Thermodynamic Equation of State and Joule-Thomson Expansion for a FRW
  Universe
  We investigate the thermodynamics for a FRW (Friedmann-Robertson-Walker)
universe in the extended phase space. From the unified first law for a FRW
universe, we derive the first law of thermodynamics, and discover that the
proper thermodynamic pressure for a FRW universe with the cosmological constant
should be defined by the combination of the work density of the matter field
and the cosmological constant. With such definition, we obtain the
thermodynamic equation of state for a FRW universe, i.e. $P=P(R_A, T)$.
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 ${\cal H}$ is negative. These investigations shed
insights on the evolution of our universe in view of thermodynamics.
