from a chance occurrence that could have occurred at any moment. If someone were to bet in the lottery an infinite number of times, eventually they’d win and their life would be changed forever. The universe simply won the lottery.

A third alternative, to both a singular explosion and a slow waking up, dates back (at least philosophically) to traditional Eastern notions of eternal cycles. The Hindus, Babylonians, dynastic Chinese, ancient Greeks, and many other cultures have advocated an ever-repeating universe in which the slate is periodically wiped clean. In the mid-1930s, Caltech physicist Richard Tolman explored a similar concept with his “oscillatory universe.” According to this model, instead of a universal beginning, the Big Bang was preceded by the “Big Crunch” of an earlier cycle. That crunch stemmed from the earlier era’s collapse, which was precipitated by a previous Big Bang, and so forth. Each era resembled a closed Friedmann model, glued by fate to its predecessors and successors.

Tolman realized, however, that his model could not produce an endless succession of viable worlds. Rather than starting afresh, each era would preserve the entropy (amount of disorder) of the previous era. Like a movie theater that never sweeps up between screenings, the universe would accumulate more and more disorderly energy. Tolman calculated that this entropy increase would make each cycle longer and longer, with higher and higher temperatures, while less and less hospitable to the development of galaxies, stars, planets, and life. Ultimately, the cosmos would recycle itself into an indefinite array of lifeless stages. We might ask a philosophical question: If a universe arises that no living being is around to observe, does it truly exist?

Note that the various cosmological theories of that period had markedly different suppositions. Both Lemaitre’s model and Eddington’s model made use of a cosmological constant term. Even though Einstein called this term his greatest blunder, it offered cosmologists greater freedom to “fine-tune” each universe model to