Do “bouncing universes” have a beginning?

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

It’s an attractive concept in part because it removes the need for a state called a singularity that 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 cyclical or “bouncy” cosmologies fall flat.

Research shows that the latest version of this theory – a cyclical model that resolves long-standing concerns about entropy – introduces a new problem (or rather, returns to an old one). The cyclic universes described under this model must have a beginning, conclude Kinney and Stein.

“People have come up with bouncing universes to make the universe infinite in the past, but what we’re showing is that one of the newer kinds of those models doesn’t work,” says Kinney, Ph.D. ., professor of physics at the UB College of Arts and Sciences. “In this new kind of model, which deals with entropy issues, even though the universe has cycles, it still has to have a beginning.”

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

The study, funded by the National Science Foundation, was published in June in the Journal of Cosmology and Astroparticle Physics. The article is entitled “Cyclic cosmology and geodetic completeness”.

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

Kinney is the author of a 2022 book called “An Infinity of Worlds,” which tells the epic story 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 Big Bang, which forged the primordial elements that later formed galaxies, stars, and planets. planets, and the atoms of our body and of all other living beings.

Cosmic inflation is a leading theory. But it focuses on what happens during and after the era of rapid expansion. It doesn’t explain what preceded that, and it doesn’t describe the conditions 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 doesn’t need to have a beginning at all. But as Kinney notes, these bouncing patterns raise their own set of unsustainable questions.

“Unfortunately, it’s been known for almost 100 years that these cyclical patterns don’t work because disorder, or entropy, builds up in the universe over time, so each cycle is different from the last. This isn’t 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 structures such as black holes, which brings back the universe back to its original homogeneous state before another bounce begins.”

“But,” he adds, “in short, we showed that by solving the entropy problem, you create a situation where the universe had to have a beginning. Our evidence shows in general that any model cyclic that removes entropy by expansion must have a beginning.”

“The idea that there was a moment before which there was nothing, no time, bothers us, and we want to know what was before that, including scientists,” Stein says. “But as far as we can tell, 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.”

Using holograms to illuminate de Sitter’s space

More information:
William H. Kinney et al, Cyclic Cosmology and Geodetic Completeness, Journal of Cosmology and Astroparticle Physics (2022). DOI: 10.1088/1475-7516/2022/06/011

Provided by the University at Buffalo

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