And this could all happen "remarkably" quickly, said study co-author Paul Steinhardt, Director of the Princeton Center for Theoretical Science at Princeton University in New Jersey.
"Going back in time 65 million years, that's when the Chicxulub asteroid hit the Earth and eliminated the dinosaurs," Steinhardt told Live Science. "On a cosmic scale, 65 million years is remarkably short."
Nothing about this theory is controversial or implausible, Gary Hinshaw, a professor of physics and astronomy at the University of British Columbia who was not involved in the study, told Live Science. However, because the model hinges on past observations of expansion alone — and because the present nature of dark energy in the universe is such a mystery — the predictions in this paper are currently impossible to test. For now, they can only remain theories.
Energy of the void
Since the 1990s, scientists have understood that the expansion of the universe is speeding up; the space between galaxies is widening faster now than it was billions of years ago. Scientists named the mysterious source of this acceleration dark energy — an invisible entity that seems to work contrary to gravity, pushing the universe's most massive objects farther apart rather than drawing them together.
Though dark energy makes up approximately 70% of the total mass-energy of the universe, its properties remain a total mystery. A popular theory, introduced by Albert Einstein, is that dark energy is a cosmological constant — an unchanging form of energy that's woven into the fabric of space-time. If that's the case, and the force exerted by dark energy can never change, then the universe should continue expanding (and accelerating) forever.
However, a competing theory suggests that dark energy doesn't need to be constant in order to fit with observations of past cosmic expansion. Rather, dark energy may be something called quintessence — a dynamic field that changes over time. (Steinhardt was one of three scientists who introduced the idea in a 1998 paper in the journal Physical Review Letters.)
Unlike the cosmological constant, quintessence can be either repulsive or attractive, depending on the ratio of its kinetic and potential energy at a given time. Over the last 14 billions years, quintessence was repulsive. For most of that period, though, it contributed insignificantly compared to radiation and matter to the expansion of the universe. That changed about five billion years when quintessence became the dominant component and its gravitational repulsion effect caused the expansion of the universe to speed up.
"The question we're raising in this paper is, 'Does this acceleration have to last forever?'" Steinhardt said. "And if not, what are the alternatives, and how soon could things change?"
The death of dark energy
In their study, Steinhardt and his colleagues, Anna Ijjas of New York University and Cosmin Andrei of Princeton, predicted how the properties of quintessence could change over the next several billion years. To do this, the team created a physical model of quintessence, showing its repellent and attractive power over time, to fit with past observations of the universe's expansion. Once the team's model could reliably reproduce the universe's expansion history, they extended their predictions into the future.
"To their surprise, dark energy in their model can decay with time," Hinshaw said. "Its strength can weaken. And if it does so in a certain way, then eventually the antigravitational property of dark energy goes away and it transitions back into something that's more like ordinary matter."
According to the team's model, the repellent force of dark energy could be in the midst of a rapid decline that potentially began billions of years ago.
In this scenario, the accelerated expansion of the universe is already slowing down today. Soon, perhaps within about 65 million years, that acceleration could stop altogether — then, within as few as 100 million years from now, dark energy could become attractive, causing the entire universe to start contracting. In other words, after nearly 14 billion years of growth, space could start to shrink.
"This would be a very special kind of contraction that we call slow contraction," Steinhardt said. "Instead of expanding, space contracts very, very slowly."
Initially, the contraction of the universe would be so slow that any hypothetical humans still alive on Earth wouldn't even notice a change, Steinhardt said. According to the team's model, it would take a few billion years of slow contraction for the universe to reach about half the size it is today.
The end of the universe?
From there, one of two things could happen, Steinhardt said. Either the universe contracts until it collapses in on itself in a big "crunch," ending space-time as we know it — or, the universe contracts just enough to return to a state similar to its original conditions, and another Big Bang — or a big "bounce" — occurs, creating a new universe from the ashes of the old one.
In that second scenario (which Steinhardt and another colleague described in a 2019 paper in the journal Physics Letters B), the universe follows a cyclical pattern of expansion and contraction, crunches and bounces, that constantly collapse and remake it. If that's true, then our current universe may not be the first or only universe, but just the latest in an infinite series of universes that have expanded and contracted before ours, Steinhardt said. And it all hinges on the changeable nature of dark energy.
How plausible is all this? Hinshaw said the new paper's interpretation of quintessence is a "perfectly reasonable supposition for what the dark energy is." Because all of our observations of cosmic expansion come from objects that are millions to billions of light-years away from Earth, current data can only inform scientists about the universe's past, not its present or future, he added. So, the universe could very well be barreling toward a crunch, and we'd have no way of knowing until long after the contraction phase began.
"I think it really just boils down to how compelling do you find this theory to be and, more importantly, how testable do you find it to be?" Hinshaw added.
Unfortunately, there is no good way to test whether quintessence is real, or whether cosmic expansion has started to slow, Steinhardt admitted. For now, it's just a matter of fitting the theory with past observations — and the authors do that capably in their new paper. Whether a future of endless growth or rapid decay awaits our universe, only time will tell.