Spatial and temporal tuning in void models for acceleration

TL;DR

This paper investigates the constraints on our position within a large cosmic void model as an alternative to dark energy, using CMB dipole and peculiar velocity data, and finds we must be very close to the void center, challenging the Copernican principle.

Contribution

The study provides a detailed analysis of how close we must be to the center of a Gpc-scale void, refining previous constraints and discussing implications for the Copernican principle and model fine-tuning.

Findings

We must be within 80 Mpc of the void center.

Only one such void likely exists within our Hubble radius.

Void models require temporal fine-tuning similar to dark energy models.

Abstract

There has been considerable interest in recent years in cosmological models in which we inhabit a very large, underdense void as an alternative to dark energy. A longstanding objection to this proposal is that observations limit our position to be very close to the void centre. By selecting from a family of void profiles that fit supernova luminosity data, we carefully determine how far from the centre we could be. To do so, we use the observed dipole component of the cosmic microwave background, as well as an additional stochastic peculiar velocity arising from primordial perturbations. We find that we are constrained to live within 80 Mpc of the centre of a void--a somewhat weaker constraint than found in previous studies, but nevertheless a strong violation of the Copernican principle. By considering how such a Gpc-scale void would appear on the microwave sky, we also show that there can be a maximum of one of these voids within our Hubble radius. Hence, the constraint on our position corresponds to a fraction of the Hubble volume of order 10^{-8}. Finally, we use the fact that void models only look temporarily similar to a cosmological-constant-dominated universe to argue that these models are not free of temporal fine-tuning.