Dark Energy After GW170817: Dead Ends and the Road Ahead
Abstract
Multimessenger gravitationalwave (GW) astronomy has commenced with the detection of the binary neutron star merger GW170817 and its associated electromagnetic counterparts. The almost coincident observation of both signals places an exquisite bound on the GW speed c_{g}/c 1 ≤5 ×10^{16} . We use this result to probe the nature of dark energy (DE), showing that a large class of scalartensor theories and DE models are highly disfavored. As an example we consider the covariant Galileon, a cosmologically viable, well motivated gravity theory which predicts a variable GW speed at low redshift. Our results eliminate any lateuniverse application of these models, as well as their Horndeski and most of their beyond Horndeski generalizations. Three alternatives (and their combinations) emerge as the only possible scalartensor DE models: (1) restricting Horndeski's action to its simplest terms, (2) applying a conformal transformation which preserves the causal structure, and (3) compensating the different terms that modify the GW speed (to be robust, the compensation has to be independent on the background on which GWs propagate). Our conclusions extend to any other gravity theory predicting varying c_{g} such as EinsteinAether, Hořava gravity, Generalized Proca, tensorvectorscalar gravity (TEVES), and other MONDlike gravities.
 Publication:

Physical Review Letters
 Pub Date:
 December 2017
 DOI:
 10.1103/PhysRevLett.119.251304
 arXiv:
 arXiv:1710.05901
 Bibcode:
 2017PhRvL.119y1304E
 Keywords:

 Astrophysics  Cosmology and Nongalactic Astrophysics;
 General Relativity and Quantum Cosmology;
 High Energy Physics  Phenomenology;
 High Energy Physics  Theory
 EPrint:
 6 pages, 2 figures. Matches PRL version