Dark Matter and Dark Energy

arXiv:astro-ph/0403324v313Apr2004DarkMatterandDarkEnergyVarunSahniInter-UniversityCentreforAstronomy&Astrophysics,Pun´e411007,[e-mail]flyreviewourcurrentunderstandingofdarkmatteranddarken-ergy.Thefirstpartofthispaperfocussesonissuespertainingtodarkmatterinclud-ingobservationalevidenceforitsexistence,currentconstraintsandthe‘abundanceofsubstructure’and‘cuspycore’issueswhichariseinCDM.IalsobrieflydescribeMOND.Thesecondpartofthisreviewfocussesondarkenergy.InthispartIdis-cussthesignificanceofthecosmologicalconstantproblemwhichleadstoapredictedvalueofthecosmologicalconstantwhichisalmost10123timeslargerthantheob-servedvalueΛ/8πG≃10−47GeV4.SettingΛtothissmallvalueensuresthattheaccelerationoftheuniverseisafairlyrecentphenomenongivingrisetothe‘cosmiccoincidence’conundrumaccordingtowhichweliveduringaspecialepochwhenthedensityinmatterandΛarealmostequal.Anthropicargumentsarebrieflydis-cussedbutmoreemphasisisplacedupondynamicaldarkenergymodelsinwhichtheequationofstateistimedependent.TheseincludeQuintessence,Braneworldmodels,ChaplygingasandPhantomenergy.Modelindependentmethodstodeter-minethecosmicequationofstateandtheStatefinderdiagnosticarealsodiscussed.TheStatefinderhastheattractiveproperty...a/aH3=1forLCDM,whichishelpfulfordifferentiatingbetweenLCDMandrivaldarkenergymodels.Thereviewendswithabriefdiscussionofthefateoftheuniverseindarkenergymodels.2VarunSahni1DarkMatterObservationsofthecosmicmicrowavebackground(CMB)andthedeuteriumabundanceintheUniversesuggestthatΩbaryonh2≃0.02,orΩbaryon≃0.04ifthecurrentHubbleexpansionrateish=H0/100km/sec/Mpc=0.7.Al-thoughΩbaryonismuchlargerthantheobservedmassinstars,Ωstars≃0.0051,itisneverthelessverymuchsmallerthanthetotalenergydensityintheuniverseinferredfromtheobservedanisotropyinthecosmicmicrowaveback-ground[193]Ωtotal≡8πGρtotal3H2=1.02±0.02.(1)BothdarkmatteranddarkenergyareconsideredessentialmissingpiecesinthecosmicjigsawpuzzleΩtotal−Ωbaryons=?(2)Althoughthenatureofneitherdarkmatter(DM)nordarkenergy(DE)iscurrentlyknown,itisfeltthatbothDMandDEarenon-baryonicinorigin,andthatDMisdistinguishedfromDEbythefactthattheformerclustersonsub-Megaparsecscales(inordertoexplaingalacticrotationcurves)whereasthelatterhasalargenegativepressure(andcanmaketheuniverseaccelerate).InadditionthereisstrongevidencetosuggestthatΩm≃1/3,ΩDE≃2/3.(3)IntheselecturesIwillbrieflyreviewsomepropertiesofbothdarkmatteranddarkenergy.ThoughtheobservationalevidencefavouringaflatUniversewithΩtotal≃1isfairlyrecent,thenatureofthe‘unseen’componentoftheuniverse(whichdominatesitsmassdensity),isalong-standingissueinmoderncosmology.Indeed,theneedfordarkmatterwasoriginallypointedoutbyZwicky(1933)whorealizedthatthevelocitiesofindividualgalaxieslocatedwithintheComaclusterwerequitelarge,andthatthisclusterwouldbegravitationallyboundonlyifitstotalmasssubstantiallyexceededthesumofthemassesofitscomponentgalaxies.Forclusterswhichhaverelaxedtodynamicalequilibriumthemeankineticandpotentialenergiesarerelatedbythevirialtheorem[50]K+U2=0,(4)whereU≃−GM2/RisthepotentialenergyofaclusterofradiusR,K≃3Mhv2ri/2isthekineticenergyandhv2ri1/2isthedispersionintheline-of-sightvelocityofclustergalaxies.(ClustersintheAbellcataloguetypicallyhaveR≃1.5h−1Mpc.)Thisrelationallowsustoinferthemeangravitational1Thissuggeststhatmostofthebaryonicmatteratz=0isnotcontainedinstarsbutmightbecontainedinhotgas[30].DarkMatterandDarkEnergy3potentialenergyifthekineticenergyisaccuratelyknown.Themass-to-lightratioinclusterscanbeaslargeasM/L≃300M⊙/L⊙.Howeversincemostofthemassinclustersisintheformofhot,x-rayemittingintraclustergas,theextentofdarkmatterintheseobjectsisestimatedtobeM/Mlum≃20,whereMlumisthetotalmassinluminousmatterincludingstarsandgas.Fig.1.TheobservedrotationcurveofthedwarfspiralgalaxyM33extendsconsid-erablybeyonditsopticalimage(shownsuperimposed);fromRoy[164].InindividualgalaxiesthepresenceofdarkmatterhasbeenconvincinglyestablishedthroughtheuseofKepler’sthirdlawv(r)=rGM(r)r(5)todeterminethe‘rotationcurve’v(r)atagivenradialdistancefromthegalacticcenter.Observationsofgalaxiestakenatdistanceslargeenoughfortheretobenoluminousgalacticcomponentindicatethat,insteadofdecliningattheexpectedratev∝r−1/2trueifM≃constant,thevelocitycurvesflattenedouttov≃constantimplyingM(r)∝r(seefig1).Thisobservationsuggeststhatthemassofgalaxiescontinuestogrowevenwhenthereisnoluminouscomponenttoaccountforthisincrease.Velocitycurveshavebeencompiledforover1000spiralgalaxiesusuallybymeasuringthe21cmemissionlinefromneutralhydrogen(HI)[148,191].TheresultsindicatethatM/L=(10−20)M⊙/L⊙inspiralgalaxiesandinellipticals,whilethisratiocanincreasetoM/L≃(200−600)M⊙/L⊙inlowsurfacebrightnessgalaxies4VarunSahni(LSB’s)andindwarfs.Forinstance,arecentmeasurementoftheDracodwarfspheroidalgalaxylocatedatadistanceofonly79kpcfromtheMilkyWayshowsthepresenceofaconsiderableamountofdarkmatterM/L|Draco=440±240M⊙/L⊙[97]!Itisinterestingthatthetotalmassofanindividualgalaxyisstillsomewhatofanunknownquantitysinceaturnaroundtothev∝r−1/2lawatlargeradiihasnotbeenconvincinglyobserved.Animportantdifferencebetweenthe