Gravitational Waves in Bimetric Gravity

Abstract: Understanding the cause of the accelerated expansion of the Universe is one of the most pressing issues in modern cosmology. An alternative to the LambdaCDM-model is bimetric gravity which is an extension of Einstein's theory of general relativity. In bimetric gravity there exists both a massive and massless graviton which gives rise to different phenomenology in a number of sectors. The LIGO observations of gravitational waves from binary mergers between black holes or neutron stars has opened up a new sector to test this theory in. We study the effects of the massive mode on the observed gravitational waveform. We place constraints on Fierz-Pauli mass, and the mixing angle between the massless and massive mode by using: (i) the observed LIGO waveform for GW150916 compared with the bimetric waveform which includes gravitational wave oscillations, (ii) absence of gravitational echoes, (iii) constraints on the luminosity distance, and (iv) constraints on the propagation speed of gravity. We note that the effects of dispersion on the massive wave packet which we detect as the echo can distort the waveform noticeably, hence it is possible that LIGO cannot detect it. Further investigation in this area is necessary. Lastly we investigate the stability of tensor perturbations in the early universe and the de Sitter point on sub- and superhorizon scales. The perturbations are found to be stable in all of these cases for all parameter values of the theory.