Dark matter.docx

Dark matter.docx

《Dark matter.docx》由会员分享，可在线阅读，更多相关《Dark matter.docx（22页珍藏版）》请在冰豆网上搜索。

Darkmatter

Darkmatter

FromWikipedia,thefreeencyclopedia

Nottobeconfusedwith antimatter, darkenergy, darkfluid,or darkflow.Forotheruses,see DarkMatter（disambiguation）

BeyondtheStandardModel

Simulated LargeHadronCollider CMSparticledetectordatadepictingaHiggsboson producedbycollidingprotonsdecayingintohadronjetsandelectrons

StandardModel

Evidence[hide]

∙Hierarchyproblem

∙Darkmatter

∙Cosmologicalconstantproblem

∙StrongCPproblem

∙Neutrinooscillation

Theories[show]

Supersymmetry[show]

Quantumgravity[show]

Experiments[show]

∙v

∙t

∙e

Partofaserieson

Physicalcosmology

∙Universe

∙BigBang

∙Ageoftheuniverse

∙Chronologyoftheuniverse

Earlyuniverse[show]

Expandinguniverse[show]

Structureformation[show]

Futureofuniverse[show]

Components[hide]

∙Lambda-CDMmodel

∙Darkenergy

∙Darkmatter

History[show]

Experiments[show]

Scientists[show]

Socialimpact[show]

∙

 Astronomyportal

∙

 Category:

Physicalcosmology

∙v

∙t

∙e

Estimateddistributionofmatterandenergyintheuniverse,today（top）andwhenthe CMB wasreleased（bottom）

In astronomy and cosmology, darkmatter isatypeof matter hypothesizedtoaccountforalargepartofthetotal massinthe universe.Darkmattercannotbeseendirectlywithtelescopes;evidentlyitneither emits norabsorbslightorother electromagneticradiation atanysignificantlevel.[1] Instead,itsexistenceandpropertiesareinferredfromitsgravitationaleffectsonvisiblematter,radiation,andthelarge-scalestructureoftheuniverse.AccordingtothePlanckmissionteam,andbasedonthe standardmodelofcosmology,thetotal mass–energy ofthe universe contains4.9%ordinarymatter,26.8%darkmatterand68.3% darkenergy.[2][3][4] Thus,darkmatterisestimatedtoconstitute84.5%ofthetotalmatterintheuniverseand26.8%ofthetotalcontentoftheuniverse.[5]

Darkmattercametotheattentionofastrophysicistsduetodiscrepanciesbetweenthemassoflargeastronomicalobjectsdeterminedfromtheirgravitationaleffectsandthemasscalculatedfromthe"luminousmatter"theycontain:

stars,gas,anddust.Itwasfirstpostulatedby JanOort in1932toaccountfortheorbitalvelocitiesofstarsinthe MilkyWay andby FritzZwicky in1933toaccountforevidenceof"missingmass"intheorbitalvelocitiesof galaxies in clusters.Subsequently,manyotherobservationshaveindicatedthepresenceofdarkmatterintheuniverse,includingtherotationalspeedsofgalaxies by VeraRubin,[6] inthe1960s–1970s, gravitationallensing ofbackgroundobjectsbygalaxyclusterssuchasthe BulletCluster,thetemperaturedistributionofhotgasingalaxiesandclustersofgalaxies,andmorerecentlythepatternofanisotropiesinthe cosmicmicrowavebackground.Accordingtoconsensusamongcosmologists,darkmatteriscomposedprimarilyofanotyetcharacterizedtypeof subatomicparticle.[7][8] Thesearchforthisparticle,byavarietyofmeans,isoneofthemajoreffortsin particlephysics today.[9]

Althoughtheexistenceofdarkmatterisgenerallyacceptedbythemainstreamscientificcommunity,thereisnogenerallyagreeddirectdetectionofit.Othertheories,including MOND and TeVeS,aresomealternativetheoriesofgravityproposedtotrytoexplaintheanomaliesforwhichdarkmatterisintendedtoaccount.

Contents

  [hide] 

∙1 Overview

∙2 Baryonicandnonbaryonicdarkmatter

∙3 Observationalevidence

o3.1 Galaxyrotationcurves

o3.2 Velocitydispersionsofgalaxies

o3.3 Galaxyclustersandgravitationallensing

o3.4 Cosmicmicrowavebackground

o3.5 Skysurveysandbaryonacousticoscillations

o3.6 TypeIasupernovaedistancemeasurements

o3.7 Lyman-alphaforest

o3.8 Structureformation

∙4 Historyofthesearchforitscomposition

o4.1 Colddarkmatter

o4.2 Warmdarkmatter

o4.3 Hotdarkmatter

o4.4 Mixeddarkmatter

∙5 Detection

o5.1 Directdetectionexperiments

o5.2 Indirectdetectionexperiments

∙6 Alternativetheories

o6.1 Modifiedgravitylaws

∙7 Popularculture

∙8 Seealso

∙9 References

∙10 Furtherreading

∙11 Externallinks

Overview[edit]

Darkmatter'sexistenceisinferredfrom gravitational effectsonvisiblematterand gravitationallensing ofbackgroundradiation,andwasoriginallyhypothesizedtoaccountfordiscrepanciesbetweencalculationsofthemassof galaxies,clustersofgalaxiesandtheentireuniversemadethrough dynamical and generalrelativistic means,andcalculationsbasedonthemassofthevisible"luminous"mattertheseobjectscontain:

 stars andthegasanddustofthe interstellarand intergalactic medium.[1]

Themostwidelyacceptedexplanationforthesephenomenaisthatdarkmatterexistsandthatitismostprobably[7]composedof weaklyinteractingmassiveparticles （WIMPs）thatinteractonlythroughgravityandthe weakforce.Alternativeexplanationshavebeenproposed,andthereisnotyetsufficientexperimentalevidencetodeterminewhichiscorrect.Manyexperimentstodetectproposeddarkmatterparticlesthroughnon-gravitationalmeansareunderway.[9]

Accordingtoobservationsofstructureslargerthan starsystems,aswellas BigBang cosmologyinterpretedundertheFriedmannequations andthe Friedmann–Lemaître–Robertson–Walkermetric,darkmatteraccountsfor26.8%ofthe mass-energy contentofthe observableuniverse.Incomparison,ordinary（baryonic）matteraccountsforonly4.9%ofthemass-energycontentoftheobservableuniverse,withtheremainderbeingattributableto darkenergy.[4] Fromthesefigures,matteraccountsfor31.7%ofthemass-energycontentoftheuniverse,and84.5%ofthematterisdarkmatter.[5]

Darkmatterplaysacentralroleinstate-of-the-artmodelingofcosmic structureformation and Galaxyformationandevolution andhasmeasurableeffectsonthe anisotropies observedinthe cosmicmicrowavebackground.Alltheselinesofevidencesuggestthatgalaxies,clustersofgalaxies,andtheuniverseasawholecontainfarmorematterthanthatwhichinteractswithelectromagneticradiation.[10]

Importantasdarkmatteristhoughttobeinthecosmos,directevidenceofitsexistenceandaconcreteunderstandingofitsnaturehaveremainedelusive.Thoughthetheoryofdarkmatterremainsthemostwidelyacceptedtheorytoexplaintheanomaliesinobservedgalacticrotation,somealternativetheoreticalapproacheshavebeendevelopedwhichbroadlyfallintothecategoriesofmodifiedgravitationallawsandquantumgravitationallaws.[11]

Baryonicandnonbaryonicdarkmatter[edit]

Fermi-LAT observationsofdwarfgalaxiesprovidenewinsightsondarkmatter.

Therearethreeseparatelinesofevidencethatthemajorityofdarkmatterisnotmadeofbaryons,ordinarymatterincludingprotons,electronsandatoms:

∙Thetheoryof BigBangnucleosynthesis,whichveryaccuratelypredictstheobserved abundanceofthechemicalelements,[12] predictsthatbaryonicmatteraccountsforaround4–5percentofcriticaldensityoftheUniverse.Incontrast,evidencefrom large-scalestructure andotherobservationsindicatesthatthetotalmatterdensityissubstantiallyhigherthanthis.

∙Largeastronomicalsearchesforgravitational microlensing,includingtheMACHO,EROSandOGLEprojects,haveshownthatonlyasmallfractionofthedarkmatterintheMilkyWaycanbehidingindarkcompactobjects;theexcludedrangecoversobjectsabovehalftheEarth'smassupto30solarmasses,excludingnearlyalltheplausiblecandidates.

∙Detailedanalysisofthesmallirregularities（anisotropies）inthe cosmicmicrowavebackground observedbyWMAP and Planck showsthataroundfive-sixthsofthetotalmatterisinaformwhichdoesnotinteractsignificantlywithordinarymatterorphotons.

Asmallproportionofdarkmattermaybe baryonicdarkmatter:

astronomicalbodies,suchas massivecompacthaloobjects,thatarecomposedofordinarymatterbutwhichemitlittleornoelectromagneticradiation.StudyofnucleosynthesisintheBigBangproducesanupperboundontheamountofbaryonicmatterintheuniverse,[13]whichindicatesthatthevastmajorityofdarkmatterintheuniversecannotbe baryons,andthusdoesnotform atoms.Italsocannotinteractwithordinarymattervia electromagneticforces;inparticular,darkmatterparticlesdonotcarryany electriccharge.

Candidatesfornonbaryonicdarkmatterarehypotheticalparticlessuchas axions,or supersymmetric particles;neutrinoscanonlyformasmallfractionofthedarkmatter,duetolimitsfrom large-scalestructure andhigh-redshiftgalaxies.Unlikebaryonicdarkmatter,nonbaryonicdarkmatterdoesnotcontributetotheformationofthe elements intheearlyuniverse（"BigBangnucleosynthesis"）[7] andsoitspresenceisrevealedonlyviaitsgravitationalattraction.Inaddition,iftheparticlesofwhichitiscomposedaresupersymmetric,theycanundergo annihilation interactionswiththemselves,possiblyresultinginobservableby-productssuchas gammarays andneutrinos（"indirectdetection"）.[14]

Nonbaryonicdarkmatterisclassifiedintermsofthemassoftheparticle（s）thatisassumedtomakeitup,and/orthetypicalvelocitydispersionofthoseparticles（sincemoremassiveparticlesmovemoreslowly）.Therearethreeprominenthypothesesonnonbaryonicdarkmatter,called colddarkmatter （CDM）, warmdarkmatter （WDM）,and hotdarkmatter （HDM）;somecombinationoftheseisalsopossible.Themostwidelydiscussedmodelsfornonbaryonicdarkmatterarebasedonthecolddarkmatterhypothesis,andthecorrespondingparticleismostcommonlyassumedtobea weaklyinteractingmassiveparticle （WIMP）.Hotdarkmattermayinclude（massive）neutrinos,butobservationsimplythatonlyasmallfracti