Does the Hubble constant tension call for new physics?

TL;DR

This paper examines whether the discrepancy in Hubble constant measurements indicates the need for new physics, exploring modifications to the standard cosmological model and assessing their Bayesian evidence.

Contribution

It provides a Bayesian analysis of early and late dark energy models to address the Hubble tension, highlighting the potential for future data to clarify the issue.

Findings

Early dark energy has marginal positive Bayesian evidence.

Late time dark energy explanations are slightly disfavoured.

A constant early energy density equation of state is constrained to 0.086+0.04/-0.03.

Abstract

The $\Lambda$ Cold Dark Matter model ($\Lambda$CDM) represents the current standard model in cosmology. Within this, there is a tension between the value of the Hubble constant, $H_0$, inferred from local distance indicators and the angular scale of fluctuations in the Cosmic Microwave Background (CMB). We investigate whether the tension is significant enough to warrant new physics in the form of modifying or adding energy components to the standard cosmological model. We find that late time dark energy explanations are slightly disfavoured whereas a pre-CMB decoupling extra dark energy component has a marginally positive Bayesian evidence. A constant equation of state of the additional early energy density is constrained to 0.086$^{+0.04}_{-0.03}$. Although this value deviates significantly from 1/3, valid for dark radiation, the latter is not disfavoured based on the Bayesian evidence. If the tension persists, future estimates of $H_0$ at the 1$\%$ level will be able to decisively determine which of the proposed explanations is favoured.