Supernovae as seen by off-center observers in a local void

TL;DR

This paper investigates how off-center positions within a large cosmic void affect supernova observations, finding that significant fine-tuning is required for such models to align with data, thus challenging the Copernican principle.

Contribution

The study constrains the observer's position in void models using supernova and CMB data, highlighting the fine-tuning problem and implications for inhomogeneous universe models.

Findings

Off-center observers can still fit supernova data within certain limits.

Data prefers observers near the center of the void, within about 1% of the scale radius.

Significant fine-tuning is necessary, challenging the Copernican principle.

Abstract

Inhomogeneous universe models have been proposed as an alternative explanation for the apparent acceleration of the cosmic expansion that does not require dark energy. In the simplest class of inhomogeneous models, we live within a large, spherically symmetric void. Several studies have shown that such a model can be made consistent with many observations, in particular the redshift--luminosity distance relation for type Ia supernovae, provided that the void is of Gpc size and that we live close to the center. Such a scenario challenges the Copernican principle that we do not occupy a special place in the universe. We use the first-year Sloan Digital Sky Survey-II supernova search data set as well as the Constitution supernova data set to put constraints on the observer position in void models, using the fact that off-center observers will observe an anisotropic universe. We first show that a spherically symmetric void can give good fits to the supernova data for an on-center observer, but that the two data sets prefer very different voids. We then continue to show that the observer can be displaced at least fifteen percent of the void scale radius from the center and still give an acceptable fit to the supernova data. When combined with the observed dipole anisotropy of the cosmic microwave background however, we find that the data compells the observer to be located within about one percent of the void scale radius. Based on these results, we conclude that considerable fine-tuning of our position within the void is needed to fit the supernova data, strongly disfavouring the model from a Copernican principle point of view.