Early dark energy at high redshifts: status and perspectives

Early dark energy models, for which the contribution to the dark energy density at high redshifts is not negligible, influence the growth of cosmic structures and could leave observable signatures that are different from the standard cosmological constant cold dark matter (Lambda CDM) model. In this paper, we present updated constraints on early dark energy using geometrical and dynamical probes. From WMAP five-year data, baryon acoustic oscillations and type Ia supernova eluminosity distances, we obtain an upper limit of the dark energy density at the last scattering surface (lss), Omega(EDE)(z(lss)) < 2.3 x 10(-2) (95% C.L.). When we include higher redshift observational probes, such as measurements of the linear growth factors, Gamma-Ray Bursts (GRBs) and Lyman-alpha-forest (Ly alpha), this limit improves significantly and becomes Omega(EDE)(z(lss)) < 1.4 x 10(-3) (95% C.L.). Furthermore, we find that future measurements, based on the Alcock-Paczynski test using the 21cm neutral hydrogenline, on GRBs and on the Ly alpha forest, could constrain the behavior of the dark energy component and distinguish at a high confidence level between early dark energy models and pure Lambda CDM. In this case, the constraints on the amount of early dark energy at the last scattering surface improve by a factor ten, when compared to present constraints. We also discuss the impact on the parameter gamma, the growth rate index, which describes the growth of structures in standard and in modified gravity models.