Abstract
A cosmological constant is still possible, according to current observations : microwave cosmological background, galaxy surveys, etc. But the fine-tuning problem is too restrictive, and doesn't allow us to make the link with the inflaton, a scalar field from the first epoch of the Universe. A large number of theories call for a scalar field, a quintessence with a dark energy that is dynamic in time. There are two main types of solution, according to the equation of state P= w xdensity: either a thawing w=-1 at the start, then w increases while remaining below -0.7 ; or a freezing, where w tends asymptotically towards -1. There are numerous modified-gravity theories to account for dark energy : Let f(R), tensor-scalar, tensor-vector, general Horndeski formalism, holographic theory, derived from the problem of information around black holes.
