The 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. H0 is not raised without the inclusion of a late universe prior, and the "S8-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 H0 and S8 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 4σ level, and thereby test whether a scalar field or some richer physics is at work.

An effective fluid description of scalar-vector-tensor theories under the sub-horizon and quasi-static approximations

“…In other words, data would be hinting at new physics not taken into consideration within the standard cosmological model [29][30][31]. For instance, a configuration of vector fields leading to an effective Early Dark Energy (EDE) fluid with equation of state w EDE (z), sound speed c 2 s,EDE , and non-vanishing anisotropic stress π EDE (z) could simultaneously soften H 0 and S 8 tensions [32].…”

Early-Universe Model Building

“…Additionally, EDE models tend to slightly increase the S 8 tensions [853,914] and can be constrained with the full shape of the galaxy power spectrum [915,916], although current data are inconclusive [887,[917][918][919]. Nevertheless, it is clear that EDE alone cannot resolve the S 8 tension, and would require a more complicated dynamics [891,920,921] or an independent mechanism [852][853][854]. Future data will play a crucial role in arbitrating the fate of the EDE scenario, as it has been shown that data from the AdvACT [909] or CMB-S4 [886] can unambiguously confirm (or exclude) the presence of an EDE that resolves the Hubble tension in the early-universe.…”

Restoring cosmological concordance with early dark energy and massive neutrinos?

“…Allali et al 2021;Ye et al 2021;Clark et al 2021). Massive neutrinos are an economical and conservative candidate in this sense, as we know from oscillation experiments at least two neutrino mass eigenstates 2022); Sabla & Caldwell (2022). 2 In the above, the CMB data is from Planck.…”