Rock 'n' Roll Solutions to the Hubble Tension

TL;DR

This paper proposes scalar field models with specific potentials that can reconcile local and CMB-inferred measurements of the Hubble constant, offering a potential solution to the Hubble tension.

Contribution

It introduces exact scalar field solutions with potentials proportional to n, providing a new approach to address the Hubble tension without relying on fluid approximations.

Findings

Best fit model with n=2 increases the Hubble constant value.

Scalar field solutions describe both evolution and fluctuations accurately.

Future measurements could distinguish these models from other solutions.

Abstract

Local measurements of the Hubble parameter are increasingly in tension with the value inferred from a $\Lambda$CDM fit to the cosmic microwave background (CMB) data. In this paper, we construct scenarios in which evolving scalar fields significantly ease this tension by adding energy to the Universe around recombination in a narrow redshift window. We identify solutions with scalar field potential $V \propto \phi^{2n}$ that have simple asymptotic behavior, both oscillatory (rocking) and rolling. These solutions consistently describe both the field evolution and its fluctuations without approximation. Our findings differ qualitatively from some of the existing literature, which rely upon a coarse-grained fluid description. Combining CMB data with low-redshift measurements, the best fit model has $n=2$ with a significantly higher value of the Hubble constant as compared to a $\Lambda$CDM fit to the same data. Future measurements of the late-time amplitude of matter fluctuations and of the reionization history could help distinguish these models from competing solutions.