Breaking a dark degeneracy with gravitational waves
Abstract
We identify a scalartensor model embedded in the Horndeski action whose cosmological background and linear scalar fluctuations are degenerate with the concordance cosmology. The model admits a selfaccelerated background expansion at late times that is stable against perturbations with a sound speed attributed to the new field that is equal to the speed of light. While degenerate in scalar fluctuations, selfacceleration of the model implies a present cosmological tensor mode propagation at lesssim95 % of the speed of light with a damping of the wave amplitude that is gtrsim5 % less efficient than in general relativity. We show that these discrepancies are endemic to selfaccelerated Horndeski theories with degenerate largescale structure and are tested with measurements of gravitational waves emitted by events at cosmological distances. Hence, gravitationalwave cosmology breaks the dark degeneracy in observations of the largescale structure between two fundamentally different explanations of cosmic acceleration—a cosmological constant and a scalartensor modification of gravity. The gravitational wave event GW150914 recently detected with the aLIGO instruments and its potential association with a weak short gammaray burst observed with the Fermi GBM experiment may have provided this crucial measurement.
 Publication:

Journal of Cosmology and Astroparticle Physics
 Pub Date:
 March 2016
 DOI:
 10.1088/14757516/2016/03/031
 arXiv:
 arXiv:1509.08458
 Bibcode:
 2016JCAP...03..031L
 Keywords:

 Astrophysics  Cosmology and Nongalactic Astrophysics;
 General Relativity and Quantum Cosmology;
 High Energy Physics  Theory
 EPrint:
 24 pages, 4 figures