Self-accelerating universe in scalar-tensor theories after GW170817

TL;DR

This paper investigates scalar-tensor theories compatible with GW170817, demonstrating their potential to explain cosmic acceleration without a cosmological constant and analyzing their observational viability.

Contribution

It identifies viable scalar-tensor theories post-GW170817 that can produce self-accelerating solutions and assesses their consistency with astrophysical and gravitational wave constraints.

Findings

Existence of scaling solutions leading to late-time de Sitter expansion.

Compatibility of these theories with gravitational wave speed constraints.

Potential for testing these theories through cosmological observations.

Abstract

The recent simultaneous detection of gravitational waves and a gamma ray burst from a neutron star merger significantly shrank the space of viable scalar-tensor theories by demanding that the speed of gravity is equal to that of light. The survived theories belong to the class of degenerate higher order scalar-tensor theories. We study whether these theories are suitable as dark energy candidates. We find scaling solutions in the matter dominated universe that lead to de Sitter solutions at late times without the cosmological constant, realising self-acceleration. We evaluate quasi-static perturbations around self-accelerating solutions and show that the stringent constraints coming from astrophysical objects and gravitational waves can be satisfied, leaving interesting possibilities to test these theories by cosmological observations.