Neutron star merger GW170817 strongly constrains doubly coupled bigravity

TL;DR

The paper investigates how recent gravitational wave observations from neutron star mergers constrain doubly coupled bigravity models, showing that such models must be either proportional or singly coupled to align with observed gravitational wave speeds.

Contribution

It demonstrates that gravitational wave speed constraints significantly limit doubly coupled bigravity models, favoring proportional backgrounds that mimic standard cosmology at the background and linear perturbation levels.

Findings

Strong bounds on gravitational wave speed restrict doubly coupled models.

Proportional backgrounds yield stable cosmologies similar to ΛCDM.

Nonlinear deviations from standard cosmology are possible.

Abstract

We study the implications of the recent detection of gravitational waves emitted by a pair of merging neutron stars and their electromagnetic counterpart, events GW170817 and GRB170817A, on the viability of the doubly coupled bimetric models of cosmic evolution, where the two metrics couple directly to matter through a composite, effective metric. We demonstrate that the bounds on the speed of gravitational waves place strong constraints on the doubly coupled models, forcing either the two metrics to be proportional at the background level or the models to become singly coupled. Proportional backgrounds are particularly interesting as they provide stable cosmological solutions with phenomenologies equivalent to that of $\Lambda$CDM at the background level as well as for linear perturbations, while nonlinearities are expected to show deviations from the standard model.