Originally Posted By: Bill S.

Although I appreciate this line of reasoning, I feel that, in terms of the physical scenario, it is little more than an exercise in semantics. Your distinction appears to be similar. Is an explosion not an expansion brought about by thermodynamic change?

Yes, an explosion is also an expansion brought about by thermodynamic changes. However, there is a difference. Consider a nuclear bomb explosion and a nuclear reactor; both are similar in thermodynamic changes. But in a reactor, the change is controlled one (here, it is controlled by us). Similarly, the model of expansion 'with an explosion' and 'without an explosion' are different.

The thermodynamic change not involving an explosion will be very slow, and the system will be in equilibrium at every instant. Such changes are reversible changes. (A perfectly reversible change is defined as a change that happens infinitely slowly, and where the system is always in equilibrium).In the case of the universe, which is an isolated system, the expansion continues for billions of years (an observed fact), and after that the reverse process or contraction takes place (in my opinion).