Coexistent physics of massive black holes in the phase transitions

The coexistent physics of de Rham-Gabada-dze-Tolley (dRGT) massive black holes and holographic massive black holes is investigated in the extended phase space where the cosmological constant is viewed as pressure. Van der Waals like phase transitions are found for both of them. Coexistent curves of reduced pressure and reduced temperature are found to be different from that of RN-AdS black holes. Coexistent curves of reduced Gibbs free energy and reduced pressure show that Gibbs free energy in the canonical ensemble decreases monotonically with the increasing pressure. The concept number density is introduced to study the coexistent physics. It is uncovered that with the increasing pressure, the number densities of small black holes (SBHs) and large black holes (LBHs) change monotonically in the contrary directions till finally reaching the same value at the critical points of the phase transitions. In other words, with the increasing pressure the number density differences between SBHs and LBHs decrease monotonically before disappearance at the critical points. Further more, HEPM thermodynamic scalar curvature differences between SBHs and LBHs are found to decrease monotonically to zero when approaching to the critical points, which is similar as a RN-AdS black hole. We propose that both the number density difference and the HEPM scalar curvature difference can be order parameters describing the SBH/LBH phase transition and judging the upcoming of critical point where a second-order phase transition takes place. These results provide us with new recognition of the massive gravity. The thermodynamics in the extended phase space of AdS black holes is enriched.