First principle study of gravitational pressure and thermodynamics of FRW universe

We make a first principle study of gravitational pressure in cosmic thermo- dynamics. The pressure is directly derived from the unified first law, in fact the Einstein field equation in spherically symmetric spacetime. By using this pressure, we obtain the thermodynamics for the FRW universe, especially presenting the gravitational equation of state for the FRW spacetime itself, i.e. P = P (R-A, T) for the first time. Furthermore, we study the Joule-Thomson expansion as an application of the thermodynamic equation of state to find the cooling-heating property of the FRW universe. We demonstrate that there is an inversion temperature for a FRW universe if its enthalpy H is negative. These investigations shed insights on the evolution of our universe in view of thermodynamics.

Automated summary

In this study, we investigate gravitational pressure in cosmic thermodynamics using a first principle approach. The pressure is derived from the unified first law and the Einstein field equation in spherically symmetric spacetime. By applying this pressure, we establish the thermodynamics of the FRW universe and obtain the gravitational equation of state for the FRW spacetime. Furthermore, we explore the cooling-heating properties of the FRW universe through the Joule-Thomson expansion, finding an inversion temperature when the enthalpy of the FRW universe is negative. These findings provide insights into the evolution of our universe from a thermodynamic perspective.