Absence of the Higuchi bound in a family of alternative linear massive spin-2 models

Abstract

There is a well known result from the Fierz-Pauli (FP) theory in de Sitter background that is the existence of a lower bound for the mass $m$ of the spin 2 particle, the Higuchi bound. It establishes that $m^2\ge 2H^2$, where $H$ is the Hubble parameter, in order to the theory presents no ghost-like instabilities. In this sense, $m$ should be unacceptable high in order to fulfill this condition at the time of the inflationary epoch of the Universe, posing a difficulty to conciliate the FP theory with cosmology. In this letter we show that the Higuchi bound can be circumvented in an alternative description of massive spin-2 particles known as $\mathcal{L}(a_1)$ models. In maximally symmetric spaces the theory has two free parameters which can be consistently chosen in order to make the model absent of a lower bound for $m$. Then, $m$ can be arbitrarily smaller than the energy scale of inflation avoiding instabilities at that time.

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