Everything started with the Big Bang. Most astrophysicists agree that the force generated by the Big Bang continues to push stars, planets, and galaxies apart from each other as our universe expands.
Where they haven’t always agreed — and in some ways, still don’t, is how quickly that is happening, and what happens far down the road. One popular theory around for decades is known as the Big Crunch — basically the opposite of the Big Bang, when everything will stop expanding and get sucked back together again.
“Galaxies are not just receding away from each other, but it speeds up over time,” says Avi Loeb, an astrophysicist at Harvard University.
The Big Crunch Theory
This idea fell out of favors in the late 1990s with the discovery of dark matter and what scientists at the time believed was proof of the accelerating expansion of the universe. The Big Crunch theory died for a quarter century.
Yet, new research using a massive amount of data has revived the possibility of the Big Crunch theory.
“We didn’t find that the universe is slowing down, we found that it is accelerating less,” says Mustapha Ishak-Boushaki, an astrophysicist at the University of Texas at Dallas. “The Pandora’s Box is open now for theories of how to understand the universe.”
Expansion from the Big Bang
To understand the Big Crunch theory, we have to look at what happened first — the Big Bang model. This widely accepted theory holds that our universe began from a massive expansion of space about 13.8 billion years ago.
This Big Bang didn’t just happen at one place and expand from there — it happened everywhere simultaneously. Ever since then, the universe has been expanding, meaning essentially, that all the galaxies are gradually moving away from each other — something that a telescope can observe.
Read More: Did the Big Bang Happen More Than Once?
What Is the Big Crunch?
Some astrophysicists believe that the universe’s expansion will eventually begin to slow down, to the point where it begins to reverse due to the forces of gravity. All the matter in the universe will clump back together, getting sucked back to the point where it started. This is the Big Crunch — or the reversal of the Big Bang.
This theory explains that many years down the road, the force of gravity will eventually prevail over the explosive force of the Big Bang. Once the universe is crunched back down to where it started, it may blast off once more into another Big Bang, repeating the whole process, though that trajectory is more theoretical.
Dark Energy and the Cosmological Constant
The problem is dark energy. Discovered in the late 1990s, dark energy plays a big role in the expansion of matter in our universe. Scientists don’t know exactly what it is, but they know dark energy is a repellent force that pushes things in our universe apart from each other at an ever-faster speed.
It works against the force of gravity, which pulls things together. And as far as researchers can tell, the repellent force is stronger than the force of gravity, Loeb says.
Albert Einstein first posited the theory of a cosmological constant in his theory of general relativity. Then in 1998, scientists who later won the Nobel Prize for their work revised this to show the cosmological constant was positive — essentially the expansion of the universe was accelerating, driven by dark energy.
If the universe does continue to expand at an accelerating rate, then rather than reverse into a Big Crunch, some scientists believe the universe will eventually reach a state known as the Big Freeze — as the heat of the universe spreads out enough, the temperature would be constant everywhere — no more hot suns and supernovas at this point.
Read More: The Universe's Expansion Could End Surprisingly Soon, Say Cosmologists
Is the Big Crunch Theory Still Relevant Today?
While the Big Crunch theory largely fell out of favor for the last quarter century or so, it may be back, due to research conducted by the Dark Energy Spectroscopic Instrument (DESI) international collaboration, including hundreds of scientists and dozens of institutions. Ishak-Boushaki presented their results first in April 2024, which reversed the commonly accepted theory in the new millennium.
“It did take some courage to go outside the usual,” he says, adding that he has been working on this question for more than two decades. “I did not think that this would happen in our lifetime.”
The data from DESI comes from the calculations of the positions and speeds that 15 million galaxies and quasars are moving at. They determined their speed and acceleration and developed a 3D model of the universe. This larger analysis revealed that while the universe is still expanding, the acceleration at which it is doing so is slowing a little.
“Eventually it will come to a zero acceleration,” Ishak-Boushaki says. Eventually, he says, gravity could regain control and begin to pull things back together.
The Future of Our Universe’s Expansion
That doesn’t mean that the Big Crunch is the only possible future, though. It’s also possible that there isn’t enough matter — and gravity — to pull things back together, even if our universe’s expansion does slow down or even stop.
Loeb still believes that these DESI calculations may be a little off. He points to one paper, still in preprint, that analyzes the DESI data with an alternative method to show that the previous model using a cosmological constant still explains the DESI data. Another preprint paper by Loeb and his colleague Xingang Chen offers another alternative possibility.
“The DESI anomaly could also be explained by tweaking the evolution of dark matter rather than dark energy,” he says.
Ishak-Boushaki points out that most papers published since the DESI’s group’s initial announcement of their result have come to similar conclusions, that evolving dark energy in time is not constant.
“The data itself is telling us otherwise,” he says. Ishak-Boushaki is excited about the future due to the DESI calculations. “The cosmological constant was a dead end,” he says. “But sooner or later we will figure out what dark energy is.”
These new discoveries by DESI, if true, could rewrite many other basic concepts of astrophysics, and also could explain ideas like string theory.
“It may change the way that we are putting the pieces of the puzzle together,” Ishak-Boushaki says.
Read More: Is the Universe Expanding?
Article Sources
Our writers at Discovermagazine.com use peer-reviewed studies and high-quality sources for our articles, and our editors review for scientific accuracy and editorial standards. Review the sources used below for this article:
Britannica. Big-bang model
The Nobel Prize. Nobel Prize in Physics 2011
Astronomy. The Big Freeze: How the universe will die
DESI data. DESI Key Publications
Joshua Rapp Learn is an award-winning D.C.-based science writer. An expat Albertan, he contributes to a number of science publications like National Geographic, The New York Times, The Guardian, New Scientist, Hakai, and others.
