$\star$ Ordinary baryon matter

The baryon matter is presented mainly in two forms: stars and the interstellar gas. The stellar component ( $He^3$) at the time of the paramount nucleosynthesis. During the first fifteen minutes of the universe, the temperature was sufficiently high to allow the synthesis of these elements (approximately hydrogen 75%, 25 % of helium and the quantifiable lithium 7 and helium 3, deuterium traces). Put has share helium, these elements are not produced by the reactions of fusion in the heart of stars, quite to the contrary, they tend to be destroyed by the nuclear reactions.

The densities relics of these elements are thus of good tracers of the densities at the time of the nucleosynthesis if one manages to find objects which too were not `` polluted '' by the stellar evolution.

It is thus possible to measure starting from relative paramount abundances an estimate of the nuclear rates of reactions having taken place when the universe had a temperature of $60 keV$ and to deduce abundances from them from baryons.

The measurement of helium 3 or lithium 7, deuterium abundances for objects not having been influenced by the stellar evolution and a good knowledge of the models of paramount nucleosynthesis make it possible to measure $\Omega_b$.

A recent measurement (Kirkman2003), based on the analysis of the absorption of the light by hydrogen on the lines of sight towards five quasars with great redshift, give a deuterium fraction on hydrogen: $D/H = (2.78^{+0.44}_{-0.38})\times 10^{-5}$ , that is to say a reduced density:

$$\Omega_b h^2 = 0.0437\pm 0.004$$ (1.27)

Another measurement making gainable once again the anisotropies diffuse bottom allows an estimate of the density of baryons. The relative height of the first and second peaks of the spectrum of fluctuations of the cosmological diffuse bottom depends on the fraction on baryons. Indeed, the Compton diffusion of the photons on the electrons which are binding to let us baryons involves an increase in the amplitudes of the oscillations as well as a deterioration of the odd peaks .

The measurement of WMAP (spergel2003) gives:

$$\Omega_b h^2 = 0.0490 \pm 0.002$$ (1.28)

These measurements show that the radiation is largely dominated today by the matter. Moreover, the visible matter represents only one small part of the total baryon matter (approximately 10%). Major the part of the universe is thus invisible for us. We will see in the next paragraph that the baryon matter in the form of gas represents it even only one small part of the matter.