# A covariant Lagrangian for stable nonsingular bounce

## TL;DR

This paper introduces a covariant effective theory that enables the construction of stable nonsingular bounce models, overcoming previous issues with ghost and gradient instabilities in cosmological bounce scenarios.

## Contribution

It proposes a new covariant Lagrangian framework with quadratic second derivatives, allowing for the explicit construction of stable nonsingular bounce models in cosmology.

## Key findings

- Successfully constructs a fully stable nonsingular bounce model
- Addresses ghost and gradient instability issues
- Provides a covariant effective theory for bounce models

## Abstract

The nonsingular bounce models usually suffer from the ghost or gradient instabilities, as has been proved recently. In this paper, we propose a covariant effective theory for stable nonsingular bounce, which has the quadratic order of the second order derivative of the field $\phi$ but the background set only by $P(\phi,X)$. With it, we explicitly construct a fully stable nonsingular bounce model for the ekpyrotic scenario.