Light-cone averages in a swiss-cheese universe

TL;DR

This paper investigates how inhomogeneities in a swiss-cheese universe affect light-cone averages and the apparent dark energy behavior, finding that voids influence the effective equation of state.

Contribution

It introduces a toy swiss-cheese cosmological model analyzing light-cone averages and shows how inhomogeneities can mimic dark energy effects.

Findings

The expansion scalar is unaffected by inhomogeneities due to spherical symmetry.

The density average along light-cones is influenced by voids and high-density structures.

The effective equation of state depends linearly on the size of inhomogeneities.

Abstract

We analyze a toy swiss-cheese cosmological model to study the averaging problem. In our model, the cheese is the EdS model and the holes are constructed from a LTB solution. We study the propagation of photons in the swiss-cheese model, and find a phenomenological homogeneous model to describe observables. Following a fitting procedure based on light-cone averages, we find that the the expansion scalar is unaffected by the inhomogeneities. This is because of spherical symmetry. However, the light-cone average of the density as a function of redshift is affected by inhomogeneities. The effect arises because, as the universe evolves, a photon spends more and more time in the (large) voids than in the (thin) high-density structures. The phenomenological homogeneous model describing the light-cone average of the density is similar to the concordance model. Although the sole source in the swiss-cheese model is matter, the phenomenological homogeneous model behaves as if it has a dark-energy component. Finally, we study how the equation of state of the phenomenological model depends on the size of the inhomogeneities, and find that the equation-of-state parameters w_0 and w_a follow a power-law dependence with a scaling exponent equal to unity. That is, the equation of state depends linearly on the distance the photon travels through voids. We conclude that within our toy model, the holes must have a present size of about 250 Mpc to be able to mimic the concordance model.