Early dark energy cosmologies

Abstract

We propose a novel parameterization of the dark energy density. It is particularly well suitedto describe a non-negligible contribution of dark energy at early times and contains only threeparameters, which are all physically meaningful: the fractional dark energy density today, theequation of state today and the fractional dark energy density at early times. As we parameterizeΩd(a) directly instead of the equation of state, we can give analytic expressions for the Hubbleparameter, the conformal horizon today and at last scattering, the sound horizon at lastscattering, and the acoustic scale as well as the luminosity distance. For an equation of state todayw0<−1, our model crosses the cosmological constant boundary. We perform numericalstudies to constrain the parameters of our model by using cosmic microwavebackground (CMB), large scale structure (LSS) and supernovae Ia (SNe Ia) data. At95% confidence, we find that the fractional dark energy density at early timesΩde<0.06. This bound tightensconsiderably to Ωde<0.04 when the latest Boomerang data are included. We find that both the gold sample of Riess et al and the SNLS data of Astier et al when combined with CMB and LSS data mildly preferw0<−1, but are well compatible with a cosmological constant.