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1、Cosmicinventoryofenergydensities:issuesandconcernsT.PadmanabhanIUCAA,PB4,Ganeshkhind,Pune-411007,IndiaAbstractThedynamicsofouruniverseischaracterisedbythedensityparametersforcos-mologicalconstant(?V),nonbaryonicdarkmatter(?wimp),radiation(?R)andbaryons(?B).Totheseparameter
2、s—whichdescribethesmoothbackgrounduniverse—oneneedstoaddatleastanotherdimensionlessnumber(～10?5)characterisingthestrengthofprimordial?uctuationsinthegravitationalpotential,inordertoensureformationofstructuresbygravi-tationalinstability.Idiscussseveralissuesrelatedtothedesc
3、riptionoftheuniverseintermsofthesenumbersandarguethatwedonotyethaveafundamentalunderstandingoftheseissues.0.1AninventoryofenergydensitiesBasedondynamicalconsiderations,wecandividetheenergydensitiescon-arXiv:gr-qc/0112068v126Dec2001tributingtotheexpansionoftheuniverseintoth
4、osedueto(i)radiationandrelativisticparticles(withanequationofstatep=wρa(bǔ)ndw=1/3),(ii)baryons(w=0),(iii)nonrelativisitic,nonbaryonicdarkmatter(w=0)and(iv)cosmologicalconstant(w≈?1).Currentobservationssuggestthatthetoptwopositionsareheldbycosmologicalconstant(?V≈0.6)andnonbar
5、yonicdarkmatter(?wimp≈0.35)forwhichwehavenolaboratoryevidence.[Thethirdpositionwillgotomassiveneutrinosiftherecentobservationalresultsarecon?rmed.]Thebaryonsintheuniversecontribute?≈0.02h?2whichisBaboutanorderofmagnitudelargerthanthebaryonsseenintheformoflumi-nousmatter,in
6、dicatingthatatleastpartofthebaryonsaredark.TheenergydensityinelectromagneticradiationisdominatedbyCMBRandcontributesabout?≈2.5×10?5h?2.RTheseenergydensitiesdrivetheexpansionofahomogeneousandisotropicuniversewhichiscompletelystructureless.Theexistenceofsmallscalestruc-1ture
7、sintheuniverseisbelievedtobeduetogravitationalinstabilitywhichhasampli?edsmallinitial?uctuations.Suchaparadigmrequirestheexistenceofnonzero?uctuationsinthegravitationalpotentialenergyduetoprimordialdensity?uctutations.Observationssuggestthatthiscanbecharacterisedbyadimensi
8、onlessnumber(～10?5)inouruniverse.Ishallnowtrytodescribesomeoftheissuesrelatedtothesenumbe
