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.

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