Probing Dark Energy Using Its Density Instead of Its Equation of State

Abstract

The variation of dark energy density with redshift, $ρ_X(z)$, provides a critical clue to the nature of dark energy. Since $ρ_X(z)$ depends on the dark energy equation of state $w_X(z)$ through an integral, $ρ_X(z)$ can be constrained more tightly than $w_X(z)$ given the same observational data. We demonstrate this explicitly using current type Ia supernova (SN Ia) 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 $w_X(z)$ extracted from current data is consistent with a cosmological constant at 68% C.L., $ρ_X(z)$ (which has far smaller uncertainties) is not. Our results clearly show the advantage of using $ρ_X(z)$, instead of $w_X(z)$, to probe dark energy.

One color figure showing w_X(z) versus rho_X(z), reconstructed model-independently from data. Submitted