New generalized Chaplygin gas as a scheme for unif

arXiv:astro-ph/0411221v410Dec2005NewgeneralizedChaplygingasasaschemeforunificationofdarkenergyanddarkmatterXinZhang1,2,Feng-QuanWu1,2,andJingfeiZhang31CCAST(WorldLaboratory),P.O.Box8730,Beijing100080,People’sRepublicofChina2InstituteofHighEnergyPhysics,ChineseAcademyofSciences,P.O.Box918(4),Beijing100049,People’sRepublicofChina3DepartmentofPhysics,LiaoningNormalUniversity,Dalian116029,People’sRepublicofChinaAbstractWeproposeinthispaperanewmodelfordescribingtheunificationofdarkenergyanddarkmatter.ThisnewmodelisafurthergeneralizationofthegeneralizedChaplygingas(GCG)model,thusdubbednewgeneralizedChaplygingas(NGCG)model.TheequationofstateoftheNGCGisgivenbyp=−˜A(a)/ρα,whereaisthescalefactorand˜A(a)=−wXAa−3(1+wX)(1+α).WeshowthattheNGCGmodelistotallydualtoaninteractingXCDMparametrizationscenario,inwhichtheinteractionbetweendarkenergyanddarkmatterischaracterizedbytheconstantα.Wediscussthecosmologicalconsequencesledbysuchanunifieddarksectorsmodel.Furthermore,weperformastatefinderanalysisonthisscenarioandshowthediscriminationbetweenthisscenarioandotherdarkenergymodels.Finally,acombinedanalysisofthedataofTypeIasupernovae,cosmicmicrowavebackground,andlargescalestructureprovidesafairlytightconstraintontheparametersoftheNGCGmodel.1I.INTRODUCTIONRecentobservationsofTypeIasupernovae(SNeIa)[1]indicatethattheexpansionoftheUniverseisacceleratingatthepresenttime.Theseresults,whencombinedwiththeobservationsofcosmicmicrowavebackground(CMB)[2]andlargescalestructure(LSS)[3],stronglysuggestthattheUniverseisspatiallyflatanddominatedbyanexoticcomponentwithlargenegativepressure,referredtoasdarkenergy[4].ThefirstyearresultoftheWilkinsonMicrowaveAnisotropyProbe(WMAP)showsthatdarkenergyoccupiesabout73%oftheenergyofourUniverse,anddarkmatterabout23%.Theusualbaryonmatterwhichcanbedescribedbyourknownparticletheoryoccupiesonlyabout4%ofthetotalenergyoftheUniverse.AlthoughwecanaffirmthattheultimatefateoftheUniverseisdeterminedbythefeatureofdarkenergy,thenatureofdarkenergyaswellasitscosmologicaloriginremainenigmaticatpresent.Sofartheconfirmedinformationaboutdarkenergyisstilllimitedandcanberoughlysummarizedasthefollowingseveralitems:itisakindofexoticenergyformwithsufficientlylargenegativepressuresuchthatdrivestheUniversetoundergoaperiodofacceleratingexpansioncurrently;itisspatiallyhomogeneousandnon-clustering;anditisinsmallpartattheearlytimeswhiledominatestheUniverseveryrecently.Theinvestigationofthenatureofdarkenergyisanimportantmissioninthemoderncosmology.Muchworkhasbeendoneonthisissue,andthereisstillalongwaytogo.Currently,thepreferredcandidatesofdarkenergyarevacuumenergy(orcosmologicalcon-stant)anddynamicalfields.ThesimplestformofdarkenergyisthecosmologicalconstantΛ.Atinypositivecosmologicalconstantwhichcannaturallyexplainthecurrentaccelera-tionwouldencountermanytheoreticalproblems,suchasthe“fine-tuning”problemandthe“coincidence”problem.Anotherpossibleformofdarkenergyisprovidedbyscalarfields.Darkenergycanbeattributedtothedynamicsofascalarfieldφ,calledquintessence[5],whichconvincinglyrealizethepresentacceleratedexpansionoftheUniversebyevolvingslowlydownitspotentialV(φ).Thetrackerversionquintessenceistosomeextentlikelytoresolvethecoincidenceproblem.Itshouldalsobepointedoutthatthecoupledquintessencemodels[6]providethisproblemwithamorenaturalsolution.However,forquintessencemodelswithflatpotentials,thequintessencefieldhastobenearlymasslessandonethusexpectsradiativecorrectionstodestabilizetheratiobetweenthismassandtheotherknownscalesofphysics.Inaddition,forthecosmologicalconstantandmanyquintessencemodels,theeventhorizonwouldleadtoapotentialincompatibilitywiththestringtheory.Otherdesignsondarkenergyroughlyincludek-essence[7],quiessence(or“X-matter”)[8],braneworld[9],tachyon[10],generalizedChaplygingas(GCG)[11,12],holographicdarkenergy[13,14],andsoforth.ThequiessenceorX-mattercomponentissimplycharacterized2byaconstant,non-positiveequationofstatewX(wX−1/3isanecessaryconditiontomaketheUniverseaccelerate).Ingeneral,inaFriedmann-Robertson-Walker(FRW)backgroundwiththepresenceofcolddarkmatter(CDM),anarbitrarybutconstantwXfordarkenergyfromtherange(−1,0)canbeachievedbyusingascalarfieldwithahyperbolicsinepotential[8].ItmaybenotedthatinprinciplethevalueofwXmaybeevenlessthan−1.Infact,byfittingtheSNeIadataintheframeworkofXCDM(X-matterwithCDM),thehintforwX−1hasbeenfound.Indeed,astudyofhigh-zSNeIa[15]findsthattheequationofstateofdarkenergyhasa99%probabilityofbeing−1ifnopriorsareplacedonΩ0m.WhentheseSNeresultsarecombinedwithCMBand2dFGRSthe95%confidencelimitsonanunevolvingequationofstateare−1.46wX−0.78[15,16]whichisconsistentwithestimatesmadebyothergroups[2,3].ThepossibilityofwX−1hasprovokedlotsofinvestigationsonthephantomdarkenergy[17].TheremarkablefeatureofthephantommodelisthattheUniversewillenditslifewitha“BigRip”(futuresingularity)withinafinitetime[18].Ontheotherhand,weconcernhereanotherinterestingproposalondarkenergy,i.e.thedarkenergycomponentmightbeexplainedbyabackgroundfluidwithanexoticequationofstate,thegeneralizedChaplygingasmodel[11].Thestrikingfeatureofthismodelisthatitall