In trying to understand the nature of the cosmos, some theorists propose that the universe expands and contracts in endless cycles.
Because this behavior is hypothesized to be perpetual, the universe should have no beginning and no end — only eternal cycles of growing and shrinking that extend forever into the future, and forever into the past.
...But a new study by University at Buffalo physicists Will Kinney and Nina Stein highlights one way that cyclic or “bouncing” cosmologies fall flat.
The research shows that the latest version of this theory — a cyclic model that resolves long-standing concerns about entropy — introduces a new problem (or rather, returns to an old one). Cyclic universes described under this model must have a beginning, Kinney and Stein conclude.
...“There are a lot of reasons to be curious about the early universe, but I think my favorite is the natural human tendency to want to know what came before,” says Stein, a UB PhD student in physics, regarding the importance of such research. “Across cultures and histories, humans have told stories about creation, about ‘in the beginning.’ We always want to know where we came from.”
...“Unfortunately, it’s been known for almost 100 years that these cyclic models don’t work because disorder, or entropy, builds up in the universe over time, so each cycle is different from the last one. It’s not truly cyclic,” Kinney says. “A recent cyclic model gets around this entropy build-up problem by proposing that the universe expands a whole bunch with each cycle, diluting the entropy. You stretch everything out to get rid of cosmic structures such as black holes, which returns the universe to its original homogenous state before another bounce begins.”
“But,” he adds, “long story short, we showed that in solving the entropy problem, you create a situation where the universe had to have a beginning. Our proof shows in general that any cyclic model which removes entropy by expansion must have a beginning.”
“The idea that there was a point in time before which there was nothing, no time, bothers us, and we want to know what there was before that — scientists included,” Stein says. “But as far as we can tell, in models that address entropy, there must have been a ‘beginning.’ There is a point for which there is no answer to the question, ‘What came before that?’ ”
And, of course, there are further research questions, Kinney says: “Our proof does not apply to a cyclic model proposed by Roger Penrose, in which the universe expands infinitely in each cycle. We're working on that one.”