Cosmological horizon thermodynamics in Gauss-Bonnet quasi-dilaton Massive Gravity
We investigate the thermodynamic properties of the cosmological apparent horizon in Gauss-Bonnet quasi-dilaton massive gravity. We derive the modified Friedmann equations and reformulate them in standard form, thereby allowing us to study the first and second laws of thermodynamics for the apparent horizon. Both equilibrium and non-equilibrium states are considered. In the equilibrium description, the first law retains the conventional form with the Bekenstein-Hawking area law for the horizon entropy, and we show that the generalized second law is satisfied under the null energy condition. In the non-equilibrium description, the Wald entropy receives a correction from the Gauss-Bonnet coupling, and the first law acquires an additional term associated with using the Wald entropy representation of the Gauss-Bonnet sector. We demonstrate that the total entropy change is non-negative provided the null energy condition, the positive horizon temperature condition, and the Gauss-Bonnet positivity constraints $\xi(\sigma)\ge0$ are simultaneously satisfied. Furthermore, we investigate the holographic entropy bound $S_{\text{inside}} \le S_{\text{horizon}}$. We demonstrate that while the idealized local thermal equilibrium assumption leads to a formal saturation or apparent breakdown during dust-dominated eras, the bound is robustly preserved across all cosmological epochs when utilizing realistic physical fluid temperatures. The condition $\xi(\sigma)\ge0$ is shown to be compatible with the stability constraints derived from tensor perturbations in our previous work. Our results establish that Gauss-Bonnet quasi-dilaton massive gravity is a consistent modified gravity theory from the perspective of horizon thermodynamics and the holographic principle.