MODERN COSMOLOGY

The dark matter problem: experimental evidence 193

The
lumdue to the contribution of the luminous matter (stars, emitting
clouds of gases) is given by

lum≤ 0. 01. (5.18)
The first evidence for dark matter (DM) comes from observations of galactic
rotation curves (circular orbital velocity versus radial distance from the galactic
centre) using stars and clouds of neutral hydrogen. These curves show an
increasing profile for small values of the radial distancerwhile for larger ones it
becomes flat, finally decreasing again. According to Newtonian mechanics this
behaviour can be explained if the enclosed mass rises linearly with galactocentric
distance. However, the light falls off more rapidly and therefore we are forced
to assume that the main part of the matter in the galaxies is made of non-shining
matter or DM which extends for a much bigger region than the luminous one. The
limit on
galacticwhich can be inferred from the study of these curves is

galactic≥ 0. 1. (5.19)

The simplest idea is to suppose that the DM is due to baryonic objects which
do not shine. However big-bang nucleosynthesis (BBN) and, in particular, a
precise determination of the primeval aboundance of deuterium provide strong
limits on the value of the baryon density [7]
B=ρB/ρcr:

B=( 0. 019 ± 0. 001 )h− 02  0. 045 ± 0. 005. (5.20)

One-third of the BBN baryon density is given by stars and the cold and warm
gas present in galaxies. The other two-thirds are probably in hot intergalactic gas,
warm gas in galaxies and dark stars such as low-mass objects which do not shine
(brown dwarfs and planets) or the result of stellar evolution (neutron stars, black
holes, white dwarfs). The latter ones are called MACHOS (MAssive Compact
Halo Objects) and can be detected in our galaxy through microlensing.
Anyway from cluster observations the ratio of baryons to total mass is
f =( 0. 075 ± 0. 002 )h− 03 /^2 ; assuming that clusters provide a good sample of
the universe, fromfand
Bin (5.20) we can infer that:

M∼ 0. 35 ± 0. 07. (5.21)

Such a value for
Mis supported by evidence, from the evolution, of the
abundance of clusters and measurements of the power spectrum of large-scale
structures.
Hence the major part of DM is non-baryonic [8]. The crucial point is that
the SM does not possess any candidate for such non-baryonic relics of the early
universe. Hence the demand for non-baryonic DM implies the existence of a new
physics beyond the SM. Non-baryonic DM divides into two classes [23, 26]: cold
DM (CDM), made of neutral heavy particles called WIMPS (Weakly Interacting
Massive Particles) or very light ones such as axions, hot DM (HDM) made
of relativistic particles as neutrinos or even warm dark matter (WDM) with
intermediate characteristics such as gravitinos.