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The speed of Galileon gravity

Brax, Philippe; Burrage, Clare; Davis, Anne-Christine

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Philippe Brax

Anne-Christine Davis


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.


Brax, P., Burrage, C., & Davis, A.-C. (2016). The speed of Galileon gravity. Journal of Cosmology and Astroparticle Physics, 2016(3), Article 004.

Journal Article Type Article
Acceptance Date Feb 6, 2016
Publication Date Mar 4, 2016
Deposit Date Apr 21, 2017
Publicly Available Date Apr 21, 2017
Journal Journal of Cosmology and Astroparticle Physics
Electronic ISSN 1475-7516
Publisher IOP Publishing
Peer Reviewed Peer Reviewed
Volume 2016
Issue 3
Article Number 004
Keywords dark energy theory, gravitational waves / theory, modified gravity
Public URL
Publisher URL
Contract Date Apr 21, 2017


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