Breaking a Dark Degeneracy with Gravitational Waves

TL;DR

This paper demonstrates that gravitational wave observations can distinguish between dark energy models that are otherwise indistinguishable using large-scale structure data, thereby breaking the dark degeneracy.

Contribution

It introduces a scalar-tensor model within Horndeski gravity that mimics standard cosmology in structure but differs in gravitational wave propagation, enabling observational tests.

Findings

Gravitational waves propagate at less than 95% of the speed of light in the model.

Wave amplitude damping is less efficient than in general relativity.

GW150914 could provide the crucial measurement to test the model.

Abstract

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 < 95% of the speed of light with a damping of the wave amplitude that is > 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.