Thermalons can mediate phase transitions between different vacua in higher curvature gravity, potentially changing the asymptotic structure of the spacetime. Treating the cosmological constant as a dynamical parameter, we study these phase transitions in the context of extended thermodynamic phase space. We find that in addition to the AdS to dS phase transitions previously studied, thermal AdS space can undergo a phase transition to an asymptotically flat black hole geometry. In the context of AdS to AdS transitions, we comment on the similarities and differences between thermalon transitions and the Hawking-Page transition.

In the context of black hole chemistry, we study the thermodynamics of asymptotically de Sitter black holes with conformal scalar hair in Einstein gravity. The hair parameter allows us to attain thermodynamic equilibrium between the event horizon and the cosmological horizon. We find that the system of the black hole and the de Sitter space surrounding it undergo a "Reverse" Hawking-Page phase transition provided we consider the grand-canonical ensemble. arXiv:1808.03349v2 [hep-th]

We study the possibility of the existence of negative mass bubbles within a de Sitter spacetime background with matter content corresponding to a perfect fluid. It is shown that there exist configurations of the perfect fluid, that everywhere satisfy the dominant energy condition, the Einstein equations and the equations of hydrostatic equilibrium, however asymptotically approach the exact solution of Schwarzschild-de Sitter space-time with a negative mass.

We present explicit expressions of the thermodynamic volume inside and outside the cosmological horizon of Eguchi-Hanson solitons in general odd dimensions. These quantities are calculable and well-defined regardless of whether or not the regularity condition for the soliton is imposed. For the inner case, we show that the reverse isoperimetric inequality is not satisfied for general values of the soliton parameter a, though a narrow range exists for which the inequality does hold. For the outer case, we find that the mass M out satisfies the maximal mass conjecture and the volume is positive. We also show that, by requiring M out to yield the mass of dS spacetime when the soliton parameter vanishes, the associated cosmological volume is always positive.

We consider negative mass solutions in inflating universes. These universes correspond to asymptotically de Sitter space-times. We show that negative mass makes perfect physical sense and could be of relevance in the early universe.Résumé : Nous évaluons les solutions de masse négative au problème d'un univers en inflation. Ces univers correspondent asymptotiquement à des espace-temps de de Sitter. Nous démontrons que la masse négative est physiquement parfaitement acceptable et qu'elle pourrait être pertinente dans la description de l'univers très jeune. [Traduit par la Rédaction]