Inflation, Higher Spins and the Swampland

TL;DR

The paper explores how an infinite tower of higher-spin states, as suggested by the Swampland Distance Conjecture, constrains inflation by imposing bounds on the inflaton's field excursion and highlights the special role of string theory excitations.

Contribution

It demonstrates that higher-spin towers impose strict bounds on inflationary field excursions and identifies string excitations as a natural way to evade these constraints.

Findings

Higher-spin towers forbid any scalar displacement at small Hubble scales.

A cutoff in the spin of the tower allows limited field excursions.

String excitations naturally circumvent the constraints due to their decreasing cutoff.

Abstract

We study the implications on inflation of an infinite tower of higher-spin states with masses falling exponentially at large field distances, as dictated by the Swampland Distance Conjecture. We show that the Higuchi lower bound on the mass of the tower automatically translates into an upper bound on the inflaton excursion. Strikingly, the mere existence of all spins in the tower forbids any scalar displacement whatsoever, at arbitrarily small Hubble scales, and it turns out therefore incompatible with inflation. A certain field excursion is allowed only if the tower has a cut-off in spin. Finally, we show that this issue is circumvented in the case of a tower of string excitations precisely because of the existence of such a cut-off, which decreases fast enough in field space.