A response to Rubin & Heitlauf: "Is the expansion of the universe accelerating? All signs still point to yes"

TL;DR

This paper defends the evidence for cosmic acceleration inferred from Type Ia supernovae against recent criticisms, showing that even with corrected assumptions, the acceleration remains statistically significant.

Contribution

It demonstrates that the evidence for isotropic cosmic acceleration persists at a 2.8 sigma level even after addressing criticisms about supernova data corrections.

Findings

Evidence for acceleration is robust at 2.8 sigma after corrections.

Incorrect velocity corrections bias the inferred acceleration.

Criticisms highlight the importance of consistent data corrections.

Abstract

We have shown (Colin et al., 2019) that the acceleration of the Hubble expansion rate inferred from Type Ia supernovae (SNe Ia) is, at $3.9\sigma$ significance, a dipole approximately aligned with the CMB dipole, while its monopole component, which can be interpreted as due to a Cosmological Constant or dark energy, is consistent with zero at $1.4\sigma$. This has been challenged by Rubin & Heitlauf (2019) who assert that the dipole arises because we made an incorrect assumption about the SNe Ia light-curve parameters (viz. took them to be sample- and redshift independent), and did not allow for the motion of the Solar system (w.r.t. the 'CMB frame' in which the CMB dipole supposedly vanishes). In fact what has an even larger impact on our finding is that we reversed the inconsistent "corrections" made for the peculiar velocities of the SNe Ia host galaxies w.r.t the CMB frame, which in fact serve to bias the lever arm of the Hubble diagram towards higher inferred values of the monopole. We demonstrate that even if all such corrections are made consistently and both sample- and redshift-dependence is allowed for in the standardisation of supernova light curves, the evidence for isotropic acceleration rises to just $2.8\,\sigma$. Thus the criticism of Rubin & Heitlauf serves only to highlight that "corrections" must be made to the SNe Ia data assuming the standard $\Lambda$CDM model, in order to recover it from the data.