The Microphysics of Early Dark Energy
  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.
