Quintessence in the Weyl-Gauss-Bonnet model

Abstract: Quintessence models have been widely examined in the context of scalar-Gauss-Bonnet gravity, a subclass of Horndeski's theory, and were proposed as viable candidates for Dark Energy. However, the relatively recent observational constraints on the speed of gravitational waves $c_{\textrm{GW}}$ have resulted in many of those models being ruled out because they predict $c_{\textrm{GW}} \neq c$ generally. While these were formulated in the metric formalism of gravity, we put forward a new quintessence model with the scalar-Gauss-Bonnet action but in Weyl geometry, where the connection is not metric compatible. We find the fixed points of the dynamical system under some assumptions and determine their stability via linear analysis. The past evolution of the Universe can be reproduced correctly, but the late Universe constraints on $c_{\textrm{GW}}$ are grossly violated. Moreover, at these later stages tensor modes suffer from the gradient instabilities. We also consider the implications of imposing an additional constraint $c_{\textrm{GW}} = c$, but this does not lead to evolution that is consistent with cosmological observations