Dark Matter in Cosmology
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Transcript of Dark Matter in Cosmology
Dark Matter in Cosmology
Alessandro Palma
Dottorato in Fisica XXII cicloCorso di Cosmologia
Prof. A. Melchiorri
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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
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Need for DM: galactic rotation curves (1)
Newton says:
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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)~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
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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)
Visible X-raysComa cluster
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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 !
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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
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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 depends on impact parameter b of the
photon wrt bending mass M
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Need for DM: lensing (2)
• Measure lensing patterns generated on “background” galaxies by clusters• Infer the mass of the bending cluster
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DM: observed relic density
bar,0 = 0.04 (from Big Bang Nucleosynthesis)
*,0 = 0.004 (from <luminosity> of Galaxy stars)
DM = mat - bar,0 – *,0 ~ 0.26 missing
“BENCHMARK MODEL”
tot ~ 1 ( CMB power spectrum)
mat~ 0.3( clusters )
~ 0.7 (1 – 0.3)
i = i /c,0
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“Identity card” of DM particles
● What is “Dark”? Not luminous, not absorbing: no EM interaction
● Stable, neutral● Must give the correct relic density (DMh) 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!
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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 M > 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!
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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 modulation signal detected in a 100-kg radiopure NaI detector+PM’s @ LNGS (7yrs of data taking until 2002)
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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