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Dark Matter in Cosmology

Dark Matter in Cosmology. Alessandro Palma Dottorato in Fisica XXII ciclo Corso di Cosmologia Prof. A. Melchiorri. Outlook. Why does Cosmology need Dark Matter (DM)? gravitational data from Galaxies and Clusters structure formation in the Universe lensing DM: observed relic density

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Dark Matter in Cosmology

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  1. Dark Matter in Cosmology Alessandro Palma Dottorato in Fisica XXII ciclo Corso di Cosmologia Prof. A. Melchiorri

  2. Outlook • Why does Cosmology need Dark Matter (DM)? • gravitational data from Galaxies and Clusters • structure formation in the Universe • lensing • DM: observed relic density • “Identity card” of DM particles • DM candidates in particle physics • Search for DM: experimental results of the DAMA experiment

  3. Need for DM: galactic rotation curves (1) Newton says:

  4. Need for DM: galactic rotation curves (2) • Two possible explanations for the constant v at large R: • Newton’s wrong: Modified Newtonian Dynamics (MOND) • there is some Dark Matter, with r(r)~1/r2 to yield M(r) ~ r, which extends well beyond the luminous disk of the Galaxy • Open questions with the Dark Matter solution: • we don’t know the dimensions of typical dark halos • DM density must fall off at a certain point, to keep the Galaxy mass finite

  5. Need for DM: cluster mass • First Zwicky, with Coma cluster (1938) • The mass of the cluster obtained using virial theorem is MUCH more than what is obtained counting for the stars + the intergalactic matter gas (visible in X band) Coma cluster Visible X-rays

  6. Need for DM: structure formation (1) • Suppose no DM: structure formation arises from evolution of density inhomogeneities of baryonic matter • Baryions are strongly coupled to photons in an equilibrium plasma until z ~1100 : no perturbation evolution for z >1100 • From z = 1100, perturbation size increases of a factor ~103 • This does not allow to observe nowadays structures !

  7. Need for DM: structure formation (2) • Suppose DM: if DM freeze-out occurs early enough, i.e. zrm ~ 3570, DM perturbations can evolve from that time • When baryions decouple at z ~ 1100, they can settle in DM potential minima (DM structures partly formed at z = 1100) • “Leap” in baryon perturbation history, it’s like starting at z = 3570 instead of 1100

  8. Need for DM: lensing (1) • Huge masses curve space and bend photons (Einstein’s GR) • This produces arcs, rings and distorted images of faraway cosmic objects • Deflection angle a depends on impact parameter b of the photon wrt bending mass M

  9. Need for DM: lensing (2) • Measure lensing patterns generated on “background” galaxies by clusters • Infer the mass of the bending cluster

  10. DM: observed relic density W “BENCHMARK MODEL” Wtot ~ 1 ( CMB power spectrum) Wmat~ 0.3( clusters ) WL~ 0.7 (1 – 0.3) Wi = ri /rc,0 Wbar,0 = 0.04 (from Big Bang Nucleosynthesis) W*,0 = 0.004 (from <luminosity> of Galaxy stars)  WDM = Wmat - Wbar,0–W*,0 ~ 0.26 missing

  11. “Identity card” of DM particles • What is “Dark”? Not luminous, not absorbing: no EM interaction • Stable, neutral • Must give the correct relic density (WDMh) calculated today • MACHO’s (MAssive Compact Halo Objects) or diffuse matter? • dark halo can contain dark compact objects, i.e. brown dwarves, neutron stars, black holes… • measures of lensing: our Galaxy’s dark halo is mostly (80%) diffuse • Hot or cold? [hot (cold) means m>> (<<) kBT at freeze-out] • Structure formation requests Cold Dark Matter!

  12. DM candidates in particle physics • Primordial black holes formed before BBN • Axions from CP-violating term in QCD Lagrangian (mass in meV range) • WIMP’s (Weakly Interacting Massive Particles) with mass between 10 GeV and a few TeV and weak-scale couplings • heavy neutrino, but… LEP implies Mn > MZ/2 and this yields too low a relic density • LSP: sneutrino but… has large annihilation x-section and is ok only if very heavy (> several 100 GeV)  uncomfortable for SUSY • LSP: neutralino …GOOD CANDIDATE!

  13. Results of the DAMA/NaI experiment • Observable: 1-year periodicity of DM flux due to combination of Sun+Earth velocity wrt to galactic halo (vsun ≈ 220 km/s) • 6.3 s modulation signal detected in a 100-kg radiopure NaI detector+PM’s @ LNGS (7yrs of data taking until 2002)

  14. References • B. Ryden, “Introduction to Cosmology”, Addison Wesley (2003) • [PDG2006] W.-M. Yao et al., J. Phys. G 33, 1 (2006) • R. Bernabei et al., “Dark Matter search”, Riv. N. Cim. 26 n.1 (2003) 1-73

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