Cosmology AS7009, 2010 Outline Lecture 6 What is dark matter? How much dark matter is there in the Universe? Evidence of dark matter Viable dark matter candidates Cold dark matter (CDM) Problems with CDM Search strategies and possible detections Alternatives to dark matter Covers chapter 8 in Ryden + extra stuff What is Dark Matter? First detection of dark matter Dark Matter Luminous Matter Fritz Zwicky (1933): Dark matter in the Coma Cluster How Do We Know That it How Much Dark Matter is Exists? There in The Universe? Cosmological Parameters + ΩΩ = ρ= ρ / ρ/ ρ Μ ΜΜ cc ~2% Inventory of Luminous material Recent measurements: (Luminous) Dynamics of galaxies ΩΩ ∼ 0.3, Ω ∼ 0.7∼ 0.7 Μ Λ Dynamics and gas properties of ΩΩ ∼ 0.005 LumLum ~98% galaxy clusters (Dark) Gravitational Lensing 1 Dynamics of Galaxies I Dynamics of Galaxies II Visible galaxy Observed Vrot Expected R Galaxy ≈ Stars + Gas + Dust + R Dark matter halo Supermassive Black Hole + Dark Matter Visible galaxy Dynamics of Galaxy Clusters Hot Gas in Galaxy Clusters Balance between High mass required to kinetic and potential keep the hot gas from energy → leaving the cluster! Virial theorem: If gas in hydrostatic → v2 R equilibrium M = Luminosity and vir G temperature profile → mass profile X-ray gas, T=10 7—10 8 K Gravitational Lensing Gravitational Lensing II 2 Suggestion for Literature Exercise: MassMass--toto--LightLight Ratios The Bullet Cluster as a proof* of the existence of dark matter M M Mass-to-light: solar L Lsolar Observed luminosity Different choices for M: → Mtot = Total mass Dynamical mass-to-light ratio Mstars = Mass of stars & stellar remnants → Stellar mass-to-light ratio * Note: Not everybody agrees that this is a proof! MassMass--toto--LightLight Ratios II Baryonic and NonNon--BaryonicBaryonic Dark Matter I What are M/LM/L--ratiosratios good for? Baryons: Ordinary matter made out of three quarks, The massmass--toto--lightlight ratio indicates how dark like protons and neutrons mattermatter--dominateddominated a certain object is. BBNS modelling + measurements of primordial abundances or CMBR analysis → Ω ≈ 0.045 Higher M/L → More dark-matter dominated baryons Ω ≈ → Baryonic 0.045 Typically: (M/L) stars < 10 (from models) But: (M/L) tot ~100 for galaxies Ω ≈ → Non-baryonic 0.25 (M/L) tot ~ 500 for galaxy clusters Ω Ω Ω ≈ (M/L) > (M/L) →→ Dark matter! M = Baryonic + Non-baryonic tottot starsstars 0.3 Baryonic and NonNon--BaryonicBaryonic MACHOs and WIMPs Dark Matter II Still missing in the local Universe: About 1/3 of the baryons →→ ΩΩ MACHO = MAssive Compact Halo Object DM, baryonic ~ 0.015 But note: The missing baryons maymay have WIMP = Weakly Interacting Massive Particle been detected at high redshift Essentially all of the nonnon--baryonsbaryons →→ ΩΩ ΩΩ DM, nonnon--baryonicbaryonic ~ 0.25 (assuming MM=0.3) But beware of misconceptions! 3 A Few Viable Dark Matter Candidates Hot and Cold Dark Matter Baryonic Non-baryonic * Hot Dark Matter (HDM) Faint stars Supersymmetric particles • • Relativistic early on (at decoupling) • Fractal H 2 • Axions • Warm/hot intergalactic gas • Sterile neutrinos Cold Dark Matter (CDM) • Hot gas around galaxies • Primordial black holes NonNon--relativisticrelativistic early on (at decoupling) • Rydberg matter • Preon stars The standard model for the nonnon--baryonicbaryonic • Quark nuggets dark matter • Mirror matter Successful in explaining the formation of • Matter in parallel branes large scale structure • Kaluza-Klein particles * or evading current constraints on the cosmic baryon density Additional Assumed CDM Problems with CDM Properties Collisionless ––interactsinteracts mainly through gravity Dark halo density profiles Dissipationless ––cannotcannot cool by radiating Dark halo substructure photons Dark halo shapes LongLong--livedlived particles Behaves as perfect fluid on large scales The angular momentum problem Adiabatic primordial density perturbations, following a scalescale--invariantinvariant power spectrum More in structure formation lecture! Dark Halo Density Profiles I Dark Halo Density Profiles II Dark halo Visible galaxy Predicted by the Cold Dark Matter Scenario (density cusp) Det går inte att visa bilden. Det finns inte tillräckligt med ledigt minne för att kunna öppna bilden eller så är bilden skadad. Starta om datorn och öppna sedan filen igen. Om det röda X:et fortfarande visas måste du kanske ta bort bilden och sedan infoga den igen. Log V Favoured by rot Density observations (density core) Radius Log Radius Spectroscopy → Rotation Curve → Halo Density Profile 4 Dark Halo Density Profiles III Dark Halo Substructure I The dark halos around galaxies form the merger of smaller halos, But there are plenty of complications… but many remnants of the smaller halos survive → The dark halos of galaxies are not perfectly smooth! NonNon--sphericalspherical dark matter halos? ~10 % of the dark matter is in clumps Central part dominated by dark baryons (a.k.a. subhalos or halo substructure) instead of CDM? Best target galaxies do not sit in typical dark halos? NN--bodybody simulations responsible for the predicted CDM halo profile prediction not reliable? Dark Halo Substructure II Should not dwarf galaxies form inside the subhalos? Naïve expectation Observed A factor of 10—100 too few satellite galaxies around the Milky Way! How to detect halo substructure Dark Halo Substructure III Dark halos can cause image splitting in quasars on angular scales of ~ 1 arcsecond (macrolensing) The solution: Dark galaxies? Dark galaxy: A dark subhalo which either lacks baryons, or inside which the baryons form very few stars Possible detections exist of galaxies with very high massmass--toto--lightlight ratios (M/L ≥≥1000), Observer Multiply-imaged Quasar but not yet in sufficient numbers to solve Lens galaxy (with dark halo) the problem 5 How to detect halo substructure II Halo substructure can cause additional splitting Alternatives to CDM of each image on angular scales of ~0.001 arcseconds (millilensing) Warm dark matter Mixed dark matter (cold + hot) SelfSelf--interactinginteracting dark matter Halo substructure SelfSelf--annihilatingannihilating or decaying dark matter Alternative theories of gravity Macrolensing (1”) Millilensing ( ≤0.001”) Zoom-in Active field of research at Stockholm Observatory! How to Search for Gravitational Microlensing by the Dark Matter Particles MACHOs Gravitational microlensing by MACHOs WIMP direct detection Recoil in detector LIGHT Annular modulation SOURCE WIMP indirect detection Obs! Fel bild! Cosmic rays from annihilating WIMPs Neutrinos from WIMP annihilation in Sun/Earth Photons (gamma, radio) from WIMP annihilation in the Galactic Centre Possible detections I Direct WIMP detection MACHO project: monitoring of 12 ××1010 66 stars in the Large Magellanic Cloud WIMP LMC MACHO Nuclear recoil Milky Way Detector --11 Very controversial detection of M compact ~10~10 MMsolar, Problem: Background of other rare reactions constituting ≈≈20% of the dark halo 6 Direct WIMP Detection Annular Modulation in Ancient Mica WIMP recoils causes chemical changes in ancient mica → Natural detector with integration time ~ 1 Gyr WIMP wind from the dark halo should show seasonal variations! Possible detections II Indirect WIMP detection by WIMP search by the DAMA experiment Neutrinos from the Sun/Earth Detected annular modulation signature → ≥10 -3 of halo fraction in WIMPs 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 Is There no Alternative to MOND Dark Matter? (MOdified Newtonian Dynamics; Milgrom 1984) ”I invite the reader (...) to test whether he/she is not left with some uneasiness as our wonderful Newtonian dynamics: a=MG/r 2 'standard' cosmology seems in fact to be so far 2 2 essentially based on MOND: a /a 0=MG/r a) a Dark Matter we do not detect in the limit of small accelerations b) a Dark Energy we do not understand → µ 2 c) a fraction of Baryons we cannot (a/a 0)a=MG/r completely find! where µ(x) ≈ 1 when x » 1 Yet everything seems to work; µ(x) ≈ x when x « 1 isn't this reminiscent of epicycles?“ L. Guzzo (2002) 7 MOND II Known problems with MOND From Stacy McGaugh’s homepage: Original MOND: Phenomenological extension of Newtonian gravity →→ No predictions for e.g. gravitational lensing or cosmic expansion Solved by Bekenstein (2004)! Fails to explain the dynamics of galaxy clusters ––somesome dark matter is still required ”’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.’ Fails to explain difference between systems of I often hear this sort of paternalistic line from well intentioned similar baryonic masses, e.g. globular clusters senior astronomers. My response is the same as Luke's, with analogous and dwarf galaxies consequences for my career.” Suggestion for literature Exercises: Alternative theories of gravity vs. Dark matter Many examples (pick oneone ):): MOND ––LotsLots of work done. Fairly easy to understand at an undergraduate level MOdified Gravity (MOG) ––SlightlySlightly more technical. Requires some understanding of tensors. Can GR explain rotation curves without dark matter? 8.