The large-scale structure of the Universe

arXiv:astro-ph/0604561v127Apr2006Thelarge-scalestructureoftheUniverseV.Springel1,C.S.Frenk2,S.D.M.White11Max-Planck-InstituteforAstrophysics,Karl-Schwarzschild-Str.1,85740Garching,Germany2InstituteforComputationalCosmology,Dep.ofPhysics,Univ.ofDurham,SouthRoad,DurhamDH13LE,UKResearchoverthepast25yearshasledtotheviewthattherichtapestryofpresent-daycosmicstructurearoseduringthefirstinstantsofcreation,whereweakrippleswereim-posedontheotherwiseuniformandrapidlyexpandingprimordialsoup.Over14billionyearsofevolution,theserippleshavebeenamplifiedtoenormousproportionsbygravi-tationalforces,producingever-growingconcentrationsofdarkmatterinwhichordinarygasescool,condenseandfragmenttomakegalaxies.Thisprocesscanbefaithfullymim-ickedinlargecomputersimulations,andtestedbyobservationsthatprobethehistoryoftheUniversestartingfromjust400,000yearsaftertheBigBang.Thepasttwoandahalfdecadeshaveseenenormousadvancesinthestudyofcosmicstructure,bothinourknowledgeofhowitismanifestinthelarge-scalematterdistribution,andinourunderstandingofitsorigin.Anewgenerationofgalaxysurveys–the2-degreeFieldGalaxyRedshiftSurvey,or2dFGRS1,andtheSloanDigitalSkySurvey,orSDSS22–havequantifiedthedistributionofgalaxiesinthelocalUniversewithalevelofdetailandonlengthscalesthatwereunthinkablejustafewyearsago.Surveysofquasarabsorptionandofgravitationallensinghaveproducedqualitativelynewdataonthedistributionsofdiffuseintergalacticgasandofdarkmatter.Atthesametime,observationsofthecosmicmicrowavebackgroundradiation,byshowingustheUniversewhenitwasonlyabout400,000yearsold,havevindicatedboldtheoreticalideasputforwardinthe1980sregardingthecontentsoftheUniverseandthemechanismthatinitiallygeneratedstructureshortlyaftertheBigBang.Thecriticallinkbetweentheearly,near-uniformUniverseandtherichstructureseenatmorerecenttimeshasbeenprovidedbydirectnumericalsimulation.Thishasmadeuseoftheunremitting1increaseinthepowerofmoderncomputerstocreateevermorerealisticvirtualuniverses:simulationsofthegrowthofcosmicstructurethatshowhowastrophysicalprocesseshaveproducedgalaxiesandlargerstructuresfromtheprimordialsoup.Together,theseadvanceshaveledtotheemergenceofa“standardmodelofcosmology”which,althoughseeminglyimplausible,hasneverthelessbeensingularlysuccessful.Figure1strikinglyillustrateshowwellthisstandardmodelcanfitnearbystructure.TheobservationalwedgeplotsatthetopandattheleftshowsubregionsoftheSDSSand2dF-GRS,illustratingthelargevolumetheycoverincomparisontotheground-breakingCenterforAstrophysics(CfA)galaxyredshiftsurvey3carriedoutduringthe1980s(thecentralsmallwedge).Theseslicesthroughthelocalthree-dimensionalgalaxydistributionrevealatremen-dousrichnessofstructure.Galaxies,groupsandclustersarelinkedtogetherinapatternofsheetsandfilamentsthatiscommonlyknownasthe“cosmicweb”4.Ahandfulofparticularlyprominentaggregationsclearlystandoutintheseimages,thelargestcontainingoftheorderof10,000galaxiesandextendingforseveralhundredmillionlightyears.Thecorrespondingwedgeplotsattherightandatthebottomshowsimilarlyconstructedsurveysofavirtualuni-verse,theresultofasimulationofthegrowthofstructureandoftheformationofgalaxiesinthecurrentstandardmodelofcosmology.Theexamplesshownwerechosenamongasetofrandom“mocksurveys”tohavelargestructuresinsimilarpositionstotherealsurveys.Thesimilarityofstructurebetweensimulationandobservationisstriking,andissupportedbyaquantitativecomparisonofclustering5.Herewereviewwhatwecanlearnfromthisexcellentmatch.Theearly1980sproducedtwoaudaciousideasthattransformedaspeculativeandnotori-ouslyuncertainsubjectintooneofthemostrapidlydevelopingbranchesofphysics.Thefirstwastheproposalthattheubiquitousdarkmatterthatdominateslarge-scalegravitationalforcesconsistsofanew(andstillunidentified)weaklyinteractingelementaryparticle.Becausethese2particlesarerequiredtohavesmallrandomvelocitiesatearlytimes,theyweredubbed“colddarkmatter”orCDM.(Hotdarkmatterisalsopossible,forexampleaneutrinowithamassofafewtensofelectronvolts.Earlycosmologicalsimulationsshowed,however,thatthegalaxydistributioninauniversedominatedbysuchparticleswouldnotresemblethatobserved6.)Thesecondideais“cosmicinflation”7,theproposalthattheUniversegrewexponentiallyformanydoublingtimesperhaps∼1035secondsaftertheBigBang,drivenbythevacuumenergydensityofaneffectivescalarfieldthatrollsslowlyfromafalsetothetruevacuum.Quantumfluctuationsinthis“inflaton”fieldareblownuptomacroscopicscalesandconvertedintogenuineripplesinthecosmicenergydensity.Theseweakseedfluctuationsgrowundertheinfluenceofgravityandeventuallyproducegalaxiesandthecosmicweb.Simplemodelsofinflationpredictthestatisticalpropertiesoftheseprimordialdensityfluctuations:theirFouriercomponentsshouldhaverandomandindependentphasesandanear-scale-invariantpowerspectrum8.InflationalsopredictsthatthepresentUniverseshouldhaveaflatgeometry.Withconcreteproposalsforthenatureofthedarkmatterandfortheinitialfluctuationdistribution,thegrowthofcosmicstructurebecame,forthefirsttime,awell-posedproblemthatcouldbetackledwiththestandardtoolsofphysics.Thebackboneofthecosmicwebistheclumpyyetfilamentarydistributionofdarkmat-ter.Thep