Anisotropy in the cosmic acceleration inferred from supernovae

TL;DR

This paper analyzes large supernova datasets and finds that the observed cosmic acceleration is anisotropic, influenced by local bulk flows and observer motion, challenging the assumption of isotropic acceleration.

Contribution

It introduces a maximum likelihood analysis revealing anisotropy in cosmic acceleration using recent supernova data, considering local motions and observer effects.

Findings

Cosmic acceleration appears anisotropic in recent supernova datasets.

Local bulk flows and observer tilt can mimic cosmic acceleration signals.

The anisotropy challenges the assumption of a uniform acceleration of the universe.

Abstract

Under the assumption that they are standard(isable) candles, the lightcurves of Type Ia supernovae have been analyzed in the framework of the standard Friedmann-Lema\^itre-Robertson-Walker cosmology to conclude that the expansion rate of the Universe is accelerating due to dark energy. While the original claims in the late 1990s were made using overlapping samples of less than 100 supernovae in total, catalogues of nearly 2000 supernovae are now available. In light of recent developments such as the cosmic dipole anomaly and the larger than expected bulk flow in the local Universe (which does not converge to the Cosmic Rest Frame), we analyze the newer datasets using a Maximum Likelihood Estimator and find that the acceleration of the expansion rate of the Universe is unequivocally anisotropic. The associated debate in the literature highlights the artifices of using supernovae as standardisable candles, while also providing deeper insights into a consistent relativistic view of peculiar motions as departures from the Hubble expansion of the Universe. The effects of our being `tilted observers' embedded in a deep bulk flow may have been mistaken for cosmic acceleration.