# Gravitational Waves in Doubly Coupled Bigravity

## TL;DR

This paper investigates gravitational waves in doubly coupled bigravity, revealing birefringence, differing propagation speeds, and beat phenomena in waveforms, with implications for detection via nano-Hertz interferometry.

## Contribution

It provides a detailed analysis of gravitational wave production and propagation in doubly coupled bigravity, highlighting birefringence and unique waveform features.

## Key findings

- One graviton propagates at a speed different from light.
- Gravitational waves exhibit beat patterns in the Jordan frame.
- Propagation speed deviations are constrained by astrophysical observations.

## Abstract

We consider gravitational waves from the point of view of both their production and their propagation in doubly coupled bigravity in the metric formalism. In bigravity, the two gravitons are coupled by a non-diagonal mass matrix and show birefrigence. In particular, we find that one of the two gravitons propagates with a speed which differs from one. This deviation is tightly constrained by both the gravitational Cerenkov effect and the energy loss of binary pulsars. When emitted from astrophysical sources, the Jordan frame gravitational wave, which is a linear combination of the two propagating gravitons, has a wave form displaying beats. The best prospect of detecting this phenomenon would come from nano-Hertz interferometric experiments.