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Microphysics of Early Dark Energy
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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.
Forward citations
Cited by 5 Pith papers
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Unifying Early and Late Dark Energy: Dynamical Requirements and Obstructions
Any unified early and late dark energy scenario with a single tracking scalar field requires a potential with three distinct slopes arranged in a steep-steeper-shallow hierarchy.
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The $H_0$ world cup. II. A comprehensive competition between proposed Hubble tension solutions
Against a common 2025-26 dataset (Planck PR4, ACT DR6, SPT-3G, DESI DR2, Pantheon+), early dark energy and early modified gravity models win the H0 competition (~3σ residual tension), while radiation and late-time sol...
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The $H_0$ World Cup. I. Summary of the baseline group stage results
In a systematic head-to-head analysis, early dark energy and early modified gravity models reduce the Hubble tension to about 3σ and are favored over ΛCDM, while radiation and late-time alternatives are not.
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Disentangling cosmic distance tensions with early and late dark energy
Early dark energy resolves CMB-BAO tension and, combined with thawing quintessence, reduces overall cosmological tensions without phantom crossing.
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The Hubble Tension and Early Dark Energy
The Hubble tension between local and early-universe expansion-rate measurements may be resolved by early dark energy that speeds up expansion before recombination while satisfying existing constraints.
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