Journal
JOURNAL OF COSMOLOGY AND ASTROPARTICLE PHYSICS
Volume -, Issue 3, Pages -Publisher
IOP Publishing Ltd
DOI: 10.1088/1475-7516/2016/03/031
Keywords
modified gravity; cosmological parameters from LSS; gravitational waves / experiments; dark energy theory
Funding
- U.K. STFC Consolidated Grant for Astronomy and Astrophysics at the University of Edinburgh
- Royal Society Wolfson Research Merit Award
- SNSF Advanced Postdoc.Mobility Fellowship [161058]
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We identify a scalar-tensor model embedded in the Horndeski action whose cosmological background and linear scalar fluctuations are degenerate with the concordance cosmology. The model admits a self-accelerated 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, self-acceleration of the model implies a present cosmological tensor mode propagation at less than or similar to 95 % of the speed of light with a damping of the wave amplitude that is greater than or similar to 5% less efficient than in general relativity. We show that these discrepancies are endemic to self-accelerated Horndeski theories with degenerate large-scale structure and are tested with measurements of gravitational waves emitted by events at cosmological distances. Hence, gravitational-wave cosmology breaks the dark degeneracy in observations of the large-scale structure between two fundamentally different explanations of cosmic acceleration - a cosmological constant and a scalar-tensor modification of gravity. The gravitational wave event GW150914 recently detected with the aLIGO instruments and its potential association with a weak short gamma-ray burst observed with the Fermi GBM experiment may have provided this crucial measurement.
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