Do “bouncing universes” have a beginning? Some cosmological models propose that the universe expands and contracts in infinite cycles, but new research finds a crucial flaw in the latest version of this theory

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 supposed to be perpetual, the universe should have no beginning or end – only eternal cycles of waxing and waning that extend forever into the future and forever in the past.

This is an attractive thought in part because it removes the need for a state called a singularity which corresponds to the “beginning of time” in other models.

But a new study by University at Buffalo physicists Will Kinney and Nina Stein highlights one way that cyclic cosmologies or “ bouncers” fall flat.

Research shows that the latest edition of this theory – a cyclical model that resolves longstanding concerns about entropy – introduces a new problem ( or pl soon, returns to an old one). Cyclic universes described under this model must have a beginning, Kinney and Stein conclude.

“People have proposed bouncing universes to make the universe infinite in the past, but this what we’re showing is that one of the newer styles of these models doesn’t work,” says Kinney, PhD, a professor of physics at the UB Faculty of Arts and Sciences. “In this new style model, which deals with entropy problems, even if the universe has cycles, it must still have a beginning.”

“There are many reasons for being curious about the early universe, but I think my favorite is the natural human tendency to want to know what happened before,” says Stein, a physics PhD student at UB, regarding the importance of such research.. “Across cultures and histories, humans have told stories about creation, about ‘the beginning’. We always want to know where we come from.”

The study, funded by the Nationwide Science Foundation, was published in June in the Journal of Cosmology and Astroparticle Physics. The article is titled “Cyclic Cosmology and Geodesic Completeness”.

If the universe had a beginning, how did it begin?

Kinney is the author of a book by 2022 called “An Infinity of Worlds”, which tells the epic tale of cosmic inflation, a competing theory about the origins of the universe. According to this model, the early universe was characterized by a period of rapid expansion from a singularity, followed by the super hot Significant Bang, which forged the primordial elements that later formed the galaxies, stars, and planets. planets, and the atoms of our body and all other living beings.

Cosmic inflation is a first program theory. But it focuses on what happens during and after the era of rapid expansion. It does not explain what preceded this, and it does not describe the situations of the initial singularity.

A truly cyclic universe would circumvent these problems: if the universe is engaged in endless cycles of expansion and contraction, it need not have a beginning at all. But as Kinney notes, these bouncy patterns raise their own set of unsustainable questions. years these cyclic patterns don’t work because disorder, or entropy, builds up in the universe over time, so each cycle is different from the last. It’s not really cyclical,” Kinney says. “A recent cyclical model gets around this problem of entropy accumulation by proposing that the universe expands with each cycle, diluting entropy. You stretch everything to get rid of cosmic constructs such as black holes, which returns the universe to its original homogeneous state before another bounce begins.”

“But”, he adds, “in short, we have shown that by solving the problem of entropy, you create a circumstance where the universe was to have a beginning. Our proof shows in general that any cyclic model that suppresses entropy by growth must have a beginning.”

“The idea that there was a moment before which there was no There was nothing, no time, bothering us, and we want to know what was there before that – including the scientists,” says Stein. “But as far as we can tell, in models that deal with entropy, there must have been a ‘beginning.’ There is a point where there is no answer to the question, ‘What came before that?’ ”

And, of course, there are other research questions, says Kinney: “Our proof does not apply to a cyclical model proposed by Roger Penrose, in which the universe expands infinitely with each cycle. We’re working on that one.”

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