The universe has been expanding for billions of years and will continue to do so. But scientists expect that one day there will be a shift, after which the universe will shrink again and eventually become one big crisis Originates. Will this go on forever? And what happened before? the great explosionWhat is the beginning of the expansion?

In an attempt to get to the heart of the origin and evolution of the universe, some theorists have suggested that the universe is expanding and contracting in an endless cycle. Since this process will be permanent, there can be no beginning and no end, but just a continuous cycle of growth and contraction extending forever, past and future.

head and tail
It is an attractive concept, because in this theory there is no need for a starting point, a state of singularity, where “time” begins in the other paradigms. But physicists Will Kenny and Nina Stein University at Buffalo (UaB) In a new study, I describe why the cyclical or “reflexive” model of the universe poses problems.

The latest version of this theory, a periodic model that appears to solve persistent entropy problems, or perturbations, has been debunked by the UaB team. There’s a problem that comes up again and again: The periodic universes this model describes must have a beginning, Kenny and Stein say.

“Many people have come up with the idea of ​​rebounding universes to explain the infinite past, but our discoveries show that one of the most recent types of this type of model is wrong,” says physics professor Kenny. “In this new paradigm, which considers the problems posed by entropy, even though the universe is of a cyclical nature, there must be a point where it all begins.”

early universe
“There are many reasons to be curious about the early universe, but my personal favorite is the basic human question of what came before all else,” Stein says of the research’s significance. “In many different cultures and periods, people have been telling each other stories about the origins of man, the world, and the universe. The “beginning of the world” is often central to that. “We have always wanted to know where we came from.”

Kenny has the book this year Infinity of Realms written. In it he describes the epic story of cosmic inflation (expansion), another theory describing the origin of the universe. It describes an early stage of the universe, in which a period of rapid expansion is followed by a singularity and a superheated Big Bang. This Big Bang formed the primordial elements, from which later originated the galaxies, stars, planets and atoms in our bodies.

The Carina Nebula. Photo: James Webb (NASA)

cosmic inflation
Cosmic inflation is a well-known model, but it focuses on what happens during and after a period of rapid expansion. It doesn’t burn toes of what happened before the expansion, nor does it explain the starting point or the original singularity.

A purely periodic model that has no beginning or end negates these problems, but Kenny shows that these “rebound universe” models raise all kinds of problems and questions of their own. “Unfortunately, it has been known for nearly a hundred years that these periodic models cannot work because of the chaos, or entropy, that builds up in the universe over time. So each subsequent cycle is different from the previous one. It cannot be a copy of the previous cycle. , so it’s not really cyclical.”

entropy problem
A recent periodic model contradicts this entropy argument by suggesting that the universe expands exponentially during each cycle, causing the universe to diminish again. Everything is being disassembled tremendously, so that cosmic structures like black holes disappear. In this way, the universe returns to its original homogeneous state before another reversal begins.”

“We have shown in our paper that the entropy problem can only be solved if we assume a situation in which the universe had a beginning. We have demonstrated that any cyclic model that dissolves entropy through expansion must have a beginning.”

At first
“The idea that in the past there was a moment when it all began, and that there was nothing before that, not even time, is a difficult concept for us. We really want to know what happened before that. Scientists have also struggled with these questions from a long time ago” , says Stein. “But as far as I can see, there must have been a ‘start’ at some point.” So there is a point somewhere in the past for which there is no answer to the question, “What happened before that?”

Of course, there are still plenty of scientific questions to answer next, according to Kenny: “Our evidence does not apply to the periodic models devised by Roger Penrose. He came up with the idea that the universe expands infinitely with each cycle. We are still working on this.” .