Prepared for submission to JHEP Black hole chemistry: thermodynamics with Lambda David Kubizˇn´ak1;2 Robert B. Mann2;1 Mae Teo3 1Perimeter Institute for Theoretical Physics, 31 Caroline St. N., Waterloo, Ontario N2L 2Y5, Canada 2Department of Physics and Astronomy, University of Waterloo, Waterloo, Ontario N2L 3G1, Canada 3Stanford Institute for Theoretical Physics, Stanford University, Stanford, CA 94305, USA E-mail:
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[email protected] Abstract: We review recent developments on the thermodynamics of black holes in extended phase space, where the cosmological constant is interpreted as thermo- dynamic pressure and treated as a thermodynamic variable in its own right. In this approach, the mass of the black hole is no longer regarded as internal energy, rather it is identified with the chemical enthalpy. This leads to an extended dictionary for black hole thermodynamic quantities, in particular a notion of thermodynamic volume emerges for a given black hole spacetime. This volume is conjectured to satisfy the reverse isoperimetric inequality|an inequality imposing a bound on the amount of entropy black hole can carry for a fixed thermodynamic volume. New thermodynamic phase transitions naturally emerge from these identifications. Namely, we show that black holes can be understood from the viewpoint of chemistry, in terms of concepts arXiv:1608.06147v3 [hep-th] 6 May 2021 such as Van der Waals fluids, reentrant phase transitions, and triple points. We also re- view the recent attempts at extending the AdS/CFT dictionary in this setting, discuss the connections with horizon thermodynamics, applications to Lifshitz spacetimes, and outline possible future directions in this field.