Cosmological gravitational particle production of massive spin-2 particles

TL;DR

This paper investigates the gravitational production of massive spin-2 particles during inflation, exploring their potential as dark matter candidates and analyzing stability conditions in cosmological backgrounds.

Contribution

It provides a detailed numerical analysis of spin-2 particle production during inflation with different couplings, and generalizes the Higuchi bound to FRW spacetimes.

Findings

CGPP can produce viable spin-2 dark matter during inflation.

Identifies parameter space where production is efficient.

Derives stability conditions and a generalized Higuchi bound.

Abstract

The phenomenon of cosmological gravitational particle production (CGPP) is expected to occur during the period of inflation and the transition into a hot big bang cosmology. Particles may be produced even if they only couple directly to gravity, and so CGPP provides a natural explanation for the origin of dark matter. In this work we study the gravitational production of massive spin-2 particles assuming two different couplings to matter. We evaluate the full system of mode equations, including the helicity-0 modes, and by solving them numerically we calculate the spectrum and abundance of massive spin-2 particles that results from inflation on a hilltop potential. We conclude that CGPP might provide a viable mechanism for the generation of massive spin-2 particle dark matter during inflation, and we identify the favorable region of parameter space in terms of the spin-2 particle's mass and the reheating temperature. As a secondary product of our work, we identify the conditions under which such theories admit ghost or gradient instabilities, and we thereby derive a generalization of the Higuchi bound to Friedmann-Robertson-Walker (FRW) spacetimes.