Probing Dark Matter and Dark Energy with Space-Bas

arXiv:astro-ph/0403229v110Mar2004Gravitationallensing:auniquetoolforcosmologyASPConferenceSeries,Vol.xxx,2003D.Valls–GabaudandJ.–P.Kneib(eds.)ProbingDarkMatterandDarkEnergywithSpace-BasedWeakLensingRichardMasseyInstituteofAstronomy,MadingleyRoad,CambridgeCB3OHA,UKAlexandreRefregierServiced’Astrophysique,CEA/Saclay,91191GifsurYvette,FranceJasonRhodesCaliforniaInstituteofTechnology,1201E.CaliforniaBlvd.,Pasadena,CA91125,USAAbstract.Weaklensingprovidesadirectmeasureofthedistributionofmassintheuniverse,andisthereforeauniquelypowerfulprobeofdarkmat-ter.Weaklensingcanalsobeusedtomeasurethetwinphenomenonofdarkenergy,viaitseffectuponthecosmologicalgrowthrateofstruc-tures.Essentialforthistechniquearewell-resolvedimagesofbackgroundgalaxiesouttolargedistances.Asaconcreteexampleofthesurveysthatwillbecomeavailablebytheendofthedecade,weconsidertheplannedSupernova/AccelerationProbe(SNAP)spacetelescope.Detailedsimula-tionsofspace-basedimages,manufacturedusingtheshapeletsformalism,enableustoquantitativelypredictthefuturesensitivitytoweaklensingshear.Thehighnumberdensityofgalaxiesresolvedfromspacewillen-ablemapsofdarkmattertobeproducedintwoandthreedimensions,witharesolutionsuperiortothatfromtheground.Suchobservationswillalsoaffordreducedsystematicsforhigh-precisionmeasurementsofweaklensingstatistics.Thesewillbeusedtosettightconstraintsoncosmologicalparameters.Inparticular,theparameterforequationofstateofdarkenergy,w,willbemeasuredusingweaklensingwithapre-cisioncomparabletoandsomewhatorthogonaltoconstraintsfromothermethods.1.IntroductionDuringitsjourneytoourtelescopes,thelightfrombackgroundgalaxiesisde-flectedbyforegroundmassconcentrations,whichactasgravitationallensesalongtheline-of-sight.Theobserveddistortionsintheshapesofdistantgalax-iesaredirectlyrelatedtothegravitationalpotentialofforegroundstructures,andthereforetotheirmass.Thiseffectisindependentofthenatureorstateoftheforegroundmassandthereforetracesthedistributionofotherwiseinvisible12Massey,Refregier,&Rhodesdarkmatter.Inrecentyears,manygroupshavemeasuredcoherentdistortionsintheshapesofgalaxiestomeasurethemassdistributionofclusters(e.g.Joffreetal.2000;Clowe,Trentham&Tonry2001;Dahleetal.2002).Othergroupshavemeasuredthedistributionoflarge-scalestructureinrandomregionsintheskytosetconstraintsoncosmologicalparameters(e.g.vanWaerbekeetal.2002;Baconetal.2002;Hoekstraetal.2002;Jarvisetal.2003).Thesecondmomentoftheresulting“cosmicshear”fieldalreadyprovidesaconstraintontheampli-tudeofthemasspowerspectrumΩmσ0.58whichiscomparabletotheconstraintssetbymoretraditionalmethodssuchastheabundanceofx-rayselectedclusters(e.g.Viana&Liddle1999;Pierpaoli,Scott&White2001).ThehigherordermomentsoftheshearfieldpromisetobreaktheΩm-σ8degeneracy(Bernardeauetal.etal.1997)inthenextgenerationoflargecosmicshearsurveys,e.g.CFHTLegacySurvey(CFHTLS,Mellieretal.2000),LSST(Tysonetal.2002a)andPan-STARRS(Kaiser,Tonry&Luppino2000).Forreviewsofthetechniquesandcurrentstatusofweaklensingmeasurements,otherthanthisvolume,seeMellieretal.(2001),Hoekstraetal.(2002),Wittman(2002)andRefregier(2003).Therapidgrowthofdedicatedsurveysandimprovementsinshearmeasure-mentmethodshasmeantthaterrorsoncosmologicalparametersfromcosmicsheararenowlimitedbysystematicratherthanstatisticalerrors.Weaklens-inghastheadvantagethatitssystematicsarisefromimperfectinstrumentsandimageanalysis,ratherthanunknownphysics(suchasthemass-temperaturerelationwhichdominatesx-rayselectedclustersamples(Huterer&White,2002)).Imageanalysistoolsarecurrentlymakingimportantadvances,fromthewell-testedmethodofKaiser,Squires&Broadhurst(1995;KSB),withworkbyRhodes,Refregier&Groth(2000;RRG),Bernstein&Jarvis(2003)and“shapelets”byRefregier(2003a).Meetingtheengineeringchallenge,however,willinevitablyrequirerisingabovetheatmosphere.ConvolutionwithalargePoint-SpreadFunction(PSF)thatislargerthanthemajorityofdistant(andthereforesmall)objectsirretrievablydestroysanyinformationtheirshapeshadcontained.Thislimitsthenumberdensityandredshiftofgalaxiesthatcanbeusedtomeasurecosmicshearwithground-basedlensingsurveys.Convolutionwiththetime-varyingatmosphericPSFfurtherlimitstherecoveryoftheshapeinformationoftheremaininggalaxiestotheaccuracywithwhichthePSFcanbemodelledfromoneexposuretothenext.Intheseproceedings,wediscussthepotentialadvancesthatawide-fieldimagerfromspaceoffersforweaklensing.AdoptingtheplannedSupernova/AccelerationProbe(SNAP)missionasaconcreteexample,wepresentdetailedshapelet-basedsimulationsusedtoestimatethelensingsensitivityofsuchobser-vations.Mostinterestinglyforcosmology,SNAP’senhancedspatialresolutionwillcapturetheshapesofbackgroundgalaxiesuptoamuchgreaterdistance.Space-basedsurveyswillthereforeprobetheevolutionofthemassdistribution,andthegrowthofstructures,overalargefractionoftimeintheevolutionoftheuniverse.ThisevolutionprovidesstrongconstraintsonbothΩmandtheequationofstateparameterofdarkenergyw.Asdiscoveredfromobserva-tionsoftypeIasuperovæ(e.g.Perlmutteretal.1999),thisdarkenergyor“quintessence”isacceleratingtheexpansionoftheuniverseandhenceretard-ingthegrowthofthemasspowers