Inference of the cosmic rest-frame from supernovae Ia^⋆

Fakultät für Physik, Universität Bielefeld, Postfach 100131, 33501 Bielefeld, Germany
e-mail: nhorstmann@physik.uni-bielefeld.de

Received: 11 November 2021
Accepted: 9 May 2022

Abstract

We determine the proper motion of the Solar System from the Pantheon sample of type Ia supernovae (SNe). The posterior distribution of the Solar System proper velocity, its direction, and relevant cosmological parameters were obtained based on the observed distance moduli, heliocentric redshifts, and positions of SNe by means of a Markov chain Monte Carlo method. We accounted for the unknown peculiar motion of SNe by including their expected covariance from linear theory. We find that the Solar System moves with v_o = 249 ± 51 km s⁻¹ towards RA = 166 ± 16 deg, Dec = 10 ± 19 deg (J2000), (all at 68% C.L.). The direction of motion agrees with the direction of the dipole observed in the cosmic microwave background (CMB) (RA = 166 deg, Dec = −7 deg). The inferred velocity is 2.4σ lower than the value inferred from a purely kinematic interpretation of the CMB dipole (370 km s⁻¹). Assuming a flat Λ cold dark matter model, we find no degeneracy of solar proper motion with other cosmological parameters. The dimensionless matter density, Ω_M = 0.305 ± 0.022, is in excellent agreement with CMB measurements. We also find no degeneracy of the solar proper motion with the SN calibration nuisance parameter. Bulk flows might be able to explain why the solar motion appears to be slower than that of nearby SNe. We conclude that a larger sample of SNe, distributed over wide areas of the sky and a broad range in redshift, will allow an independent and robust test of the kinematic nature of the CMB dipole.