Mass spectrum and linear perturbations of ghost-free multi-spin-2 theory

TL;DR

This paper analyzes a ghost-free multi-spin-2 field theory, deriving the mass spectrum and perturbation behavior around Einstein backgrounds, revealing a universal massless mode and bounds on massive modes.

Contribution

It develops an all-order vielbein expansion for multi-spin-2 theories and analyzes the mass spectrum around Einstein backgrounds, providing bounds on the masses of the modes.

Findings

The theory has a non-tachyonic spectrum with one massless and N-1 massive spin-2 modes.

The massless mode is universal across solutions.

Bounds on the massive mode eigenvalues are derived in terms of theory parameters.

Abstract

Motivated by the spin-2 nature of gravity, we consider a theory of N interacting spin-2 fields, formulated in terms of vielbeins which have been argued to be free of the Boulware-Deser ghost. This is the only known such theory with nontrivial multi-field interactions. We develop an all-order expansion of the vielbein in terms of the metric and Lorentz field perturbations, enabling an expansion of the action in arbitrary backgrounds to any order. The mass matrix is then analysed around proportional vielbein solutions corresponding to vacuum Einstein spacetimes. It is shown that the theory exhibits a non-tachyonic spectrum with one massless and N-1 massive spin-2 modes, regardless of the sign of the cosmological constant. While the massless mode is universal, the non-zero masses cannot generally be determined analytically, but we obtain the lower and upper bounds on each mass eigenvalue in terms of the parameters of the theory.