Today, he is trying to use the Second Law of Thermodynamics to state that the universe cannot always have existed. "If a closed system is not in the equilibrium configuration, the most probable consequence is that the entropy of the system will increase monotonically in successive instants of time" Princeton Guide to Advanced Physics [Emphasis mine]
The equations of motion (as they are understood) are time-reversible. Entropy is a statistical phenomenon. When "entropy increases" it simply means the number of possible configurations that correspond to the observed state is increasing. There are simply more disordered states than ordered ones. A common illustration of the principle is the shuffling of a deck of cards. If you shuffle a deck of cards, it is highly unlikely that they will come out Ace of hearts, 2 of hearts, 3 of hearts, ... and so one through diamonds, spades and clubs. However, the cards are just as likely to come out in that configuration as they are to come out in any other configuration specified in advance.
Now, if you shuffle decks of cards endlessly, there will be occassions that full order is the result. Simlarly, if the physical universe is eternal in some sense, then "pockets" of low entropy are expected to occur.