How did the universe begin? Study reveals why the ‘bouncing’ theory is wrong

A new study proves there still must be a beginning to "bouncing" universes that go through cycles of expansion and contraction, with infinite Big Bangs or Big Crunches.
Paul Ratner
Cosmic Cliffs
Cosmic Cliffs


There's still quite a bit of disagreement among cosmological theories about how the universe began. In the more commonly-accepted hypothesis, the universe started from a singularity, a point of infinite density and gravity, inflating rapidly as a result of the Big Bang, and kept expanding until reaching the shape we find ourselves in today, about 13.8 billion years later. In another view, the Big Bang is just one of an endless cycle of expansion and contraction, ending with a “Big Crunch” and restarting with a Big Bang. A new study offers a caveat that challenges the cyclic universe theory, proposing that even the so-called “bouncing” universes must have a beginning.

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The study from University at Buffalo (UB) physicists Will Kinney and Nina Stein zeroes in on the most recent model of the “bouncing” cosmology, as outlined in the 2019 study 'a new kind of cyclic universe'. The UB scientists find that the model, which smooths out some of the previous concerns, still must include a starting point.

"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," explained Professor Kinney, in a press release. "In this new type of model, which addresses problems with entropy, even if the universe has cycles, it still has to have a beginning,” he maintained.

The Big Bounce

The cyclic universe theories, also called Big Bounce models, while marginalized at the expense of the cosmic inflation theory, have one notable advantage. Big Bounce models are based on the premise that the universe would not need a beginning if it were genuinely involved in an ongoing loop of periods of expansion and contraction. It would stretch forever into the past and similarly into the future in an infinite sequence of universes.

A new universe would go through a period of inflation and then contract or collapse. But the collapsing universe would essentially rebound before it even got to the singularity, thus avoiding the Big Bang as imagined by inflationists and instead experiencing something akin to a Big Bounce. At that point, the universe would stop getting smaller and start expanding again. And on and on, it would go through the eons.

Not having to have a cosmic beginning, the Big Bounce theories get around some issues that arise from the cosmic inflation theory, which doesn’t concern itself with what happens before the singularity and the Big Bang.

The problem with the Big Bounce

The new study, published in the Journal of Cosmology and Astroparticle Physics, offers pertinent questions about the viability of the current cyclic universe model.

According to Kinney, the model has been trying to get around one major issue for almost 100 years — bouncing universe models have a problem with disorder or entropy. As entropy accumulates in the universe over a period of time, it would make each cycle different from any other.

“It’s not truly cyclic,” Kinney maintained, according to the press release. “A recent cyclic model gets around this entropy buildup problem by proposing that the universe expands a whole bunch with each cycle, diluting the entropy.” As Kinney added, the stretching out of the universe would expand it to the point of getting rid of cosmic structures like black holes, which would return the universe to its “original homogenous state before another bounce begins.”

What the study from Kinney and Stein shows is that in the way the recent model solved the entropy problem, a beginning must still be added to the universe, disrupting the model’s supposedly cyclic nature. A cyclic universe can’t be stretched without end into the past.

What’s before the singularity?

If there was a beginning to the universe, even in bouncing universe models, as their work demonstrated, the scientists want to find out what happened before that, before the singularity. “The idea that there was a point in time before which there was nothing, no time, bothers us, and we want to know what there was before that — scientists included,” elaborated Stein, the study’s co-author and a Ph.D. student in physics.

In further describing the significance of their research, Stein expressed her enthusiasm for this area of study: “There are a lot of 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,” she stated in the press release. Stein also pointed out that creation stories are commonly found in different cultures and histories of people. People want to have a conception of what was present in the beginning. “We always want to know where we came from,” she added.

The Roger Penrose exception

The scientists behind the new study pointed out that while they stand by their critique of the Big Bounce universes, their work doesn’t apply to the cyclic universe model proposed by the physicist Sir Roger Penrose. Penrose, a Nobel Prize Laureate, who advocates for a model of the cyclic universe that he called conformal cyclic cosmology. In his concept of a bouncing universe, each cycle, which he called an “aeon,” expands to infinity without periods of contraction. Instead, the theory proposes that the universe expands until all matter decays, sucked up by black holes, which also eventually disappear due to the emission of Hawking radiation. As uniformity is restored, the former universe becomes physically identical to the Big Bang of the next aeon, and a new universe is born.

“We’re working on that one,” said Kinney, referring to their work in applying their theory to Penrose’s cyclic model.

Check out their paper titled “Cyclic cosmology and geodesic completeness.”


We consider recently proposed bouncing cosmological models for which the Hubble parameter is periodic in time, but the scale factor grows from one cycle to the next as a mechanism for shedding entropy. Since the scale factor for a flat universe is equivalent to an overall conformal factor, it has been argued that this growth corresponds to a physically irrelevant rescaling, and such bouncing universes can be made perfectly cyclic, extending infinitely into the past and future. We show that any bouncing universe which uses growth of the scale factor to dissipate entropy must necessarily be geodesically past-incomplete, and therefore cannot be truly cyclic in time.