Dark energy in Horndeski theories after GW170817: A review

By R. Kase, S. Tsujikawa. International Journal of Modern Physics D, 2018

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The gravitational wave (GW) event GW170817 from a binary neutron star merger together with the electromagnetic counterpart showed that the speed of GWs [Formula: see text] is very close to that of light for the redshift [Formula: see text]. This places tight constraints on dark energy models constructed in the framework of modified gravitational theories. We review models of the late-time cosmic acceleration in scalar–tensor theories with second-order equations of motion (dubbed Horndeski theories) by paying particular attention to the evolution of dark energy equation of state and observables relevant to the cosmic growth history. We provide a gauge-ready formulation of scalar perturbations in full Horndeski theories and estimate observables associated with the evolution of large-scale structures, cosmic microwave background and weak lensing by employing a so-called quasi-static approximation for the modes deep inside the sound horizon. In light of the recent observational bound of [Formula: see text], we also classify surviving dark energy models into four classes depending on different structure-formation patterns and discuss how they can be observationally distinguished from each other. In particular, the nonminimally coupled theories in which the scalar field [Formula: see text] has a coupling with the Ricci scalar [Formula: see text] of the form [Formula: see text], including [Formula: see text] gravity, can be tightly constrained not only from the cosmic expansion and growth histories but also from the variation of screened gravitational couplings. The cross-correlation of integrated Sachs–Wolfe signal with galaxy distributions can be a key observable for placing bounds on the relative ratio of cubic Galileon density to total dark energy density. The dawn of GW astronomy will open up a new window to constrain nonminimally coupled theories further by the modified luminosity distance of tensor perturbations.

Subfields: Astronomy and Astrophysics, Nuclear and High Energy Physics

View Paper: https://www.semanticscholar.org/paper/5481af5a82610129465b6b28a399524d901c361e

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Citations in the corpus

Horndeski theory and beyond: a review (2019, background): “logical solutions and hence can potentially be viable [169, 175]. In light of GW170817, we have seen that some of free functions in DHOST Lagrangians can be strongly constrained upon imposing cGW = 1 [195, 196, 197, 198, 199, 200, 201, 202]. However, one must be careful about the range of the validi”
Effective field theory of dark energy: A review (2019, unknown): “z < 10−2 [184, 335].”
Dark energy and modified gravity in degenerate higher-order scalar–tensor (DHOST) theories: A review (2018, background): “[46–59] and further references in the review [60]).”