What can we learn about dark energy evolution?
Résumé
We examine some of the constraints that can be obtained on a two parameters model of dark energy in which the characteristic parameter of the equation of state $w (z) = P(z) / \rho (z)$ does not vary smoothly with time as usually assumed, but undergoes a transition between two values over a period that can be significantly shorter than the Hubble time. We find that the most recent SNIa survey allows a transition between $w \sim -0.2$ to $w \sim -1$ (the first value being somewhat arbitrary) at redshift as low as $0.1$, despite the fact that data extend beyond $z \sim 1$. Surveys with precision anticipated for space experiments would allow to improve this constraint but not by much, as a transition occurring at redshift as low as $\sim 0.17$ could still remain undistinguishable from a standard cosmological constant. The addition of a prior on the matter density $\Omega_\MAT = 0.27$ improves the constraints, although in a rather limited way. This suggests that Hubble diagram of distant SNIa might hardly reveal the actual nature of dark energy at redshift above $0.2$. Even deep space experiments would fail to identify a rapid transition at redshift above $0.5$. This suggests that only the local dynamics of the quintessence can be caught by SNIa Hubble diagram. The reason for this phenomenon is investigated. On the contrary without any prior, we found that existing constraints from combining CMB and SNIa already reject a transition at redshift below $1.5$.