# Unitary null energy condition violation in $P(X)$ cosmologies

## TL;DR

This paper investigates the constraints of unitarity on NEC violation in P(X) cosmologies, revealing that a non-singular bounce requires higher-scale operators or unnatural parameters, challenging the viability of simple NEC-violating models.

## Contribution

It provides a detailed analysis of unitarity constraints on NEC violation in P(X) theories, highlighting the necessity of higher-scale operators or unnatural coefficients for a consistent bounce.

## Key findings

- Tree-level unitarity constrains NEC violation in P(X) theories.
- Higher-scale irrelevant operators are needed for a consistent bounce.
- Imposing shift symmetry or unnatural coefficients can evade unitarity constraints.

## Abstract

A non-singular cosmological bounce in the Einstein frame can only take place if the Null Energy Condition (NEC) is violated. We explore situations where a single scalar field drives the NEC violation and derive the constraints imposed by demanding tree level unitarity on a cosmological background. We then focus on the explicit constraints that arise in P(X) theories and show that constraints from perturbative unitarity make it impossible for the NEC violation to occur within the region of validity of the effective field theory without also involving irrelevant operators that arise at a higher scale that would enter from integrating out more massive degrees of freedom. Within the context of P(X) theories we show that including such operators allows for a bounce that does not manifestly violate tree level unitarity, but at the price of either imposing a shift symmetry or involving technically unnatural small operator coefficients within the low-energy effective field theory.