invariantspectrumofprimordialdensityperturbations.BothoftheseobservationshintstronglythattheUniversebeganwithinflation[3],aperiodofacceleratedexpansionintheveryearliestUniverse,drivenbythevacuumenergyassociatedwithsomenewultra-high-energyphysics.Evenmorerecently,thepolarizationoftheCMBhasbeendetected[4]andDarkMatterandDarkEnergy3beguntobemappedonsmallscales[5]anddetectedthroughitscross-correlationwiththetemperature[6,7].Thesmall-scaleresultsareconsistentwithexpectationsbasedonmodelsthatfitthetemperatureresults,