Fermionic Isocurvature Perturbations

TL;DR

This paper explores how stable massive fermions during inflation can produce observable isocurvature perturbations, offering a new cosmological probe for theories involving fermionic fields.

Contribution

It introduces a novel mechanism for fermionic isocurvature perturbations during inflation and provides detailed calculations including renormalization and non-Gaussianity estimates.

Findings

Large amplitude scale-invariant isocurvature perturbations can be generated.

Non-Gaussianities in fermionic perturbations may be observable.

Gravitational Ward identities suppress certain fermionic contributions.

Abstract

Isocurvature perturbations in the inflationary literature typically involve quantum fluctuations of bosonic field degrees of freedom. In this work, we consider isocurvature perturbations from fermionic quantum fluctuations during inflation. When a stable massive fermion is coupled to a non-conformal sector different from the scalar metric perturbations, observably large amplitude scale invariant isocurvature perturbations can be generated. In addition to the computation of the isocurvature two-point function, an estimate of the local non-Gaussianities is also given and found to be promising for observations in a corner of the parameter space. The results provide a new class of cosmological probes for theories with stable massive fermions. On the technical side, we explicitly renormalize the composite operator in curved spacetime and show that gravitational Ward identities play an important role in suppressing certain contributions to the fermionic isocurvature perturbations.