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

Abstract

Inhomogeneous universe models have been proposed as an alternativeexplanation for the apparent acceleration of the cosmic expansion thatdoes not require dark energy. In the simplest class of inhomogeneousmodels, we live within a large, spherically symmetric void. Severalstudies have shown that such a model can be made consistent with manyobservations, in particular the redshift-luminosity distance relationfor type Ia supernovae, provided that the void is of Gpc size and thatwe live close to the center. Such a scenario challenges the Copernicanprinciple that we do not occupy a special place in the universe. Weuse the first-year Sloan Digital Sky Survey-II supernova search dataset as well as the Constitution supernova data set to put constraintson the observer position in void models, using the fact thatoff-center observers will observe an anisotropic universe.We first show that a spherically symmetric void can give good fits tothe supernova data for an on-center observer, but that the two datasets prefer very different voids. We then continue to showthat the observer can be displaced at least fifteen percent ofthe void scale radius from the center and still give an acceptable fit to thesupernova data. When combined with the observed dipole anisotropy ofthe cosmic microwave background however, we find that the datacompells the observer to be located within about one percent of thevoid scale radius. Based on these results, we conclude thatconsiderable fine-tuning of our position within the void is needed tofit the supernova data, strongly disfavouring the model from aCopernican principle point of view.