Probing the dark energy: methods and strategies

TL;DR

The paper investigates how to probe dark energy through the expansion history and growth of structure, parameterized by the equation-of-state $w(z)$, and assesses the relative power of distance and growth probes. It compares type Ia supernovae, galaxy/cluster counts, and CMB anisotropy, and develops a Fisher-matrix framework to optimize the redshift distribution and parametrization of $w(z)$. The results show that a high-quality SN Ia sample (about 2000 SNe up to $z\sim1.5$) can constrain $w$ to $\sigma_w\approx 0.05$ (with complementary CMB information), while detecting redshift evolution in $w$ is substantially harder and requires careful treatment of priors and systematics. The analysis also derives optimal strategies (e.g., two- or three-Delta-function redshift placements for SN surveys) and highlights the importance of preventing systematic errors to realize the full potential of low-redshift probes against CMB cross-checks.

Abstract

The presence of dark energy in the Universe is inferred directly from the accelerated expansion of the Universe, and indirectly, from measurements of cosmic microwave background (CMB) anisotropy. Dark energy contributes about 2/3 of the critical density, is very smoothly distributed, and has large negative pressure. Its nature is very much unknown. Most of its discernible consequences follow from its effect on evolution of the expansion rate of the Universe, which in turn affects the growth of density perturbations and the age of the Universe, and can be probed by the classical kinematic cosmological tests. Absent a compelling theoretical model (or even a class of models), we describe the dark energy by an effective equation-of-state w=p_X/ρ_X which is allowed to vary with time. We describe and compare different approaches for determining w(t), including magnitude-redshift (Hubble) diagram, number counts of galaxies and clusters, and CMB anisotropy, focusing particular attention on the use of a sample of several thousand type Ia supernova with redshifts z\lesssim 1.7, as might be gathered by the proposed SNAP satellite. Among other things, we derive optimal strategies for constraining cosmological parameters using type Ia supernovae. While in the near term CMB anisotropy will provide the first measurements of w, supernovae and number counts appear to have the most potential to probe dark energy.