The Dark Matter Problem

TheThe DarkDark MatterMatter ProblemProblem OutlineOutline

WhatWhat isis darkdark matter?matter? HowHow muchmuch darkdark mattermatter isis therethere inin thethe Universe?Universe? EvidenceEvidence ofof darkdark mattermatter ViableViable darkdark mattermatter candidatescandidates ColdCold darkdark mattermatter (CDM)(CDM) ProblemsProblems withwith CDMCDM SearchSearch strategiesstrategies andand possiblepossible detectionsdetections AlternativesAlternatives toto darkdark mattermatter WhatWhat isis DarkDark Matter?Matter?

Dark Matter Luminous Matter FirstFirst detectiondetection ofof darkdark mattermatter

FritzFritz ZwickyZwicky (1933):(1933): DarkDark mattermatter inin thethe ComaComa ClusterCluster HowHow MuchMuch DarkDark MatterMatter isis ThereThere inin TheThe Universe?Universe?

ΩΩ == ρρ // ρρ Μ Μ c ~2% RecentRecent measurements:measurements: (Luminous) Ω ∼ 0.3, Ω ∼ 0.7 ΩΜ ∼ 0.3, Ω Λ ∼ 0.7 ΩΩ ∼∼ 0.0050.005 Lum ~98% (Dark) HowHow DoDo WeWe KnowKnow ThatThat itit Exists?Exists?

CosmologicalCosmological ParametersParameters ++ InventoryInventory ofof LuminousLuminous materialmaterial DynamicsDynamics ofof galaxiesgalaxies DynamicsDynamics andand gasgas propertiesproperties ofof galaxygalaxy clustersclusters GravitationalGravitational LensingLensing DynamicsDynamics ofof GalaxiesGalaxies II

Galaxy ≈ Stars + Gas + Dust + Supermassive Black Hole + Dark Matter DynamicsDynamics ofof GalaxiesGalaxies IIII

Visible galaxy

Observed

Vrot

Expected R

R Dark matter halo Visible galaxy DynamicsDynamics ofof GalaxyGalaxy ClustersClusters

Balance between kinetic and potential energy → Virial theorem: v2 R M = vir G HotHot GasGas inin GalaxyGalaxy ClustersClusters

High mass required to keep the hot gas from leaving the cluster!

If gas in hydrostatic equilibrium → Luminosity and temperature profile → mass profile X-ray gas, T=10 7—10 8 K GravitationalGravitational LensingLensing GravitationalGravitational LensingLensing IIII BaryonicBaryonic andand NonNon --BaryonicBaryonic DarkDark MatterMatter II Baryons: Ordinary matter made out of three quarks, like protons and neutrons BBNS modelling + measurements of primordial abundances → Ω ≈ 0.045 or CMBR analysis baryons Ω ≈ → Baryonic 0.045

Ω ≈ → Non-baryonic 0.25

Ω Ω Ω ≈ M = Baryonic + Non-baryonic 0.3 BaryonicBaryonic andand NonNon --BaryonicBaryonic DarkDark MatterMatter IIII StillStill missingmissing inin thethe locallocal Universe:Universe: AboutAbout 1/31/3 ofof thethe baryonsbaryons →→ ΩΩ DM, baryonic ~~ 0.0150.015 ButBut note:note: TheThe missingmissing baryonsbaryons maymay havehave beenbeen detecteddetected atat highhigh redshiftredshift EssentiallyEssentially allall ofof thethe nonnon --baryonsbaryons →→ ΩΩ ΩΩ DM, non -baryonic ~~ 0.250.25 (assuming(assuming M=0.3)=0.3) MACHOsMACHOs andand WIMPsWIMPs

MACHOMACHO == MAssiveMAssive CompactCompact HaloHalo ObjectObject WIMPWIMP == WeaklyWeakly InteractingInteracting MassiveMassive ParticleParticle

But beware of misconceptions! AA FewFew ViableViable DarkDark MatterMatter CandidatesCandidates Baryonic Non-baryonic * • Faint stars • Supersymmetric particles

• Fractal H 2 • Axions • Warm intergalactic gas • Sterile neutrinos • Rydberg matter • Primordial black holes • Preon stars • Quark nuggets • Mirror matter • Matter in parallel branes * or evading current constraints on the cosmic baryon density HotHot andand ColdCold DarkDark MatterMatter

HotHot DarkDark MatterMatter (HDM)(HDM) RelativisticRelativistic atat decouplingdecoupling

ColdCold DarkDark MatterMatter (CDM)(CDM) NonNon --relativisticrelativistic atat decouplingdecoupling TheThe standardstandard modelmodel forfor thethe nonnon -- baryonicbaryonic darkdark mattermatter SuccessfulSuccessful inin explainingexplaining thethe formationformation ofof largelarge scalescale structurestructure AdditionalAdditional AssumedAssumed CDMCDM PropertiesProperties

CollisionlessCollisionless –– interactsinteracts mainlymainly throughthrough gravitygravity DissipationlessDissipationless –– cannotcannot coolcool byby radiatingradiating photonsphotons LongLong --livedlived particlesparticles BehavesBehaves asas perfectperfect fluidfluid onon largelarge scalesscales AdiabaticAdiabatic primordialprimordial densitydensity perturbations,perturbations, followingfollowing aa scalescale --invariantinvariant powerpower spectrumspectrum ProblemsProblems withwith CDMCDM

DarkDark halohalo densitydensity profilesprofiles DarkDark halohalo substructuresubstructure DarkDark halohalo shapesshapes TheThe angularangular momentummomentum problemproblem DarkDark HaloHalo DensityDensity ProfilesProfiles II

Dark halo Visible galaxy

Log Vrot Density

Radius Log Radius Spectroscopy → Rotation Curve → Halo Density Profile DarkDark HaloHalo DensityDensity ProfilesProfiles IIII

Predicted by the Cold Dark Matter Scenario (density cusp)

Favoured by observations (density core) DarkDark HaloHalo DensityDensity ProfilesProfiles IIIIII

ButBut therethere areare plentyplenty ofof complicationscomplications …… NonNon --sphericalspherical darkdark mattermatter halos?halos? CentralCentral partpart dominateddominated byby darkdark baryonsbaryons insteadinstead ofof CDM?CDM? BestBest targettarget galaxiesgalaxies dodo notnot sitsit inin typicaltypical darkdark halos?halos? NN--bodybody simulationssimulations responsibleresponsible forfor thethe predictedpredicted CDMCDM halohalo profileprofile predictionprediction notnot reliable?reliable? DarkDark HaloHalo SubstructureSubstructure II Dark halos are not perfectly smooth! ≤ × Msubhalos 0.1 Mhalo DarkDark HaloHalo SubstructureSubstructure IIII Should not dwarf galaxies form inside the subhalos?

Naïve expectation Observed A factor of 10—100 too few satellite galaxies around the Milky Way! DarkDark HaloHalo SubstructureSubstructure IIIIII

TheThe solution:solution: DarkDark galaxies?galaxies? DarkDark galaxy:galaxy: AA darkdark subhalosubhalo whichwhich eithereither lackslacks baryons,baryons, oror insideinside whichwhich thethe baryonsbaryons formform veryvery fewfew starsstars PossiblePossible (but(but veryvery shaky)shaky) detectionsdetections exist:exist: GalaxiesGalaxies withwith veryvery highhigh massmass --toto --lightlight ratiosratios PossiblePossible gravitationalgravitational lensinglensing detectionsdetections HowHow toto detectdetect halohalo substructuresubstructure Dark halos can cause image splitting in quasars on angular scales of ~ 1 arcsecond (macrolensing)

Observer Multiply-imaged Quasar Lens galaxy (with dark halo) HowHow toto detectdetect halohalo substructuresubstructure IIII Halo substructure can cause additional splitting of each image on angular scales of ~0.01 arcseconds (mesolensing)

Halo substructure

Macrolesing (1”) Mesolensing (≤0.01”)

Zoom-in AlternativesAlternatives toto CDMCDM

WarmWarm darkdark mattermatter MixedMixed darkdark mattermatter (cold(cold ++ hot)hot) SelfSelf --interactinginteracting darkdark mattermatter SelfSelf --annihilatingannihilating oror decayingdecaying darkdark mattermatter FuzzyFuzzy darkdark mattermatter HowHow toto SearchSearch forfor thethe DarkDark MatterMatter ParticlesParticles

GravitationalGravitational microlensingmicrolensing byby MACHOsMACHOs WIMPWIMP directdirect detectiondetection RecoilRecoil inin detectordetector AnnularAnnular modulationmodulation WIMPWIMP indirectindirect detectiondetection CosmicCosmic raysrays fromfrom annihilatingannihilating WIMPsWIMPs NeutrinosNeutrinos fromfrom WIMPWIMP annihilationannihilation inin Sun/EarthSun/Earth PhotonsPhotons (gamma,(gamma, radio)radio) fromfrom WIMPWIMP annihilationannihilation inin thethe GalacticGalactic CentreCentre GravitationalGravitational MicrolensingMicrolensing byby MACHOsMACHOs

LIGHT SOURCE

Obs! Fel bild! PossiblePossible detectionsdetections II MACHOMACHO project:project: monitoringmonitoring ofof 1212 ××1010 6 starsstars inin thethe LargeLarge MagellanicMagellanic CloudCloud LMC

MACHO

Milky Way

-1 DetectionDetection ofof MM compact ~10~10 MMsolar , constitutingconstituting ≈≈20%20% ofof thethe darkdark halohalo DirectDirect WIMPWIMP detectiondetection

WIMP

Nuclear recoil

Detector

Problem: Background of other rare reactions DirectDirect WIMPWIMP DetectionDetection inin AncientAncient MicaMica

WIMP recoils causes chemical changes in ancient mica → Natural detector with integration time ~ 1 Gyr AnnularAnnular ModulationModulation

WIMP wind from the dark halo should show seasonal variations! PossiblePossible detectionsdetections IIII WIMP search by the DAMA experiment Detected annular modulation signature → ≥10 -3 of halo fraction in WIMPs IndirectIndirect WIMPWIMP detectiondetection byby NeutrinosNeutrinos fromfrom thethe Sun/EarthSun/Earth

WIMP, χ Neutrino detector ν

Earth χ + χ→ν

WIMPs may accumulate in the potential well of the Sun/Earth, and annihilate to produce neutrinos IsIs ThereThere nono AlternativeAlternative toto DarkDark Matter?Matter? ”I invite the reader (...) to test whether he/she is not left with some uneasiness as our wonderful 'standard' cosmology seems in fact to be so far essentially based on a) a Dark Matter we do not detect b) a Dark Energy we do not understand c) a fraction of Baryons we cannot completely find! Yet everything seems to work; isn't this reminiscent of epicycles? “ L. Guzzo (2002) MONDMOND ((MOdifiedMOdified NewtonianNewtonian Dynamics;Dynamics; MilgromMilgrom 1984)1984)

Newtonian dynamics: a=MG/r 2 2 2 MOND: a /a 0=MG/r in the limit of small accelerations → µ 2 (a/a 0)a=MG/r where µ(x) ≈ 1 when x » 1 µ(x) ≈ x when x « 1 MONDMOND IIII

From Stacy McGaugh’s homepage:

”’You do not know the Power of the Dark Side. Join me, and together we can use dark matter to make galaxy rotation curves flat.’ I often hear this sort of paternalistic line from well intentioned senior astronomers. My response is the same as Luke's, with analogous consequences for my career.” ProblemsProblems withwith MONDMOND

OriginalOriginal MOND:MOND: PhenomenologicalPhenomenological extensionextension ofof NewtonianNewtonian gravitygravity →→ NoNo predictionspredictions forfor e.g.e.g. gravitationalgravitational lensinglensing oror cosmiccosmic expansionexpansion Solved by Bekenstein (2004)! FailsFails toto explainexplain thethe dynamicsdynamics ofof galaxygalaxy clustersclusters –– somesome darkdark mattermatter isis stillstill requiredrequired FailsFails toto explainexplain differencedifference betweenbetween systemssystems ofof similarsimilar baryonicbaryonic masses,masses, e.g.e.g. globularglobular clustersclusters andand dwarfdwarf galaxiesgalaxies LateLate breakingbreaking news:news: FailsFails toto explainexplain observedobserved gravitationalgravitational imageimage splittingsplitting (a(a 0 hashas toto vary)vary)