Self-accelerating universe in scalar-tensor theories after GW170817
  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.
