The Early Dark Sector, the Hubble Tension, and the Swampland
  We consider the interplay of the Early Dark Energy (EDE) model, the Swampland
Distance Conjecture (SDC), and cosmological parameter tensions. EDE is a
proposed resolution of the Hubble tension relying upon a near-Planckian scalar
field excursion, while the SDC predicts an exponential sensitivity of masses of
other fields to such an excursion, $m\propto e^{-c|\Delta \phi|/M_{\rm pl}}$
with $c\sim{\cal O}(1)$. Meanwhile, EDE is in tension with large-scale
structure (LSS) data, due to shifts in the standard $\Lambda$CDM parameters
necessary to fit the cosmic microwave background (CMB). One might hope that a
proper treatment of the model, e.g., accounting for the SDC, may ameliorate the
tension with LSS. Motivated by these considerations, we introduce the Early
Dark Sector (EDS) model, wherein the mass of dark matter is exponentially
sensitive to super-Planckian field excursions of the EDE scalar. The EDS model
exhibits new phenomenology in both the early and late universe, the latter due
to an EDE-mediated dark matter self-interaction. This dark matter-philic "fifth
force", while constrained to be small, remains active in the late universe and
is not screened in virialized halos. We find that the new interaction with dark
matter partially resolves the LSS tension. However, the marginalized posteriors
are nonetheless consistent with $f_{\rm EDE}=0$ at 95$\%$ CL once the Dark
Energy Survey Year 3 measurement of $S_8$ is included. We study constraints on
the model from Atacama Cosmology Telescope data, and find a factor of two
improvement on the error bar on the SDC parameter $c$, along with an increased
preference for the EDE component. We discuss the implications of these
constraints for the SDC, and find the tightest observational constraints to
date on a swampland parameter, suggesting that an EDE description of
cosmological data is in tension with the SDC.
