The effects of peculiar velocities in SN Ia environments on the local $H_0$ measurement

Abstract

The discrepancy between estimates of the Hubble Constant ($H_0$) measured from local ($z\lesssim 0.1$) scales and from scales of the sound horizon is a crucial problem in modern cosmology. Peculiar velocities ($v_{pec}$) of standard candle distance indicators can systematically affect local $H_0$ measurements. We here use 2MRS galaxies to measure the local galaxy density field, finding a notable $z$ < 0.05 under-density in the SGC-6dFGS region of 27 $\pm$ 2 %. However, no strong evidence for a 'Local Void' pertaining to the full 2MRS sky coverage is found. Galaxy densities are used to measure a density parameter, $\Delta {\varphi }_{+-}$, which we introduce as a proxy for $v_{pec}$ which quantifies density gradients along a SN line-of-sight. $\Delta {\varphi }_{+-}$ is found to correlate with local $H_0$ estimates from 88 Pantheon SNeIa (0.02 < $z$ < 0.05). Density structures on scales of $\sim$ 50 Mpc are found to correlate strongest with $H_0$ estimates in both the observational data and in mock data from the MDPL2-Galacticus simulation. Using trends of $H_0$ with $\Delta {\varphi }_{+-}$, we can correct for the effects of density structure on local $H_0$ estimates, even in the presence of biased $v_{pec}$. However, the difference in the inferred $H_0$ estimate with and without the peculiar velocity correction is limited to < 0.1 %. We conclude that accounting for environmentally-induced peculiar velocities of SNIa host galaxies does not resolve the tension between local and CMB-derived $H_0$ estimates.

15 pages, 14 figures. Accepted for publication in MNRAS

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• A 5% measurement of the Hubble constant from Type II supernovae
• Constrained realizations of 2MRS density and peculiar velocity fields: growth rate and local flow
• The Local Hole: a galaxy under-density covering 90% of sky to ~200 Mpc
• Analysis of galaxy kinematics based on Cepheids from the Gaia DR2 Catalogue
• Non-comoving Cold Dark Matter in a $\Lambda$CDM background
• Can Hubble tension be eased by invoking a finite range for gravity?