Constraints on Dark Energy state equation with varying pivoting redshift

TL;DR

This study investigates how the choice of pivoting redshift affects constraints on dark energy's equation of state parameters, revealing that optimal pivot points and parameter intervals depend on neutrino mass assumptions.

Contribution

It introduces an analysis of the dependence of dark energy parameter constraints on pivoting redshift, considering neutrino mass effects, using comprehensive cosmological data and CosmoMC fitting.

Findings

Optimal pivot redshift differs with neutrino mass assumptions.

Parameter intervals shift with pivot redshift, affecting overlap.

Neutrino mass influences constraints mainly through w_a.

Abstract

We assume the DE state equations w(a) = w_0+w_a(a_p-a), and study the dependence of the constraints on w_0 and w_a coefficients on the pivoting redshift 1+z_p=1/a_p. Coefficients are fitted to data including WMAP7, SNIa (Union 2.1), BAO's (including WiggleZ and SDSS results) and H_0 constraints. The fitting algorithm is CosmoMC. We find specific differences between the cases when neutrino mass is allowed or disregarded. More in detail: i) The z_p value yielding uncorrelated constraints on w_0 and w_a is different in the two cases, holding ~0.25 and ~0.35, respectively. (ii) If we consider the intervals allowed to w_0, we find that they shift when z_p increases, in opposite directions for vanishing or allowed neutrino mass. This leads to no overlap between 1sigma intervals already at z_p >~0.4. (iii) The known effect that a more negative state parameter is required to allow for neutrino mass displays its effects on w_a, rather than on w_0. (iv) The w_0-w_a constraints found by using any pivot z_p can be translated into constraints holding at a specific z_p value (0 or the z_p where errors are uncorrelated). When we do so, error ellipses exhibit a satisfactory overlap.