Cosmology AS7009, 2009 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 + Ω = ρ / ρ Μ Μ c ~2% Inventory of Luminous material Recent measurements: (Luminous) Dynamics of galaxies Ω ∼ 0.3, Ω ∼ 0.7 ΩΜ ∼ 0.3, Ω Λ ∼ 0.7 Μ Λ Dynamics and gas properties of Ω ∼ 0.005 Lum ~98% (Dark) galaxy clusters 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: Mass -to -Light 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! Mass -to -Light Ratios II Baryonic and Non -Baryonic Dark Matter I What are M/L -ratios good for? Baryons: Ordinary matter made out of three quarks, The mass -to -light ratio indicates how dark like protons and neutrons matter -dominated a certain object is. BBNS modelling + measurements of primordial abundances → Ω ≈ 0.045 or CMBR analysis baryons Higher M/L → More dark-matter dominated 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 = Baryonic + Non-baryonic 0.3 (M/ L) tot > (M/ L) stars Dark matter! Baryonic and Non -Baryonic MACHOs and WIMPs Dark Matter II Still missing in the local Universe: About 1/3 of the baryons → Ω MACHO = MAssive Compact Halo Object DM, baryonic ~ 0.015 But note: The missing baryons may have WIMP = Weakly Interacting Massive Particle been detected at high redshift Essentially all of the non -baryons → Ω Ω DM, non -baryonic ~ 0.25 (assuming M=0.3) But beware of misconceptions! 3 A Few Viable Dark Matter Candidates Hot and Cold Dark Matter Ve ry P op Baryonic Non-baryonic * ula r! 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 Non -relativistic early on (at decoupling) • Rydberg matter • Preon stars • Quark nuggets The standard model for the non -baryonic dark matter • Mirror matter Successful in explaining the formation of • Matter in parallel branes Successful in explaining the formation of large scale structure • Kaluza-Klein particles * or evading current constraints on the cosmic baryon density Additional Assumed CDM Problems with CDM Properties Collisionless – interacts mainly through gravity Dark halo density profiles Dissipationless – cannot cool by radiating Dark halo substructure photons Dark halo shapes Long -lived particles Behaves as perfect fluid on large scales The angular momentum problem Adiabatic primordial density perturbations, following a scale -invariant 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) 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! Non -spherical 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? N-body 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 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 mass -to -light 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) Self -interacting dark matter Halo substructure Self -annihilating 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 ×10 6 stars in the Large Magellanic Cloud WIMP LMC MACHO Nuclear recoil Milky Way Detector -1 Very controversial detection of M compact ~10 Msolar , 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 – some 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 one ): MOND – Lots of work done . Fairly easy to understand at an undergraduate level MOdified Gravity (MOG) – Slightly more technical . Requires some understanding of tensors . Can GR explain rotation curves without dark matter ? 8.