A LooKK at the Higuchi Bound

Abstract

Massive spin 2 fields on de Sitter must satisfy the Higuchi bound, $m^2\geq 2H^2$. Compactifications of higher dimensional gravity to $\mathrm{dS}_4$ come with a tower of Kaluza-Klein gravitons whose masses are fixed by the internal geometry, so the bound becomes a constraint on the compactification. We propose that the KK gravitons of a consistent compactification never violate it, and test this in a few examples. On a circle stabilized by Casimir energy the bound holds unless the circle shrinks below the species scale, and on a warped interval with negative tension branes the tower is gapped at $m^2\geq \tfrac{9}{4}H^2$ for any length of the interval. On group manifolds, we argue that if the internal curvature is not parametrically larger than the Hubble scale, the bound can apparently be violated by smooth deformations. The violation disappears once one promotes these deformations to moduli, which are immediately extremized when solving the 10d equations of motion.