  We define thermodynamic pressure $P$ by work density $W$ as the conjugate
quantity of thermodynamic volume $V$ from field equation. We derive the
equations of state $P$=$P(V, T)$ for the Friedmann-Robertson-Walker (FRW)
universe in Einstein gravity and a modified gravity respectively. We find that
the equation of state from Einstein gravity shows no $P$-$V$ phase transition,
while the equation of state from the modified gravity does, where the critical
exponents are the same as those in mean field theory.
