# Implication of GW170817 for cosmological bounces

## TL;DR

This paper constructs stable nonsingular bounce cosmological models within beyond Horndeski and DHOST theories, constrained by the GW170817 event which requires gravitational wave speed to equal light speed, simplifying model building.

## Contribution

It develops stable bounce models in modified gravity theories under the GW170817 constraint, linking gravitational wave observations with early universe cosmology.

## Key findings

- Models are stable throughout evolution.
- Constraint $c_T=1$ simplifies model construction.
- Models are consistent with gravitational wave speed observations.

## Abstract

The detection of GW170817 and its electromagnetic counterpart has revealed the speed of gravitational waves coincides with the speed of light, $c_T=1$. Inspired by the possibility that the physics implied by GW170817 might be related with that for the primordial universe, we construct the spatially flat stable (throughout the whole evolution) nonsingular bounce models in the beyond Horndeski theory with $c_T=1$ and in the degenerate higher-order scalar-tensor (DHOST) theory with $c_T=1$, respectively. Though it constricts the space of viable models, the constraint of $c_T=1$ makes the procedure of building models simpler.