# Numerical binary black hole mergers in dynamical Chern-Simons: I. Scalar field

## TL;DR

This paper develops a numerical scheme to simulate binary black hole mergers in dynamical Chern-Simons gravity, revealing new phenomenology and estimating bounds on the theory's parameters from gravitational wave observations.

## Contribution

It introduces a well-posed perturbation scheme for beyond-GR theories and applies it to simulate BBH mergers in dynamical Chern-Simons gravity, uncovering new merger phenomenology.

## Key findings

- Good agreement with analytic predictions at early times
- Discovery of a burst of dipole radiation during merger
- Estimated bounds on dCS length scale from LIGO data

## Abstract

Testing general relativity in the non-linear, dynamical, strong-field regime of gravity is one of the major goals of gravitational wave astrophysics. Performing precision tests of general relativity (GR) requires numerical inspiral, merger, and ringdown waveforms for binary black hole (BBH) systems in theories beyond GR. Currently, GR and scalar-tensor gravity are the only theories amenable to numerical simulations. In this article, we present a well-posed perturbation scheme for numerically integrating beyond-GR theories that have a continuous limit to GR. We demonstrate this scheme by simulating BBH mergers in dynamical Chern-Simons gravity (dCS), to linear order in the perturbation parameter. We present mode waveforms and energy fluxes of the dCS pseudoscalar field from our numerical simulations. We find good agreement with analytic predictions at early times, including the absence of pseudoscalar dipole radiation. We discover new phenomenology only accessible through numerics: a burst of dipole radiation during merger. We also quantify the self-consistency of the perturbation scheme. Finally, we estimate bounds that GR-consistent LIGO detections could place on the new dCS length scale, approximately $\ell \lesssim \mathcal{O}(10)~\mathrm{km}$.