Kavli Affiliate: Edward W. Kolb
| First 5 Authors: Edward W. Kolb, Andrew J. Long, Evan McDonough, ,
| Summary:
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, $m_{3/2} lesssim H$, 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 ${cal N}=1$, $d=4$,
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 4-dimensional
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.
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