Dark Matter in Cosmology
Alessandro Palma
Dottorato in Fisica XXII ciclo
Corso di Cosmologia
Prof. A. Melchiorri
Outlook
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Why does Cosmology need Dark Matter (DM)?
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gravitational data from Galaxies and Clusters
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structure formation in the Universe
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lensing
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DM: observed relic density
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“Identity card” of DM particles
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DM candidates in particle physics
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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)
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Two possible explanations for the constant v at large R:
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Newton’s wrong: Modified Newtonian Dynamics (MOND)
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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
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Open questions with the Dark Matter solution:
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we don’t know the dimensions of typical dark halos
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DM density must fall off at a certain point, to keep the Galaxy mass
finite
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Need for DM: cluster mass
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First Zwicky, with Coma cluster (1938)
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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
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Need for DM: structure formation (1)
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Suppose no DM: structure formation arises from evolution of
density inhomogeneities of baryonic matter
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Baryions are strongly coupled to photons in an equilibrium plasma
until z ~1100 : no perturbation evolution for z >1100
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From z = 1100, perturbation size increases of a factor ~103
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This does not allow to observe nowadays structures !
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Need for DM: structure formation (2)
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Suppose DM: if DM freeze-out occurs early enough, i.e. zrm ~
3570, DM perturbations can evolve from that time
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When baryions decouple at z ~ 1100, they can settle in DM
potential minima (DM structures partly formed at z = 1100)
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“Leap” in baryon perturbation history, it’s like starting at z =
3570 instead of 1100
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Need for DM: lensing (1)
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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
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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 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
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“Identity card” of DM particles
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What is “Dark”? Not luminous, not absorbing: no EM interaction
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Stable, neutral
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Must give the correct relic density (WDMh) calculated today
MACHO’s (MAssive Compact Halo Objects) or diffuse matter?
–
–
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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]
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Structure formation requests Cold Dark Matter!
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DM candidates in particle physics
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Primordial black holes formed before BBN
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Axions from CP-violating term in QCD Lagrangian (mass in meV range)
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WIMP’s (Weakly Interacting Massive Particles) with mass between 10
GeV and a few TeV and weak-scale couplings
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heavy neutrino, but… LEP implies Mn > MZ/2 and this yields too low a
relic density
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LSP: sneutrino but… has large annihilation x-section and is ok only if
very heavy (> several 100 GeV)  uncomfortable for SUSY
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LSP: neutralino …GOOD CANDIDATE!
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Results of the DAMA/NaI experiment
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Observable: 1-year periodicity of DM flux due to combination of
Sun+Earth velocity wrt to galactic halo (vsun ≈ 220 km/s)
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6.3 s modulation signal detected in a 100-kg radiopure NaI
detector+PM’s @ LNGS (7yrs of data taking until 2002)
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References
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B. Ryden, “Introduction to Cosmology”, Addison Wesley (2003)
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[PDG2006] W.-M. Yao et al., J. Phys. G 33, 1 (2006)
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R. Bernabei et al., “Dark Matter search”, Riv. N. Cim. 26 n.1
(2003) 1-73
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