Early dark energy cosmologies

We propose a novel parameterization of the dark energy density. It is particularly well suited to describe a non-negligible contribution of dark energy at early times and contains only three parameters, which are all physically meaningful: the fractional dark energy density today, the equation of state today and the fractional dark energy density at early times. As we parameterize Omega(d)(a) directly instead of the equation of state, we can give analytic expressions for the Hubble parameter, the conformal horizon today and at last scattering, the sound horizon at last scattering, and the acoustic scale as well as the luminosity distance. For an equation of state today w(0) < -1, our model crosses the cosmological constant boundary. We perform numerical studies to constrain the parameters of our model by using cosmic microwave background (CMB), large scale structure (LSS) and supernovae Ia (SNe Ia) data. At 95% confidence, we find that the fractional dark energy density at early times. Omega(e)(d) < 0.06. This bound tightens considerably to. Omega(e)(d) < 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 prefer w(0) < -1, but are well compatible with a cosmological constant.