Inflation with the Gauss-Bonnet term in the Palatini formulation

TL;DR

This paper investigates the effects of the Gauss-Bonnet term coupled to the inflaton within the Palatini formulation of gravity, revealing modifications to the inflaton's kinetic term and gravitational waves that resemble Chern-Simons effects but with negative signs.

Contribution

It provides an exact solution for the connection in the Palatini formulation with the Gauss-Bonnet term and analyzes the resulting modifications to inflationary dynamics and gravitational waves.

Findings

The inflaton kinetic term is modified similarly to Chern-Simons but with a negative sign.

Gravitational wave sector modifications mirror Chern-Simons effects with negative signs.

Differences from metric formulation are small unless the kinetic term approaches zero.

Abstract

We consider the Gauss-Bonnet term coupled to the inflaton in the Palatini formulation of gravity. Unlike in the metric formulation, the Gauss-Bonnet term is not always a total derivative. We solve for the connection and insert it into the action, exactly for the spatially flat FLRW spacetime, and using the gradient approximation and order reduction for a general spacetime. We consider three cases: when the connection is unconstrained, and when non-metricity or torsion is put to zero. In all cases, the leading order change to the inflaton kinetic has the same form as that generated by the Chern-Simons term, but a negative sign. The modification of the gravitational wave sector also has the same form as in the Chern-Simons case but with a negative sign, except possibly for zero torsion, depending on the coupling and the potential. Within the range of validity of our approximations, differences from the metric formulation are small unless the kinetic term flips sign or is close to doing so.