Transient Parity Violation during Inflation: Implications for PTA Gravitational Waves

TL;DR

This paper explores how a transient parity violation during inflation could amplify primordial gravitational waves, producing distinctive spectral and polarization features detectable by pulsar timing arrays, distinguishing them from astrophysical sources.

Contribution

It introduces a model of transient parity violation during inflation that predicts a unique gravitational wave spectrum with specific polarization signatures, serving as a cosmological template.

Findings

Amplifies primordial gravitational waves at small scales with a blue spectral slope n_T ≈ 2.

Predicts large linear polarization and specific Stokes correlations in the gravitational wave background.

Provides a framework to differentiate primordial signals from astrophysical sources in PTA data.

Abstract

We investigate the consequences of a transient phase of enhanced parity violation during inflation. Modeling this phase through a time-localized Chern--Simons-like coupling, we show that it amplifies primordial gravitational waves at small scales, producing a robust spectral shape with a blue growth of effective slope $n_T \simeq 2$, largely insensitive to microscopic details. This prediction lies in the range explored by recent pulsar timing array (PTA) analyses under cosmological power-law interpretations, while differing from the canonical supermassive black hole binary expectation. Our framework thus provides a predictive cosmological template to benchmark astrophysical versus primordial origins of the signal, consistent with cosmic microwave background bounds. The signal also exhibits large linear polarization and non-trivial Stokes correlations, corresponding to an almost maximally phase-coherent helicity state. Such features are difficult to realize in classical stochastic backgrounds, and their detection would provide circumstantial evidence for a primordial, coherently generated origin of the gravitational-wave background.