Bounds on Gravitational Wave Production from Unitarity in an Early NEC-Violating Model

TL;DR

This paper investigates how unitarity constraints limit the amplitude and tilt of primordial gravitational waves in an early universe model with NEC violation, providing broad applicability across similar cosmological scenarios.

Contribution

It introduces a unitarity-based framework to derive bounds on gravitational wave properties in NEC-violating cosmologies, independent of transition details.

Findings

Strong bounds on gravitational wave amplitude

Constraints on spectral tilt of primordial waves

Applicability to various NEC-violating models

Abstract

We study a cosmological scenario featuring an early phase of null energy condition (NEC) violation. Within this framework, we show that perturbative unitarity bounds place strong constraints on both the amplitude and the spectral tilt of primordial gravitational waves. Our analysis is largely insensitive to the detailed realization of the transition between the NEC-violating phase and subsequent cosmological phases, allowing our results to be extended to a broader class of models. Finally, the perturbative unitarity approach employed here is applicable to a wide range of cosmological scenarios.