## The road to black hole thermodynamics with Λ

*by Dmitry Chernyavsky and Kamal Hajian*

What are volume and pressure in black hole thermodynamics? That is the question!

What do the gas in a balloon and a black hole have in common? For a regular CQG reader the answer should be obvious; both can be described within the framework of thermodynamics. However we know that the gas in balloon is characterised by volume and pressure, as well as other thermodynamic quantities. So, a natural question arises about analogues of the volume and pressure for a black hole.

Answering this question, black hole physicists have noticed that if the universe is filled with a non-zero cosmological constant Λ, this mysterious entity can be absorbed in the energy-momentum tensor of matter, and its contribution resembles a perfect fluid with a pressure proportional to Λ. Continuing with this analogy, one can also introduce a ‘thermodynamic volume’ for a black hole. For instance, the appropriate volume which satisfies the first law of thermodynamics for the Schwarzschild black hole is equal to the volume of a ball with the same radius, but in flat space! Using the notions of the black hole pressure *P* and volume *V*, it is standard to vary the cosmological constant generalising the first law of black hole thermodynamics by *V δP*.

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