Generalized Gravitational Phase Transition in Novel 4D Einstein-Gauss-Bonnet Gravity

In this paper, the possible existence of a thermal phase transition from asymptotic anti-de Sitter (AdS) to de Sitter (dS) geometries in vacuum in the context of the novel fourdimensional Einstein-Gauss-Bonnet (4D EGB) gravity is presented. The phase transition proceeds via a thermalon (the Euclidean section of a bubble thin shell) formation, having a black hole in the dS spacetime and a thermal AdS spacetime in the exterior, without introducing any matter field. From the analysis, it is found that, using regularized novel 4D EGB gravity, the gravitational phase transitions occur for all allowed values of the 4D EGB coupling. In the 4D EGB gravity, the existence of the above phase transition is over a narrow range of the 4D EGB coupling, with particular critical values. In contrast, in five-dimensional (5D) EGB gravity, a 5D sector needs a wider range of its coupling for the existence of a thermalon, and there also exist critical values for the emergence of the above phase transition.