On the determination of dark energy

TL;DR

This paper discusses the challenges in understanding dark energy, emphasizing the importance of model-independent tests, the impact of inhomogeneities, and the potential of void models to explain cosmic acceleration.

Contribution

It introduces model-independent tests for dark energy, analyzes the effects of inhomogeneities, and evaluates void models that challenge the Copernican principle.

Findings

Small inhomogeneities can increase the deceleration parameter by at least 10%.

Backreaction effects from density perturbations are significant.

Void models may explain cosmic acceleration without dark energy.

Abstract

I consider some of the issues we face in trying to understand dark energy. Huge fluctuations in the unknown dark energy equation of state can be hidden in distance data, so I argue that model-independent tests which signal if the cosmological constant is wrong are valuable. These can be constructed to remove degeneracies with the cosmological parameters. Gravitational effects can play an important role. Even small inhomogeneity clouds our ability to say something definite about dark energy. I discuss how the averaging problem confuses our potential understanding of dark energy by considering the backreaction from density perturbations to second-order in the concordance model: this effect leads to at least a 10% increase in the dynamical value of the deceleration parameter, and could be significantly higher. Large Hubble-scale inhomogeneity has not been investigated in detail, and could conceivably be the cause of apparent cosmic acceleration. I discuss void models which defy the Copernican principle in our Hubble patch, and describe how we can potentially rule out these models.