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 , 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 , Large Scale Structure and Supernovae Ia data .
At 95 \% confidence , we find that the fractional dark energy density at early times \Omega _ { d } ^ { e } < 0.06 .
This bound tightens considerably to \Omega _ { d } ^ { e } < 0.04 when the latest Boomerang data is 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 .