The universe could “hop” for eternity. But it still had to start somewhere

From the smallest bacterium to the largest galaxy, death looms on the horizon; even if the time scales are too large in cosmic terms for us to really comprehend. Eventually even the universe itself should come to an end – when the last light dies and the cold, dense clumps of dead stars are all that remains.

At least that’s how it is according to current cosmological models. What if our universe didn’t die a cold death, but instead collapsed, reinflated, and collapsed again and again, like a giant cosmic lung?

It’s not exactly a widely accepted theory, but for some cosmologists, our universe could be just one in a long line of births, deaths, and rebirths that has no beginning or end—not a big bang, but a big bang.

Now, physicists have shown that the latest iteration of the big bounce hypothesis — which had solved significant problems with previous iterations — still has pretty big limitations.

“People have suggested universe hopping 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,” said University of Buffalo physicist Will Kinney.

“In this new type of model that addresses problems with entropy, even if the universe has cycles, it must still have a beginning.”

Currently, the most accepted model of our universe sees it as emanating from a point of origin called a singularity. About 13.8 billion years ago, the universe as we know it started expanding out of an incredibly dense chunk of time and space… for some reason.

Unfortunately, the models supporting a “Big Bang” explanation have little to say about what such a singularity might look like.

The big bounce hypothesis could alternatively circumvent the problem of a singularity by eliminating it entirely. A collapsing universe would instead recover before ever reaching such a landmark moment.

However, the hypothesis was not without its own problems. An endlessly “hopping” universe should also have an endlessly increasing entropy, the measure of the disorder in the universe. So if the big bang was just one in an eternal series of bangs, the entropy would have to have been really high; but it wasn’t. If the universe had high entropy at the Big Bang, it could not exist as we know it.

In 2019, the Big Bounce was given a reprieve with the release of a revised model that included a solution to this significant hurdle that had stalled the hypothesis for decades. The researchers found that the expansion of the universe with each cycle dilutes the entropy enough to return the universe to its pristine state before the next jump.

This was a big deal, seemingly putting the Big Bounce back on the table as a plausible cosmological model; but now other scientists have done what scientists do best. They put a new hole in the revised model.

Kinney and his colleague, physicist Nina Stein, also from the University at Buffalo, performed a series of calculations and found that a cyclic universe cannot extend infinitely into the past.

“In short, we showed that when you solve the entropy problem, you create a situation where the universe must have a beginning,” Kinney explained. “In general, our proof shows that any cyclic model that removes entropy by expansion must have a beginning.”

This does not mean that the cyclic universe is dead in water. The team notes that their work does not apply to physicist Roger Penrose’s model of the cyclic universe, called conformal cyclic cosmology. According to his version of a repeating universe, each cycle expands indefinitely with no period of contraction. This is pretty complicated stuff and will require more poking around.

For now, however, it appears that the Big Bounce will require at least a little more thought to remain viable.

“The idea that there was a point in time before which there was nothing, no time, bothers us and we want to know what was before that, including scientists,” Stein said. “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?'”

The research was published in the Journal of Cosmology and Astroparticle Physics.