Catastrophic Production of Slow Gravitinos

Abstract

We study gravitational particle production of the massive spin-3/2 Rarita-Schwinger field, and its close relative, the gravitino, in FRW cosmological spacetimes. For masses lighter than the value of the Hubble expansion rate after inflation, m 3/2 H, we find catastrophic gravitational particle production, wherein the number of gravitationally produced particles is divergent, caused by a transient vanishing of the helicity-1/2 gravitino sound speed. In contrast with the conventional gravitino problem, the spectrum of produced particles is dominated by those with momentum at the UV cutoff. This suggests a breakdown of effective field theory, which might be cured by new degrees of freedom that emerge in the UV. We study the UV completion of the Rarita-Schwinger field, namely N = 1, d = 4, supergravity. We reproduce known results for models with a single superfield and models with an arbitrary number of chiral superfields, find a simple geometric expression for the sound speed in the latter case, and extend this to include nilpotent constrained superfields and orthogonal constrained superfields. We find supergravity models where the catastrophe is cured and models where it persists. Insofar as quantizing the gravitino is tantamount to quantizing gravity, as is the case in any UV completion of supergravity, the models exhibiting catastrophic production are prime examples of 4dimensional effective field theories that become inconsistent when gravity is quantized, suggesting a possible link to the Swampland program. We propose the Gravitino Swampland Conjecture, which is consistent with and indeed follows from the KKLT and Large Volume scenarios for moduli stabilization in string theory.

The unbearable lightness of charged gravitini

“…For a different line of investigation concerning massless gravitini based on post-inflationary cosmological considerations, see[20][21][22][23].…”

Salvage of Too Slow Gravitinos

“…One might be tempted to diagonalize N so as to decouple the two fermions, but in general the mixing matrices depend on time, thus a unitary transformation to the basis diagonalising N would also be time-dependent, which by time derivative gives a contribution to the mass matrix, rendering M non-diagonal. Though on general grounds, we will not provide an analytical expression of the physical fermions in terms of {Ψ 1 , Ψ 2 }, as long as we consider the catastrophic production in [17,18] it is not necessary to carry out the entire diagonalization of (2.18).…”

“…Recently, [17,18] have reconsidered this process. The equation of motion of the fermion θ describing the longitudinal mode in the vaccuum after reheating can be written as…”

“…where M P is the reduced Planck mass, m 3/2 the gravitino mass, while ρ, p denote the energy density and pressure of the matter system. It was then noticed in [17,18] that a catastrophic gravitino production occurs whenever the above sound speed vanishes.…”

Gravitino thermal production

“…Assuming gauge mediated supersymmetry breaking, a different production mechanism (freeze-out) is possible [30,31,32,33]. Recently, an alternative scenario, where the so-called "catastrophic" non-thermal production of slow gravitinos has attracted attention [34,35,36,37,38,39].…”