After nearly 13.8 billion years of uninterrupted expansion, the Universe may soon come to a standstill and then slowly begin to contract, according to new research published in the journal Proceedings of the National Academy of Sciences suggests.
In the new paper, three scientists attempt to model the nature of dark energy – a mysterious force that appears to be causing the Universe to expand ever faster – based on past observations of cosmic expansion.
In the team model, dark energy is not a constant force of nature, but an entity called quintessence, which can decay over time.
Researchers have found that even though the expansion of the Universe has been accelerating for billions of years, the repulsive force of dark energy may be weakening.
According to their model, the acceleration of the Universe could end rapidly within the next 65 million years – then, within 100 million years, the Universe could stop expanding altogether, and instead , it could enter an era of slow contraction that would end billions of years away. now with the death – or perhaps the rebirth – of time and space.
And all of this could 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 Chicxulub asteroid struck the Earth and eliminated the dinosaurs“, Steinhardt told Live Science. “On a cosmic scale, 65 million years is remarkably short.
nothing about it the 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 relies solely on past observations of the expansion – and because the current nature of dark energy in the Universe is such a mystery – the predictions in this paper are currently untestable. For now, they can only remain theories.
Since the 1990s, scientists have understood that the expansion of the Universe is accelerating; the space between galaxies is expanding faster today than it was billions of years ago.
Scientists have named the mysterious source of this accelerating dark energy – an unseen entity that appears to operate the opposite of gravitymoving the most massive objects in the Universe away rather than bringing them closer.
Although dark energy accounts for about 70% of the total mass of energy in the Universe, its properties remain a complete mystery.
A popular theory, introduced by Albert Einsteinis that dark energy is a cosmological constant – an unchanging form of energy that is woven into the fabric of space-time. If this is the case, and the force exerted by dark energy can never change, then the Universe should continue to expand (and accelerate) forever.
However, a competing theory suggests that dark energy need not be constant to match observations of past cosmic expansion.
Rather, dark energy can be something called quintessence – a dynamic field that changes over time. (Steinhardt was one of three scientists who introduced the idea in a 1998 article in the journal Physical examination letters.)
Unlike the cosmological constant, the quintessence can be repulsive or attractive, depending on the ratio of its kinetic and potential energy at a given instant. For the past 14 billion years, the quintessence was repulsive.
For most of that time, however, it contributed insignificantly compared to radiation and matter to the expansion of the Universe. This changed about five billion years ago, when the quintessence became the dominant component and its gravitational repulsion effect accelerated the expansion of the universe.
“The question we’re asking in this article is, ‘Must this acceleration 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 her colleagues, Anna Ijjas of New York University and Cosmin Andrei of Princeton, predicted how quintessence properties could change over the next billion years.
To do this, the team created a physical model of the quintessence, showing its repulsive and attractive power over time, to match past observations of the expansion of the Universe. Once the team’s model was able to reliably reproduce the history of the expansion of the universe, they extended their predictions into the future.
“To their surprise, the dark energy in their model can decay over time,” Hinshaw said. “Its strength can weaken. And if it does that in a certain way, the antigravity property of dark energy eventually wears off and it becomes something more like ordinary matter again.”
According to the team’s model, the repulsive 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 in about 65 million years, this acceleration could come to a complete halt – then, in just 100 million years, dark energy could become attractive, causing the entire universe to contract. .
In other words, after almost 14 billion years of growth, space may start to shrink.
“It would be a very special kind of contraction that we call a slow twitch,” Steinhardt said. “Instead of expanding, space contracts very, very slowly.”
Initially, the contraction of the Universe would be so slow that any hypothetical human 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 its current size.
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 spacetime 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 great “rebound” – occurs, creating a new Universe from the ashes of the old.
In this second scenario (which Steinhardt and another colleague described in a 2019 article in the journal Physics Letters B), the Universe follows a cyclical pattern of expansion and contraction, crunches and bounces, constantly collapsing and reshaping.
If that’s true, then our current universe may not be the first or only universe, but just the latest in an endless series of universes that expanded and contracted before ours, Steinhardt said. And it all depends on the changing nature of dark energy.
How plausible is all this? Hinshaw said the new paper’s interpretation of quintessence is a “perfectly reasonable guess about what dark energy is.”
Because all of our observations of cosmic expansion come from objects millions or even billions of light-years from Earth, current data can only tell scientists about the Universe’s past, not about its present or its future, he added.
So the Universe could very well be headed for a crisis, and we would have no way of knowing it long after the contraction phase had begun.
“I think it really comes down to how compelling you find this theory and, more importantly, how testable do you find it?” Hinshaw added.
Unfortunately, there’s no good way to test if the quintessence is real or if the cosmic expansion has started to slow down, Steinhardt admitted. For now, it’s just a matter of adapting the theory to past observations – and the authors do that competently in their new paper.
Whether a future of endless growth or rapid decline awaits our Universe, alone weather will say.