Microphysics of Early Dark Energy

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.

28 pages, 19 figures; new text, figures and references added, minor typographical changes to match published version; Data available at https://doi.org/10.5281/zenodo.7101869

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Cited by in corpus (5)

• Restoring cosmological concordance with early dark energy and massive neutrinos?
• Updated constraints from the effective field theory analysis of BOSS power spectrum on Early Dark Energy
• Non-oscillating Early Dark Energy and Quintessence from Alpha-Attractors
• Holographic energy density, dark energy sound speed, and tensions in cosmological parameters: $H_0$ and $S_8$
• An effective fluid description of scalar-vector-tensor theories under the sub-horizon and quasi-static approximations