A cyclical universe with multiple Big Bangs could explain one of the greatest mysteries in cosmology today.

Cosmologists know there is something seriously wrong with the standard Big Bang cosmological model. The magnitude of the cosmological constant—a mathematical representation of the energy of empty space—predicted by this theory differs from the measured value by a factor of 10120.

Last week, a paper published online by the journal Science contained a resolution to this disturbing discrepancy. Its authors contend that a very old cyclic universe, one with several “Big Bangs” in its past, each marking the beginning of a new universe, could explain the observed value of the cosmological constant.

“You can have a cosmological constant which is very, very slowly decreasing, over many, many cycles,” said the study’s lead author, Princeton physics professor Paul Steinhardt. “So you have many, many cycles when it was large. But then if you wait long enough, it gets smaller and smaller and smaller.”

The disparity between theory and reality lies in calculating vacuum energy—the energy of empty space—which comes from quantum fluctuations that bring pairs of fundamental particles into and out of existence.

“If you’re trying to add up the total energy of the universe, you’d be adding up all these contributions,” Steinhardt said. “The problem is, when you look at any one of the contributions, they’re exponentially huge compared to the actual observed value of the cosmological constant.”

In the past, physicists sought a natural symmetry to show that vacuum energy was negligible, Steinhardt said. But they abandoned this search a decade ago when they found that the universe is accelerating in its expansion, giving it a positive vacuum energy.

In the new paper, Steinhardt and his colleague, Cambridge professor Neil Turok, propose that if the universe is very old—far older than the widely believed age of 14 billion years—and Big Bangs occur cyclically, the cosmological constant may have once been large, then gradually decreased over many cycles. They propose that the rate of change of the constant decreases as it gets smaller; so, for most of the time, the universe has a small cosmological constant, as ours does now.

A primary advantage of the model, said Steinhardt, is that the conditions we observe are typical, whereas standard models predict only a tiny fraction of a universe will be conducive to life. Proponents of these theories often justify our extraordinarily unlikely conditions with the anthropic principle, which argues that we could only exist in these rare conditions. Therefore, it’s unsurprising that, since we do exist, we observe these rare conditions.

UCLA astronomy professor Ned Wright said that even though there is currently no way to verify the model, the authors have a strong hypothesis.

“This paper by Steinhardt and Turok combines two speculative ideas, and the combination is more interesting than the individual pieces,” Wright said via e-mail, referring to the notions of a cyclic universe and the small, positive cosmological constant over time.

He added that future evidence of an inflationary period—an early epoch of rapid expansion absent here, but integral to the standard Big Bang model—could overturn the new theory.

Steinhardt concedes that the model may eventually be falsified, but he said it gives scientists a new way of approaching cosmological questions.

“It’s premature,” he said. “But it sort of opens my mind up to this new way of solving certain problems by using the idea that the universe is much older, and so things may not be what you might typically expect—they may have been evolving over time in some regular, predictable way.”

Originally published May 9, 2006

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