After nearly 13.8 billion years of continuous expansion, the universe may soon stop, then slowly begin to shrink, according to new research published in the Proceedings of the National Academy of Sciences.
In the new article, three scientists try to model the nature of dark energy – a mysterious force that seems to be causing the universe to expand faster and faster – based on past observations of cosmic expansion.
In the team model, dark energy is not a constant force of nature, but a unit called quintessence, which can disintegrate over time.
Researchers have found that although the expansion of the universe has been accelerating for billions of years, the repulsive power of dark energy can weaken.
According to their model, the acceleration of the universe could end quickly within the next 65 million years – then, within 100 million years, the universe could stop expanding completely and instead enter an era of slow contraction that ends billions of years from now with the death – or perhaps rebirth – of time and space.
And all of this can happen “remarkably quickly,” said study co-author Paul Steinhardt, director of the Princeton Center for Theoretical Sciences at Princeton University in New Jersey.
“Going back 65 million years, then the asteroid Chicxulub hit Earth and eliminated the dinosaurs,” Steinhardt told Live Science. “On a cosmic scale, 65 million years is remarkably short.”
Nothing in this theory is contradictory or implausible, said Gary Hinshaw, a professor of physics and astronomy at the University of British Columbia who was not involved in the study, he told Live Science.
However, because the model depends only on past observations of expansion – and because the current nature of dark energy in the universe is such a mystery – the predictions in this article are currently impossible to test. For now, they can only remain theories.
Energy of emptiness
Since the 1990s, scientists have learned that the expansion of the universe is accelerating; the space between galaxies is now expanding faster than it was billions of years ago.
Scientists have named the mysterious source of this acceleration dark energy – an invisible being that seems to work against gravity, pushing the most massive objects in the universe farther instead of bringing them together.
Although dark energy makes up approximately 70 percent of the total energy mass of the universe, its properties remain a complete mystery.
A popular theory introduced by Albert Einstein is that dark energy is a cosmological constant – an invariable form of energy that is woven into the fabric of space-time. If this is the case and the power exerted by the dark energy can never change, then the universe must continue to expand (and accelerate) forever.
However, a competitive theory suggests that dark energy does not have to be constant to be combined 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 to present the idea in a 1998 article 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. For the last 14 billion years, quintessence has been repulsive.
For most of this period, however, he contributed little compared to radiation and matter to the expansion of the universe. This changed about five billion years ago, when quintessence became the dominant component and the effect of gravitational repulsion accelerated the expansion of the universe.
“The question we raise in this document is, ‘Should this acceleration last forever?'” Steinhard said. “And if not, what are the alternatives and how soon can things change?”
The death of dark energy
In their study, Steinhardt and his colleagues, Anna Ijas of New York University and Cosmin Andrew of Princeton, predicted how the properties of quintessence could change over the next few billion years.
To do this, the team created a physical model of quintessence, showing its repulsive and attractive power over time, to match past observations of the expansion of the universe. Once the team model could reliably replicate the history of the expansion of the universe, they expanded their predictions into the future.
“To their surprise, the dark energy in their model may disintegrate over time,” Hinshaw said. “Its power can weaken. And if it does it a certain way, then eventually the antigravity property of dark energy disappears and it goes back to something more like ordinary matter.”
According to the team’s model, the repulsive force of dark energy may 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 may stop completely – then, after only 100 million years, dark energy can become attractive, causing the entire universe to begin to shrink.
In other words, after nearly 14 billion years of growth, space may begin to shrink.
“It would be a very special kind of contraction, which we call slow contraction,” Steinhard said. “Instead of expanding, space is shrinking very, very slowly.”
Initially, the shrinking of the universe would have been so slow that all hypothetical people still alive on Earth would not even notice the change, Steinhard said. According to the team’s model, it will take several billion years of slow contraction to reach the universe about half the size it is today.
The end of the universe?
From there, one of two things can happen, Steinhard said. Either the universe shrinks until it collapses into a big “squeak”, ending space-time as we know it – or the universe shrinks enough to return to a state similar to its original conditions, and another Big Bang – or a big “rebound” – arises, creating a new universe from the ashes of the old.
In this second scenario (which Steinhard and another colleague described in a 2019 article in Physics Letters B), the universe follows a cyclical pattern of expansion and contraction, crunching and bouncing that are constantly collapsing and reworking.
If this is true, then our current universe may not be the first or only universe, but simply the latest in an endless series of universes that expanded and contracted before ours, Steinhard said. And it all depends on the changing nature of dark energy.
How plausible is all this? Hinshaw said the interpretation of the quintessence in the new newspaper was “a perfectly reasonable assumption of what dark energy is.”
Because all of our observations of cosmic expansion come from objects millions to billions of light-years from Earth, current data can only inform scientists about the past of the universe, not its present or future, he added.
So the universe can very well go to rupture, and we can’t know until the contraction phase begins.
“I think it really comes down to how convincing you think this theory is and, more importantly, how much you test it?” Hinshaw added.
Unfortunately, there is no good way to test whether quintessence is real or whether space expansion has begun to slow, Steinhardt admitted. For now, it’s just a matter of combining theory with past observations – and the authors do so skillfully in their new article.
Whether the future of endless growth or rapid disintegration awaits our universe, only time will tell.
This article was originally published by Live Science. Read the original article here.
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