Intrinsic uncertainty on the nature of dark energy

TL;DR

This paper discusses the fundamental limit on measuring dark energy's equation of state due to intrinsic cosmic variance caused by large-scale structure, which introduces a systematic uncertainty of several percent.

Contribution

It models local density perturbations using the Lemaître-Tolman-Bondi solution to quantify the intrinsic measurement uncertainty of dark energy parameters.

Findings

Large-scale structure causes a several percent variation in inferred w.

The uncertainty is systematic and due to cosmic variance.

Modeling with Lemaître-Tolman-Bondi reveals intrinsic measurement limits.

Abstract

We argue that there is an intrinsic noise on measurements of the equation of state parameter $w=p/\rho$ from large-scale structure around us. The presence of the large-scale structure leads to an ambiguity in the definition of the background universe and thus there is a maximal precision with which we can determine the equation of state of dark energy. To study the uncertainty due to local structure, we model density perturbations stemming from a standard inflationary power spectrum by means of the exact Lema\^{i}tre-Tolman-Bondi solution of Einstein's equation, and show that the usual distribution of matter inhomogeneities in a $\Lambda$CDM cosmology causes a variation of $w$ -- as inferred from distance measures -- of several percent. As we observe only one universe, or equivalently because of the cosmic variance, this uncertainty is systematic in nature.