For a system that is completely isolated from its surroundings, basic thermodynamics requires that the quasi-static heat flux dQ and the entropy change dS be related by: dQ = TdS and since the system is isolated, dQ=dS=0 Therefore, an isolated system should be isentropic, that is, it's entropy must remain a constant. The UNiverse is itself an isolated system (please don't bring up branes and stuff like that, let's keep it classical) - so there is no heat input or output. Consequently, the entropy of the Universe should also be constant. However, my understanding of the second law is that the entropy of the Universe is always increasing. Further, many cosmological models require that the entropy of the universe increase drastically immediately after big bang. Can someone please explain if I am wrong somewhere and if so, how do I reconcile the two?