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 supernovae luminosity distances, we obtain an upper limit of the
dark energy density at the last scattering surface (lss), $\Omega_{\rm
EDE}(z_{\rm lss})<2.3\times10^{-2}$ (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 (\lya), this limit improves
significantly and becomes $\Omega_{\rm EDE}(z_{\rm lss})<1.4\times10^{-3}$ (95%
C.L.). Furthermore, we find that future measurements, based on the
Alcock-Paczy\'nski test using the 21cm neutral hydrogen line, on GRBs and on
the \lya 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.
