Evidence from Type Ia Supernovae for an Accelerati

arXiv:astro-ph/0008057v13Aug2000EvidencefromTypeIaSupernovaeforanAcceleratingUniverseAlexeiV.Filippenko∗DepartmentofAstronomy,UniversityofCalifornia,Berkeley,CA94720-3411([e-mail])AdamG.Riess∗SpaceTelescopeScienceInstitute,3700SanMartinDr.,Baltimore,MD21218([e-mail])(∗OnbehalfoftheHigh-zSupernovaSearchTeam)Abstract.WereviewtheuseofTypeIasupernovaeforcosmologicaldistancedeter-minations.Low-redshiftSNeIa(z∼0.1)demonstratethattheHubbleexpansionislinear,thatH0=65±2(statistical)kms−1Mpc−1,andthatthepropertiesofdustinothergalaxiesaresimilartothoseofdustintheMilkyWay.Wefindthatthelightcurvesofhigh-redshift(z=0.3–1)SNeIaarestretchedinamannerconsistentwiththeexpansionofspace;similarly,theirspectraexhibitslowertemporalevolution(byafactorof1+z)thanthoseofnearbySNeIa.Theluminositydistancesofourfirstsetof16high-redshiftSNeIaare,onaverage,10–15%fartherthanexpectedinalowmass-density(ΩM=0.2)universewithoutacosmologicalconstant.Preliminaryanalysisofoursecondsetof9SNeIaisconsistentwiththis.Ourworksupportsmodelswithpositivecosmologicalconstantandacurrentaccelerationoftheexpansion.Weaddressthemainpotentialsourcesofsystematicerror;atpresent,noneofthemappearstoreconcilethedatawithΩΛ=0andq0≥0.ThedynamicalageoftheUniverseisestimatedtobe14.2±1.7Gyr,consistentwiththeagesofglobularstarclusters.INTRODUCTIONSupernovae(SNe)comeintwomainvarieties(seereference[1]forareview).ThosewhoseopticalspectraexhibithydrogenareclassifiedasTypeII,whilehydrogen-deficientSNearedesignatedTypeI.SNeIarefurthersubdividedac-cordingtotheappearanceoftheearly-timespectrum:SNeIaarecharacterizedbystrongabsorptionnear6150˚A(nowattributedtoSiII),SNeIblackthisfeaturebutinsteadshowprominentHeIlines,andSNeIchaveneithertheSiIInortheHeIlines.SNeIaarebelievedtoresultfromthethermonucleardisruptionofcarbon-oxygenwhitedwarfs,whileSNeIIcomefromcorecollapseinmassivesupergiantstars.ThelattermechanismprobablyproducesmostSNeIb/Icaswell,buttheprogenitorstarspreviouslylosttheirouterlayersofhydrogenorevenhelium.IthaslongbeenrecognizedthatSNeIamaybeveryusefuldistanceindicatorsforanumberofreasons;see[2,3],andreferencestherein).(1)Theyareexceedinglyluminous,withpeakabsolutebluemagnitudesaveraging−19.2iftheHubblecon-stant,H0,is65kms−1Mpc−1.(2)“Normal”SNeIahavesmalldispersionamongtheirpeakabsolutemagnitudes(σ∼0.3mag).(3)Ourunderstandingofthepro-genitorsandexplosionmechanismofSNeIaisonareasonablyfirmphysicalbasis.(4)LittlecosmicevolutionisexpectedinthepeakluminositiesofSNeIa,anditcanbemodeled.ThismakesSNeIasuperiortogalaxiesasdistanceindicators.(5)OnecanperformlocaltestsofvariouspossiblecomplicationsandevolutionaryeffectsbycomparingnearbySNeIaindifferentenvironments.ResearchonSNeIainthe1990shasdemonstratedtheirenormouspotentialascosmologicaldistanceindicators.Althoughtherearesubtleeffectsthatmustindeedbetakenintoaccount,itappearsthatSNeIaprovideamongthemostaccuratevaluesofH0,q0(thedecelerationparameter),ΩM(thematterdensity),andΩΛ(thecosmologicalconstant,Λc2/3H20).Therearenowtwomajorteamsinvolvedinthesystematicinvestigationofhigh-redshiftSNeIaforcosmologicalpurposes.The“SupernovaCosmologyProject”(SCP)isledbySaulPerlmutteroftheLawrenceBerkeleyLaboratory,whilethe“High-ZSupernovaSearchTeam”(HZT)isledbyBrianSchmidtoftheMt.StromloandSidingSpringsObservatories.Oneofus(A.V.F.)hasworkedwithbothteams,buthisprimaryallegianceisnowwiththeHZT.InthispaperwepresentresultsfromtheHZT.HOMOGENEITYANDHETEROGENEITYThetraditionalwayinwhichSNeIahavebeenusedforcosmologicaldistancedeterminationshasbeentoassumethattheyareperfect“standardcandles”andtocomparetheirobservedpeakbrightnesswiththoseofSNeIaingalaxieswhosedistanceshavebeenindependentlydetermined(e.g.,Cepheids).TherationaleisthatSNeIaexhibitrelativelylittlescatterintheirpeakblueluminosity(σB≈0.4–0.5mag;[4]),andevenlessif“peculiar”orhighlyreddenedobjectsareeliminatedfromconsiderationbyusingacolorcut.Moreover,theopticalspectraofSNeIaareusuallyratherhomogeneous,ifcareistakentocompareobjectsatsimilartimesrelativetomaximumbrightness([5]andreferencestherein).Over80%ofallSNeIadiscoveredthroughtheearly1990swere“normal”[6].FromaHubblediagramconstructedwithunreddened,moderatelydistantSNeIa(z∼0.1)forwhichpeculiarmotionsshouldbesmallandrelativedistances(asgivenbyratiosofredshifts)areaccurate,Vaughanetal.[7]findthatMB(max)=(−19.74±0.06)+5log(H0/50)mag.(1)Inaseriesofpapers,Sandageetal.[8]andSahaetal.[9]combinesimilarrelationswithHubbleSpaceTelescope(HST)CepheiddistancestothehostgalaxiesofsevenSNeIatoderiveH0=57±4kms−1Mpc−1.Overthepastdecadeithasbecomeclear,however,thatSNeIadonotconstituteaperfectlyhomogeneoussubclass(e.g.,[1,10]).Inretrospectthisshouldhavebeenobvious:theHubblediagramforSNeIaexhibitsscatterlargerthanthephoto-metricerrors,thedispersionactuallyriseswhenreddeningcorrectionsareapplied(undertheassumptionthatallSNeIahaveuniform,veryblueintrinsiccolorsatmaximum;[11,12]),andtherearesomesignificantoutlierswhoseanomalousmag-nitudescannotpossiblybeexplainedbyextinctionalone.Spectroscopicandphotometricpeculiaritieshavebeennotedwithincreasingfre-quen