cosmic inventory of energy densities issues and concerns

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1、Cosmicinventoryofenergydensities:issuesandconcernsT.PadmanabhanIUCAA,PB4,Ganeshkhind,Pune-411007,IndiaAbstractThedynamicsofouruniverseischaracterisedbythedensityparametersforcos-mologicalconstant(ΩV),nonbaryonicdarkmatter(Ωwimp),radiation(ΩR)andbaryons(ΩB).Totheseparameter

2、s—whichdescribethesmoothbackgrounduniverse—oneneedstoaddatleastanotherdimensionlessnumber(∼10−5)characterisingthestrengthofprimordialfluctuationsinthegravitationalpotential,inordertoensureformationofstructuresbygravi-tationalinstability.Idiscussseveralissuesrelatedtothedesc

3、riptionoftheuniverseintermsofthesenumbersandarguethatwedonotyethaveafundamentalunderstandingoftheseissues.0.1AninventoryofenergydensitiesBasedondynamicalconsiderations,wecandividetheenergydensitiescon-arXiv:gr-qc/0112068v126Dec2001tributingtotheexpansionoftheuniverseintoth

4、osedueto(i)radiationandrelativisticparticles(withanequationofstatep=wρandw=1/3),(ii)baryons(w=0),(iii)nonrelativisitic,nonbaryonicdarkmatter(w=0)and(iv)cosmologicalconstant(w≈−1).Currentobservationssuggestthatthetoptwopositionsareheldbycosmologicalconstant(ΩV≈0.6)andnonbar

5、yonicdarkmatter(Ωwimp≈0.35)forwhichwehavenolaboratoryevidence.[Thethirdpositionwillgotomassiveneutrinosiftherecentobservationalresultsareconfirmed.]ThebaryonsintheuniversecontributeΩ≈0.02h−2whichisBaboutanorderofmagnitudelargerthanthebaryonsseenintheformoflumi-nousmatter,in

6、dicatingthatatleastpartofthebaryonsaredark.TheenergydensityinelectromagneticradiationisdominatedbyCMBRandcontributesaboutΩ≈2.5×10−5h−2.RTheseenergydensitiesdrivetheexpansionofahomogeneousandisotropicuniversewhichiscompletelystructureless.Theexistenceofsmallscalestruc-1ture

7、sintheuniverseisbelievedtobeduetogravitationalinstabilitywhichhasamplifiedsmallinitialfluctuations.Suchaparadigmrequirestheexistenceofnonzerofluctuationsinthegravitationalpotentialenergyduetoprimordialdensityfluctutations.Observationssuggestthatthiscanbecharacterisedbyadimensi

8、onlessnumber(∼10−5)inouruniverse.Ishallnowtrytodescribesomeoftheissuesrelatedtothesenumbe