Challenges to selfacceleration in modified gravity from gravitational waves and largescale structure
Abstract
With the advent of gravitationalwave astronomy marked by the aLIGO GW150914 and GW151226 observations, a measurement of the cosmological speed of gravity will likely soon be realised. We show that a confirmation of equality to the speed of light as indicated by indirect Galactic observations will have important consequences for a very large class of alternative explanations of the latetime accelerated expansion of our Universe. It will break the dark degeneracy of selfaccelerated Horndeski scalartensor theories in the largescale structure that currently limits a rigorous discrimination between acceleration from modified gravity and from a cosmological constant or dark energy. Signatures of a selfacceleration must then manifest in the linear, unscreened cosmological structure. We describe the minimal modification required for selfacceleration with standard gravitationalwave speed and show that its maximum likelihood yields a 3σ poorer fit to cosmological observations compared to a cosmological constant. Hence, equality between the speeds challenges the concept of cosmic acceleration from a genuine scalartensor modification of gravity.
 Publication:

Physics Letters B
 Pub Date:
 February 2017
 DOI:
 10.1016/j.physletb.2016.12.048
 arXiv:
 arXiv:1602.07670
 Bibcode:
 2017PhLB..765..382L
 Keywords:

 Astrophysics  Cosmology and Nongalactic Astrophysics;
 General Relativity and Quantum Cosmology;
 High Energy Physics  Phenomenology;
 High Energy Physics  Theory
 EPrint:
 5 pages