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 endless cycles.

Because this behavior is assumed to be eternal, the universe should have no beginning and no end—just eternal cycles of growth and contraction that extend forever into the future, and forever into the past.

It is an appealing 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 from University at Buffalo physicists Will Kinney and Nina Stein highlights one way that cyclical or “bouncing” cosmologies fall flat.

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

“People proposed bounce universes to make the universe infinite into 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 in the UB College of Arts and Sciences . “In this new kind of model, which addresses problems with entropy, 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 my favorite is the natural human tendency to want to know what came before,” says Stein, a UB Ph.D. student in physics, regarding the importance of research. “Across cultures and histories, people have told stories about creation, about ‘in the beginning.’ We will always know where we came from.”

The study, funded by the National Science Foundation, was published in June i Journal of Cosmology and Astroparticle Physics. The paper is entitled “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 of the universe’s origins. Under this model, the early universe was marked by a period of rapid expansion from a singularity, followed by the super-hot Big Bang, which forged the primordial elements that went on to make galaxies and stars and planets, and the atoms in our bodies and everything. other living things.

Cosmic inflation is a leading theory. But it focuses on what happens during and after the age of rapid expansion. It does not explain what came before it, and it does not describe the conditions of the original 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 jumping models raise their own series 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 truly cyclical,” says Kinney. “A more recent cyclical 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 it all out to get rid of cosmic structures like black holes, which return the universe to its original homogeneous state before a new bounce begins.”

“But,” he adds, “long story short, we showed that when you solve the entropy problem, you create a situation where the universe had to have a beginning. Our proof generally shows that any cyclical model that removes entropy by expansion must have a beginning. “

“The idea that there was a time before 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, 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 further research questions, says Kinney: “Our evidence does not apply to a cyclical model proposed by Roger Penrose, where the universe expands infinitely in each cycle. We are working on that.”

Using holograms to illuminate the de Sitter room

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

Provided by the University at Buffalo

Citation: Do ‘bouncing universes’ have a beginning? (2022, August 5) retrieved August 6, 2022 from

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