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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 hypothesized to be perpetual, the 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.

It's an appealing concept in part because it removes the need for a state called a singularity that corresponds to "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 or "bouncing" cosmologies fall flat.

The research shows that the latest version of this theory—a cyclic model that resolves long-standing concerns about —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.

"People proposed bouncing 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 type of model, which addresses problems with entropy, even if the universe has cycles, it still has to have a beginning."

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