The speed of Galileon gravity

Brax, Philippe and Burrage, Clare and Davis, Anne-Christine (2016) The speed of Galileon gravity. Journal of Cosmology and Astroparticle Physics, 2016 (03). 004/1-004/17. ISSN 1475-7516

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We analyse the speed of gravitational waves in coupled Galileon models with an equation of state ωphgr=−1 now and a ghost-free Minkowski limit. We find that the gravitational waves propagate much faster than the speed of light unless these models are small perturbations of cubic Galileons and the Galileon energy density is sub-dominant to a dominant cosmological constant. In this case, the binary pulsar bounds on the speed of gravitational waves can be satisfied and the equation of state can be close to -1 when the coupling to matter and the coefficient of the cubic term of the Galileon Lagrangian are related. This severely restricts the allowed cosmological behaviour of Galileon models and we are forced to conclude that Galileons with a stable Minkowski limit cannot account for the observed acceleration of the expansion of the universe on their own. Moreover any sub-dominant Galileon component of our universe must be dominated by the cubic term. For such models with gravitons propagating faster than the speed of light, the gravitons become potentially unstable and could decay into photon pairs. They could also emit photons by Cerenkov radiation. We show that the decay rate of such speedy gravitons into photons and the Cerenkov radiation are in fact negligible. Moreover the time delay between the gravitational signal and light emitted by explosive astrophysical events could serve as a confirmation that a modification of gravity acts on the largest scales of the Universe.

Item Type: Article
Keywords: dark energy theory, gravitational waves / theory, modified gravity
Schools/Departments: University of Nottingham, UK > Faculty of Science > School of Physics and Astronomy
Identification Number:
Depositing User: Burrage, Clare
Date Deposited: 21 Apr 2017 13:59
Last Modified: 04 May 2020 17:42

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