Cosmological correlators from the Inflation end to CMB sky via reheating

TL;DR

This paper explores how the post-inflationary reheating phase affects primordial cosmological correlators, revealing that non-minimal coupling can produce observable effects on large scales, unlike conformal coupling.

Contribution

It demonstrates that non-minimal coupling during reheating can cause superhorizon enhancements in correlators, providing a new observational window into reheating physics.

Findings

Conformal coupling confines reheating signatures to subhorizon scales.

Non-minimal coupling can induce superhorizon tachyonic enhancement.

Reheating effects on correlators depend critically on the coupling and equation of state.

Abstract

We investigate the imprint of post-inflationary evolution on primordial cosmological correlators by relaxing the standard assumption of instantaneous reheating. We compute the power spectrum and bispectrum for a conformally coupled and non-minimally coupled ($\xi \neq \frac{1}{6},0$) scalar field with cubic self-interaction, across a finite reheating epoch characterized by an effective equation of state $w$, and reheating temperature $T_{\rm reh}$. We find that for a conformally coupled scalar field, the signatures of reheating, such as the modification to oscillatory features, are strictly confined to the subhorizon regime. On superhorizon scales, the correlators receive negligible corrections at leading order. In sharp contrast, for a non-minimally coupled field, we identify a distinct phenomenology where non-trivial values of $\xi$ and reheating equation of state $w$, can induce a tachyonic enhancement of the field modes on superhorizon scales. This enhancement modifies both the power spectrum and bispectrum, marking a sharp deviation from the instantaneous transition limit. Our results demonstrate that to the leading order while conformal coupling shields large-scale correlations from the expansion history, non-minimal coupling breaks this degeneracy and opens an observable window into the physics of reheating.