The Hubble tension and fifth forces: a cosmic screenplay

TL;DR

This paper investigates how fifth forces in modified gravity theories can influence the cosmic distance ladder and potentially reduce the Hubble tension, showing some models can reconcile local and cosmic measurements.

Contribution

It introduces effective models of fifth forces affecting the Hubble constant estimation, demonstrating their potential to alleviate the Hubble tension while maintaining consistency with distance indicators.

Findings

Fifth force models can increase the probability of agreement between local and Planck $H_0$ values to over 20%.

Some models reduce the Hubble tension but increase discrepancies between Cepheid and TRGB distance estimates.

Certain models can reconcile data with a probability exceeding 20%, encouraging further theoretical investigation.

Abstract

Fifth forces are ubiquitous in modified theories of gravity. In this paper, we analyze their effect on the Cepheid-calibrated cosmic distance ladder, specifically with respect to the inferred value of the Hubble constant ($H_0$). We consider a variety of effective models where the strength, or amount of screening, of the fifth force is estimated using proxy fields related to the large-scale structure of the Universe. For all models considered, the local distance ladder and the Planck value for $H_0$ agrees with a probability $\gtrsim 20 \, \%$, relieving the tension compared to the concordance model with data being excluded at $99 \, \%$ confidence. The alleviated discrepancy comes partially at the cost of an increased tension between distance estimates from Cepheids and the tip of the red-giant branch (TRGB). Demanding also that the consistency between Cepheid and TRGB distance estimates is not impaired, some fifth force models can still accommodate the data with a probability $\gtrsim 20 \, \%$. This provides incentive for more detailed investigations of fundamental theories on which the effective models are based, and their effect on the Hubble tension.