THE LAWS OF THERMODYNAMICS AND THERMODYNAMIC STABILITY OF THE MODIFIED CHAPLYGIN GAS

Laws of thermodynamics have been examined for the universe filled with a perfect fluid, obeying an adiabatic equation of state p = gamma rho - A/rho(alpha) (called modified Chaplygin gas), where gamma, A and alpha are positive constants and rho and p are energy density and thermodynamic pressure respectively. Using general thermodynamics, the behavior of temperature and the thermodynamic stability has been discussed for modified Chaplygin gas. A scenario is obtained such that the thermal equation of state depends on both temperature and volume and there will be thermodynamic stability during the expansion process so that the fluid cools down through the expansion without any phase transition (or passing through any critical point).