Misner-Sharp energy and P-V criticality in quasi-topological cosmology

We presented a sound foundation of thermodynamics for a Friedmann-Robertson-Walker (FRW) universe from the first principle in original work [33]. Based on such an approach, we explore the thermodynamics of cosmology in quasi-topology gravity. Starting from the unified first law, we first obtain the well-defined Misner-Sharp energy in quasi-topology cosmology. We demonstrate that the Misner-Sharp energy is equal to ρV inside the apparent horizon. Further, the unified first law requires extra terms for generalized force and conjugate generalized position, which are identified as thermodynamic pressure and thermodynamic volume, respectively. Hence we naturally derive the equation of state of the FRW universe in quasi-topology gravity, and show that it undergoes P-V phase transitions. We calculate the critical exponents for the phase transition, which may be beneficial to probe the micro theory of quasi-topology gravity.