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1. INTRODUCTION: THE CONTENTS OF THE UNIVERSE

Cosmology is a rather young field still undergoing a very fast evolution. Twenty years ago the nature of the microwave background was still a matter of debate, although it was generally believed that the origin was mainly from the Big Bang, the exact shape of the spectrum was still uncertain. The measurement of the spectrum by COBE, which was concomitant to the Gush et al. measurement (1990), showing that the spectrum was a nearly perfect blackbody has been a fundamental result in modern cosmology, by establishing in a definitive way one of the most critical prediction of the standard hot Big Bang picture. The determination of cosmological parameters is a central question in modern cosmology and it has become more central after the next fundamental result established by COBE: the first robust detection of the CMB fluctuations, nearly thirty years after the prediction of their presence (Sachs and Wolf, 1967). This detection has opened a new area with the perspective of reaching high "precision cosmology". However, it is also important to mention the fact that the Inflation paradigm (Guth, 1981) has represented an enormous attraction for theorist towards the field of cosmology, opening the perspective of properly testing high energy physics from cosmological data, while such a physics will probably remain largely unaccessible from laboratory experiment. Even if the data from the CMB fluctuations were not taken at face values as a proof of inflation the need for new physics appear very strongly (it is interesting to mention that the origin of the asymmetry between matter and antimatter was a fundamental problem which solution involves physics of the very early universe).

Moreover, the presence for non-baryonic dark matter can be now considered as a well established fact of modern physics. This was far from being obvious twenty years ago. By present days the abundances of light elements is well constrained by observations, consistent with a restricted range of baryon abundance (O'Meara et al., 2001):

Equation 1 (1)

where h is the Hubble constant in unit of 100 km/s/Mpc. The above baryon abundance is in full agreement with what can be inferred from CMB (Le Dour et al., 2000; Benoît et al., 2002b). There are differences in matter density estimations, but nearly all of them lead to a cosmological density parameter in the range [0.2-1.], and therefore those estimates imply the presence of a non-baryonic component of the density of the universe. An other implication is that most baryons are dark: the amount of baryons seen in the Universe is mainly in form of stars:

Equation 2 (2)

much less than predicted by primordial nucleosynthesis. This picture, the presence of two dark components in the Universe, has gained considerable strength in the last twenty years, first of all because the above numbers have gained in robustness. However, it is now believed that a third dark constituent has been discovered: the dark energy.

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