An Introduction to Quintessence

Abstract

A large number of observations seem to indicate that in addition to baryonic and dark matter, a large fraction of the universe energy density is under the form of some unknown form of matter with negative pressure. This form of matter, usually called “dark energy” may be a cosmological constant, but this explanation is not satisfactory for several reasons. Moreover, the exact nature of this dark energy is unknown: what observation suggest is that (i) dark energy is likely to be the dominant form of matter in the universe1 (ΩDE ∼ 0.7), (ii) the expansion of the universe is probably accelerating today, so that the pressure-to-density ratio wDE is smaller than −3 , and (iii) it is unclustered. The idea of quintessence is to explain this dark energy with a scalar field which would behave closely to a cosmological today, just as the inflaton behaved as a cosmological constant at very early times. After a short overview of the difficulties to explain the dark energy by a cosmological constant (Sec. 8.1), we shall explain the main characteristic we need for a scalar field to behave as dark energy today (Secs. 8.2-8.3). We shall then give a few considerations about this model from the point of view of high energy physics (Sec. 8.4). Then the cosmological consequences of such a scalar field will be briefly discussed in Sec. 8.5, and we shall conclude by giving the observational status and prospect of dark energy (Sec. 8.6).