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DarkMatterintheUniverse?A(mostly)PictorialIntroductionEdwardDawUniversityofSheffieldASpiralGalaxy-NGC3198InUrsaMajor(GreatBear/Plough/BigDipper)constellation.Distanceis9.1Mpc,or30millionlightyears.Forcomparison,distancefromEarthtothegalacticcentreisafactorof~1000less,~30thousandlightyears33thousandlightyearsDust,Gas,andDebrisSombreroGalaxy-NGC4594,inVirgo.HubbleSpaceTelescopeImage://antwrp.gsfc.nasa.gov/apodInOrion,0.9mtelescope,KittPeakNationalObservatory,U.S.A.Imageis2.7by1.8lightyears,distanceis1.6thousandlightyearsPinkbackgroundishydrogenemmissionfromIC434emmissionnebula.Question:Isthemassintheuniverseallobservablethroughemmissionorabsorbsionofelectromagneticradiation?DarkMatter...ismatterthatdoesnotshineorabsorblight,andhasthereforeescapeddirectdetectionbyelectromagnetictransducersliketelescopes,radioantennas,x-raysatellites...Itturnsoutthatthereisstrongexperimentalevidencethatthereismorethan4timesasmuchdarkmatterasluminousmatterintheobservableuniverseEvidenceforDarkMatterUsethefactthatmassiveobjects,eveniftheyemitnolight,exertgravitationalforcesonothermassiveobjects.Studythemotions(dynamics)ofvisibleobjectslikestarsingalaxies,andlookforeffectsthatarenotexplicablebythemassoftheotherlightemmittingorabsorbingobjectsaroundthem.m1m2r12RotationofStarsaroundGalacticCentresWecanmeasurehowfaststarsrotatearoundgalacticcentresbylookingatthefrequencyshiftofknownspectrallinesoriginatinginthestarsduetotheDopplereffect.Star’smotiontowardsyou,relativetothegalacticcentrealterswavelengthoflightNeedWorld’slargesttelescope...SomeResultsvrDISKRADIUSThisiswhatweexpect.......butherearesometypicalresultsWavelengthshiftsareaboutapartin106.Thesearehardmeasurements.PossibleInterpretations-Maybethereismorematteringalaxiesthatwehavenotobserved.(WHAT?Faintstars?Planets?Rocks?Gas?Dust?ExoticParticles?)-MaybeNewton’slawofgravitationiswrongforverylargedistancesorverysmallaccelerations.(Aalternativetheory(MOND)hasbeenseriouslyproposed,cannotyetrulethisout)Needabout10timesasmuchdarkmatterasvisiblemattertoexplaintherotationcurvediscrepancy!GravitationalLensingofLightBendingoflightingravitationalfieldscanmakelensesoutofmassiveobjectsLENSINGOBJECTUSSOURCENOLENSLENSStrongorcloselens,expectaringoflight,oraringofimagesinthepresenceofthelens.Whennotresolved,expectincreasedintensity.GalaxyCluster0024-1654,HubbleDeepField1.4millionlightyearsReconstructedMatterDistributioninCL0024-1654[Tysonetal.,Ap.J.498L107-110,1998]HowMuchDarkMatterinClusters?Needabout10timesasmuchdarkmatterintheoverallclusterasobservableintheindividualgalaxiestoexplaintheobservedmicrolensingGalaxiesaremadeupofabout10percentvisiblematterand90percentdarkmatter,whosenatureiscurrentlynotunderstood.Galaxyclustersthathavebeenmeasuredaremadeupofabout10percentvisiblematterand90percentdarkmatter,whichmeansthatonagalaxyandclusterscale,onlyafewpercent,perhapslower,ofthematterissolidlyunderstood....ORyoucouldconsidermodifyingNewton’slawofgravitation,butthemodificationsproposedtoexplainbothrotationcurvesandclusterlensingaresocomplicatedthatnobodyexcepttheauthorclaimstounderstandthem!WhatCouldConstitutetheDarkMatter(1)?IDEA1:Rocks-frompebblestogiantplanetslikeJupiter.Ifthereareenoughofthem,theycouldmakeupthedarkmatter.Jupiter-sizeandaboveplanetsareaseriouscontender,andarecalledMACHOsbythecommunity-MAssiveCompactHaloObjects.IDEA2:NeutrinosLight,neutralparticlesofwhichatleastsomehaveasmallmass.Producedinenormousnumbersinstarsandpossiblyatthebigbang.Ifthereareenoughofthem,theycould(maybe)bethedarkmatter.WhatCouldConstitutetheDarkMatter(2)?IDEA3:BlackHolesDon’temitsignificantamountsoflight,canbeverymassive.Wouldneedlotsofthem.IDEA4:CosmicStringsDensefilamentarystructuresthatsometheoriststhinkcouldthreadtheuniverse,givingrisetoitspresent-daylumpiness.Currentlydisfavouredbycosmologicaldata,butmaycomebackintovoguesometime.WhatCouldConstitutetheDarkMatter(3)?IDEA5:AxionsVerylightparticles,massaround1/1,000,000,000,000ofanelectron.Neededforbuildingmostrealisticmodelsoftheneutronfromstandardmodelparticlephysics.Notdetected.Tobethedarkmatter,thereshouldbearound10,000,000,000,000percubiccentimetrehereonEarth.IDEA6:WIMPS(fortherestofthistalk)Particleshavingmassroughlythatofanatomicnucleus,couldbeaslightascarbonorasheavyas7nucleiofxenon.Needafewperlitretoconstitutedarkmatter.Unlikenucleus,onlyinteractWEAKLYwithothermatter,throughthesamemechanismthatisresponsiblefornuclearbeta-decay.DetectableWIMPInteractionsWIMPsinteractWEAKLYwithordinarymatter.Ordinarymatterconsistsofprotons,neutrons,andelectrons.Thinkofthisinteractionasclassical,‘billiardball’scattering,exceptthattheprobabilityofscatteringoccuringisabsolutelytiny.ThebilliardballsaretheincomingWIMPandanatomicnucleus.TheWIMPcollideswiththenucleus,causingittorecoil.INCOMINGWIMPTRAJECTORYDetectingtheWIMPInteractionsThenucleusrecoilsafterbeinghit.Thekineticenergyitgainsisenoughtoionizetheatom,creatingapositivelychargednucleusandafreeelectron.+NUCLEUSe-EXAMPLEDETECTIONSCHEME:Thestrucknucleusispartofamolecu