Trans-Planckian censorship constraints on properties and cosmological applications of axion-like fields

TL;DR

This paper applies the Trans-Planckian Censorship Conjecture to constrain axion-like fields, revealing limitations for inflation models and viability for quintessence models under certain initial conditions.

Contribution

It derives a new bound on axion decay constants using TCC and compares implications for inflation and quintessence models.

Findings

Axion decay constants must satisfy f√N ≲ 0.6 M_pl for TCC consistency.

Inflation models cannot produce enough e-foldings while satisfying TCC.

Quintessence models can be compatible with TCC if initial conditions are finely tuned.

Abstract

We use the Trans-Planckian Censorship Conjecture (TCC) to constrain the decay constants $f$ characterizing a set of N identical axion-like fields with cosine potentials, improving upon the precision of other Swampland conjectures and existing string-theoretic arguments. We find that consistency with the TCC requires any such set of axion-like fields to satisfy $f\sqrt{N} \lesssim 0.6M_{pl}$, where $M_{pl}$ is the reduced Planck mass. We show that this bound makes models of axion-driven inflation incapable of simultaneously producing the required number of e-foldings and the observed scalar spectral tilt. In contrast, we find that models of axion quintessence can be simultaneously compatible with the TCC and observational data, provided that the axions' initial field values are set near the maxima of their potentials to within roughly $\pm \frac{\pi}{5}f$.