Parity breaking signatures from a Chern-Simons coupling during inflation: the case of non-Gaussian gravitational waves

TL;DR

The paper analyzes parity-breaking signatures from a Chern-Simons term coupled to the inflaton during slow-roll inflation. While power-spectrum parity violations remain suppressed to avoid ghost instabilities, the authors show that the tensor-tensor-scalar bispectrum ⟨γγζ⟩ acquires a nonzero, potentially observable parity-violating component proportional to the second derivative f''(φ) of the CS coupling, with a pronounced peak in the squeezed limit. They derive explicit expressions, connect the amplitude to a dimensionless coefficient Π ∝ H^2 f''(φ), and identify theoretical constraints that can still allow sizable signals when Θ/ε is large. The study also discusses squeezed-limit consistency relations, gauge considerations, and observational pathways in CMB bispectra that could test non-minimal CS couplings during inflation.

Abstract

Considering high-energy modifications of Einstein gravity during inflation is an interesting issue. We can constrain the strength of the new gravitational terms through observations of inflationary imprints in the actual universe. In this paper we analyze the effects on slow-roll models due to a Chern-Simons term coupled to the inflaton field through a generic coupling function $f(φ)$. A well known result is the polarization of primordial gravitational waves (PGW) into left and right eigenstates, as a consequence of parity breaking. In such a scenario the modifications to the power spectrum of PGW are suppressed under the conditions that allow to avoid the production of ghost gravitons at a certain energy scale, the so-called Chern-Simons mass $M_{CS}$. In general it has been recently pointed out that there is very little hope to efficiently constrain chirality of PGW on the basis solely of two-point statistics from future CMB data, even in the most optimistic cases. Thus we search if significant parity breaking signatures can arise at least in the bispectrum statistics. We find that the tensor-tensor-scalar bispectra $\langle γγζ\rangle$ for each polarization state are the only ones that are not suppressed. Their amplitude, setting the level of parity breaking during inflation, is proportional to the second derivative of the coupling function $f(φ)$ and they turn out to be maximum in the squeezed limit. We comment on the squeezed-limit consistency relation arising in the case of chiral gravitational waves, and on possible observables to constrain these signatures.

Figures (1)

• Figure 1: Plot of the shape of the correlator $\langle \gamma_{R}( \vec{k}_1) \gamma_{R}(\vec{k}_2) \zeta(\vec{k}_3) \rangle$. The quantity $F(1,x_2,x_3) x^2_2 x_3^2$ in terms of $x_2 = \frac{k_2}{k_1}$ and $x_3 = \frac{k_3}{k_1}$ is shown. The figure is normalized to have value $1$ for equilateral configurations $x_2=x_3=1$.