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