Cosmic variance and the measurement of the local Hubble parameter

TL;DR

This paper examines the discrepancy between local and cosmic microwave background measurements of the Hubble constant, showing that local gravitational effects and cosmic variance partly explain the tension, but a new physics model might be needed if the discrepancy persists.

Contribution

It introduces a method to account for local gravitational potential effects on Hubble constant measurements, reducing the tension between different cosmological constraints.

Findings

Local gravitational potential accounts for part of the Hubble tension.

Cosmic variance limits local Hubble measurements to about 2.4%.

A rare fluctuation would be needed to fully explain the observed discrepancy.

Abstract

There is an approximately 9% discrepancy, corresponding to 2.4sigma, between two independent constraints on the expansion rate of the universe: one indirectly arising from the cosmic microwave background and baryon acoustic oscillations, and one more directly obtained from local measurements of the relation between redshifts and distances to sources. We argue that by taking into account the local gravitational potential at the position of the observer this tension - strengthened by the recent Planck results - is partially relieved and the concordance of the standard model of cosmology increased. We estimate that measurements of the local Hubble constant are subject to a cosmic variance of about 2.4% (limiting the local sample to redshifts z>0.010) or 1.3% (limiting it to z>0.023), a more significant correction than that taken into account already. Nonetheless, we show that one would need a very rare fluctuation to fully explain the offset in the Hubble rates. If this tension is further strengthened, a cosmology beyond the standard model may prove necessary.