Microphysics of Early Dark Energy

TL;DR

This paper explores alternative microphysics for Early Dark Energy, showing that anisotropic sound speed can alleviate Hubble and structure growth tensions while fitting CMB data, and proposes future CMB-S4 tests.

Contribution

It introduces non-scalar field microphysics for EDE, demonstrating potential to resolve key cosmological tensions and distinguish underlying physics with future observations.

Findings

Anisotropic sound speed in EDE can reduce H0 and S8 tensions.

EDE with non-scalar microphysics fits CMB data well.

Future CMB-S4 observations can distinguish microphysical models at 4σ.

Abstract

Early Dark Energy (EDE) relies on scalar field dynamics to resolve the Hubble tension, by boosting the pre-recombination length scales and thereby raising the CMB-inferred value of the Hubble constant into agreement with late universe probes. However, the collateral effect of scalar field microphysics on the linear perturbation spectra appears to preclude a fully satisfactory solution. $H_0$ is not raised without the inclusion of a late universe prior, and the "$S_8$-tension", a discrepancy between early- and late-universe measurements of the structure growth parameter, is exacerbated. What if EDE is not a scalar field? Here, we investigate whether different microphysics, encoded in the constitutive relationships between pressure and energy density fluctuations, can relieve these tensions. We show that EDE with an anisotropic sound speed can soften both the $H_0$ and $S_8$ tensions while still providing a quality fit to CMB data. Future observations from the CMB-S4 experiment may be able to distinguish the underlying microphysics at the $4\sigma$ level, and thereby test whether a scalar field or some richer physics is at work.