Do “bouncing universes” have a beginning?

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 infinite cycles.

Since this behavior is hypothesized to be perpetual, the universe should have no beginning and no end, only eternal cycles of growth and contraction that extend forever into the future and forever into the past.

It is an attractive concept in part because it eliminates the need for a state called a singularity that corresponds to the “beginning of time” in other models.

But a new study by physicists Will Kinney and Nina Stein of the University at Buffalo highlights one way in which cyclical or “bouncing” cosmologies fall apart.

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 cyclical universes described under this model must have a beginning, Kinney and Stein conclude.

“People proposed bouncing universes to make the universe infinite in the past, but what we show is that one of the newest types of these models doesn’t work,” says Kinney, Ph.D., professor of physics at the Faculty of Education of the UB. Arts and Sciences. “In this new kind of model, which addresses entropy issues, even if the universe has cycles, it still has to have a beginning.”

“There are many reasons to be curious about the early universe, but I think the one I like the most is the natural human tendency to want to know what came before,” says Stein, a UB Ph.D. physics student, regarding the importance of research. “Throughout cultures and histories, humans have told stories about creation, about ‘in the beginning’. We always want to know where we came from.”

The study, funded by the National Science Foundation, was published in June Astroparticle Cosmology and Physics Magazine. The paper is titled “Cyclic Cosmology and Geodesic Completeness”.

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

Kinney is the author of a 2022 book titled “An Infinity of Worlds,” which tells the epic story of cosmic inflation, a competing theory about the origins of the universe. Under this model, the early universe was characterized by a period of rapid expansion from a singularity, followed by the superhot Big Bang, which forged the primordial elements that formed galaxies and stars and planets, and the atoms of our bodies and 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 does not explain what happened before, nor does it describe the conditions of the initial singularity.

A truly cyclic universe would avoid these problems: if the universe is engaged in endless cycles of expansion and contraction, it need not have a beginning. But, as Kinney points out, these rebound models raise their own variety of unsustainable questions.

“Unfortunately, it’s been known for almost 100 years that these cyclical models 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 solves this problem of entropy accumulation by proposing that the universe expands a lot with each cycle, diluting entropy. Everything is stretched to get rid of cosmic structures like black holes, which return the universe 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 proof shows in general that any cyclical model that removes the “entropy by expansion must have a beginning”.

“The idea that there was a point in time before which there was nothing, no time, disturbs us, and we want to know what was there before, scientists included,” says Stein. “But as far as we can tell, there must have been a ‘beginning’. There is a point for which there is no answer to the question, ‘What came before?'”

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


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

Provided by the University at Buffalo

Summons: Do “bouncing universes” have a beginning? (2022, August 5) retrieved August 6, 2022 from https://phys.org/news/2022-08-universes.html

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