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.

Dark Energy

“…It should be mentioned that w(z) is related with f (z) through an integration [719,720,721] f (z) = exp 3…”

“…Therefore, most researchers have chosen to study dark energy by constraining w from observations. However, in a series of works [515,719,720,721,722,723,724], Wang and collaborators argued that, due to the smearing effect [712] arising from the multiple integrals relating w(z) to the luminosity distance of SN d L (z), it is difficult to constrain w using the SN data [711]. On the contrary, since using the dark energy density ρ de can minimize the smearing effect by removing one integral, ρ de can be constrained more tightly than w given the same observational data (see Fig.…”

“…It should be mentioned that there was a debate about the reliability of this ansatz [765,766]. Another interesting polynomial fit is the polynomial interpolation, which was proposed by Wang [515,345,719,720,721,722,723,724]. It choose different redshift points z i = i * z max /n(i = 1, 2, · · · , n), and interpolate f(z) by using its own values at these redshift points.…”

“…In [721], Wang and Freese demonstrated that ρ de (z) can be constrained more tightly than w(z) given the same observational data by using the Tonry/Barris SNIa sample [320,321]. In [723], by utilizing the nearby+SDSS+ESSENCE+SNLS+HST set of 288 SNIa and the Constitution set of 397…”

Falsifying Paradigms for Cosmic Acceleration

“…Refs. [83,84] discuss the advantages of using either w(z) or ρ DE (z) to describe dark energy. Our choice to use w(z) is motivated by the fact that the model classes we consider are separated by the allowed values of w: w = −1 for ΛCDM and −1 ≤ w ≤ 1 for quintessence.…”