First-order thermodynamics of Horndeski gravity

We extend to the Horndeski realm the irreversible thermodynamics description of gravity previously studied in first generation scalar-tensor theories. We identify a subclass of Horndeski theories as an out-of--equilibrium state, while general relativity corresponds to an equilibrium state. In this context, we identify an effective heat current, temperature of gravity, and shear viscosity in the space of theories. The identification is accomplished by recasting the field equations as effective Einstein equations with an effective dissipative fluid, with Einstein gravity as the equilibrium state, following Eckarts first-order thermodynamics.