Probing dark energy using its density instead of its equation of state

The variation of dark energy density with redshift, rho x(z), provides a critical clue to the nature of dark energy. Since rho x(z) depends on the dark energy equation of state wx (z) through an integral, rho x (z) can be constrained more tightly than wx (z) given the same observational data. We demonstrate this explicitly using current type la supernova (SN la) data [the Tonry/Barris sample], together with the Cosmic Microwave Background (CMB) shift parameter from CMB data (WMAP, CBI, and ACBAR), and the large scale structure (LSS) growth factor from 2dF galaxy survey data. We assume a flat universe, and use Markov Chain Monte Carlo (MCMC) technique in our analysis. We find that, while wx (z) extracted from current data is consistent with it cosmological constant at 68% C.L., rho x(z) (which has far smaller uncertainties) is not. Our results clearly show the advantage Of Using rho x (z), instead of wx (z), to probe the variation of dark energy. (c) 2005 Elsevier B.V. All rights reserved.