# Fixing extensions to General Relativity in the non-linear regime

## TL;DR

This paper proposes a new method inspired by viscous hydrodynamics to address well-posedness issues in extended theories of gravity, enabling better analysis of their strong-field regimes such as gravitational waves.

## Contribution

It introduces a systematic approach to stabilize higher derivative gravity theories, ensuring well-posed initial value problems in 3+1 dimensions.

## Key findings

- Controlled higher order derivatives in gravity theories.
- Applied method to non-commutative geometry gravity.
- Applied method to Chern-Simons modified gravity.

## Abstract

The question of what gravitational theory could supersede General Relativity has been central in theoretical physics for decades. Many disparate alternatives have been proposed motivated by cosmology, quantum gravity and phenomenological angles, and have been subjected to tests derived from cosmological, solar system and pulsar observations typically restricted to linearized regimes. Gravitational waves from compact binaries provide new opportunities to probe these theories in the strongly gravitating/highly dynamical regimes. To this end however, a reliable understanding of the dynamics in such a regime is required. Unfortunately, most of these theories fail to define well posed initial value problems, which prevents at face value from meeting such challenge. In this work, we introduce a consistent program able to remedy this situation. This program is inspired in the approach to "fixing" viscous relativistic hydrodynamics introduced by Israel and Stewart in the late 70's. We illustrate how to implement this approach to control undesirable effects of higher order derivatives in gravity theories and argue how the modified system still captures the true dynamics of the putative underlying theories in 3+1 dimensions. We sketch the implementation of this idea in a couple of effective theories of gravity, one in the context of Non-commutative geometry, and one in the context of Chern-Simons modified General Relativity.