The Early Dark Sector, the Hubble Tension, and the Swampland

Abstract

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 ∝ e −c|∆φ|/M pl with c ∼ O(1). Meanwhile, EDE is in tension with large-scale structure (LSS) data, due to shifts in the standard ΛCDM 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, which manifests as an enhanced gravitational constant on small scales. This EDE-induced 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 fEDE = 0 at 95% CL once the Dark Energy Survey Year 3 measurement of S8 is included. We additionally 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.

Constraints on Early Dark Energy from the Axion Weak Gravity Conjecture

“…We will then show that this conjecture leads to a tight, n-independent constraint on EDE models, which for typical parameter values amounts to a bound f < 0.008M Pl on the axion decay constant. This value is smaller than those typically considered in previous studies of EDE, and the resulting bound is tighter than those previously noted in discussions of swampland constraints on EDE [18][19][20]. We will elaborate on possible ways to evade this constraint by taking advantage of various "loopholes" and uncertainties in the parameter values, and we will conclude with a discussion of directions for future research.…”

“…One recent paper[20] considered the cosmological consequences of a coupling e −αχ ψψ between a fermionic dark matter field ψ and the EDE axion χ. Such couplings have been observed in string compactifications[67] and are a prediction of the refined swampland distance conjecture[68], but they are not necessarily required by the original swampland distance conjecture[66] because the periodicity of the axion under χ → χ + 2πf means that χ → ∞ is not an infinite distance limit in moduli space.…”

Hints of Early Dark Energy in Planck, SPT, and ACT data: new physics or systematics?

“…A fully satisfactory resolution of the 'S 8 tension', if not due to systematic errors, e.g. from galaxy assembly bias and baryonic effects [69], may require a more complicated EDE dynamics [70][71][72] or an independent mechanism [22,73,74].…”

Testing dark energy after pre-recombination early dark energy

“…[10][11][12][13][14]. In early dark energy (EDE) scenario [15], see also [16][17][18][19][20][21][22][23][24][25][26][27][28][29][30], since the scalar field energy has a non-negligible fraction before recombination, the sound horizon…”