From the smallest bacteria to the largest galaxies, death is imminent; even from a cosmic perspective, the timescales are too large for us to really understand. Ultimately, even the universe itself should come to an end — when the last ray of light is gone, all that’s left is the icy, dense chunk of the Dead Star.
That is, at least, how it looks under the current cosmological model. What if our universe didn’t freeze to death, but instead collapsed, re-expanded and collapsed again and again like a giant cosmic lung?
It’s not a widely accepted theory, but to some cosmologists, our universe may just be one in a series of births, deaths and rebirths with no beginning or end — not a big bang, but a big bounce.
Now physicists have shown that the latest iteration of the Great Bounce Hypothesis — which addresses major problems from previous iterations — still has significant limitations.
“People have proposed bouncing the universe to make cosmic infinity a thing of the past, but what we show is that one of the latest types of these models doesn’t work,” said physicist Wilkinney of the University at Buffalo.
“In this new model for solving the entropy problem, even if the universe has cycles, it still has to have a beginning.”
Currently, the most accepted model of our universe suggests that it emerged from a point of origin called a singularity. About 13.8 billion years ago, the universe as we know it began expanding out of an impossibly dense time and space…for some reason.
Unfortunately, the models supporting the “big bang” explanation have little to say about what this singularity looks like.
As an alternative, the big bounce hypothesis can circumvent the singularity problem by eliminating the singularity entirely. Instead, a collapsing universe will bounce back before it reaches such a model-breaking moment.
However, this assumption is not without its own problems. A constantly “bouncing” universe should also have increasing entropy, a measure of disorder in the universe. If the Big Bang was just one in a series of eternal explosions, then the entropy should be very high; but it is not. In fact, if the universe had high entropy at the Big Bang, it couldn’t have existed as we know it.
In 2019, the big rally eased with the release of a revised model containing solutions to this significant obstacle that has held back the hypothesis for decades. The researchers found that each cycle of cosmic expansion dilutes entropy enough to allow the universe to return to its original state before the next bounce.
It’s a huge deal that seems to put the big rally back on the table as a plausible cosmic model. But now, other scientists do what scientists do best. They poked a new hole in the revised model.
Kinney and his colleague, physicist Nina Stein, also of the University at Buffalo, performed a series of calculations and found that a looping universe couldn’t go back indefinitely.
“Long story short, we show that in solving the entropy problem, you create a situation where the universe must have had a beginning,” Kinney explained. “Our proof shows in general that any cyclic model that eliminates entropy through inflation must have a beginning.”
This does not mean that the circular universe is dead in water. The team noted that their work does not apply to physicist Roger Penrose’s model of the cyclic universe, known as conformal cyclic cosmology. According to his version of the repeating universe, each cycle expands infinitely, with no periods of contraction. This is rather complicated stuff and needs further research.
For now, however, the big rally seems to require at least a little more thought to remain viable.
“There was a point in time when there was nothing, and the idea of no time bothered us, and we wanted to know what was there—including scientists,” Stein said. “But as far as we know, there must have been a ‘beginning.’ There’s a question left unanswered, ‘What happened before that?'”
The research has been published in journal of cosmology and astroparticle physics.