Can the Expansion of the Universe Stop?

Can the Expansion of the Universe Stop?
An artist's impression of star formation in the early universe, several hundred million years after the Big Bang. (Image credit: NASA)

New research shows that in just 100 million years, the universe may begin to shrink. After nearly 13.8 billion years of uninterrupted expansion, the universe may soon come to a standstill, then slowly begin to contract, according to a new study published in the journal Proceedings of the National Academy of Sciences.

In the new paper, three scientists attempt to model the nature of dark energy, a mysterious force that's causing the universe to expand faster, based on past observations of cosmic expansion. In the team's model, dark energy is not a fixed force of nature, but an entity called quintessence that can decay over time.

Researchers have found that although the expansion of the universe has been accelerating for billions of years, the driving force of dark energy may weaken.

According to their models, the acceleration of the universe could end rapidly in the next 65 million years.

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 even faster — based on past observations of cosmic expansion. In the team's model, dark energy is not a fixed force of nature, but an entity called quintessence that can decay over time.

According to Steinhardt, one of two things can happen from there. Either the universe shrinks until it collapses in on itself in a great "crash" that ends space-time as we know it - or the universe shrinks enough to return to a state similar to its original conditions, resulting in another Big Bang - or a massive collapse that gives birth to a new universe from the ashes of the old one. "vault".

"Back to 65 million years ago, when the asteroid Chicxulub crashed into Earth and wiped out the dinosaurs," Steinhardt told Live Science. “On a cosmic scale, 65 million years is pretty short.”

Gary Hinshaw, professor of physics and astronomy at the University of British Columbia, who was not involved in the study, told Live Science that nothing about this theory is controversial or implausible.

However, because the model depends solely on past observations of expansion – and 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 as theories.

Since the 1990s, scientists have realized 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 called the mysterious source of this acceleration dark energy — an invisible entity that works against gravity, pushing the universe's largest objects apart rather than bringing them together.

Although dark energy accounts for about 70% of the total mass energy 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 unchanging form of energy woven into the fabric of space-time.

If that's the case, and the force exerted by dark energy can never change, the universe should continue to expand (and accelerate) forever.

Dark energy proposes a rival theory that does not need constant but to align with recent cosmic expansion observations. Rather, dark energy can be something called conciseness – a dynamic field that changes over time. (Steinhardt was one of three scientists to introduce the idea in a 1998 paper in Physical Review Letters.)

Contrary to the cosmological constant, conciseness can be repulsive or attractive, depending on the ratio of its kinetic and potential energy at a given time. In the last 14 billion years, conciseness has been atrocious. For most of this period, radiation and matter contributed insignificantly to the expansion of the universe, Steinhardt said.

That changed about five billion years ago, when conciseness became the dominant component and gravitational repulsion caused the expansion of the universe to accelerate.

“The question we raise in this post is, 'Does this momentum have to last forever? said Steinhardt. “And if not, what are the alternatives and how soon can things change?”

The Death of Dark Energy

In their work, Steinhardt and colleagues, Anna Ijjas of New York University and Cosmin Andrei of Princeton, predicted how the properties of the essence might change over the next few billion years.

To do this, the team created a physical conciseness model to match past observations of the expansion of the universe, showing its repulsive and attractive force over time. After the team's model was able to reliably reproduce the universe's expansion history, they extended their predictions into the future.

"To their surprise, the dark energy in their models can degrade over time," said Hinshaw. “It can weaken. And if it does this in a certain way, then eventually the antigravitational property of dark energy disappears and it reverts to something more like ordinary matter.”

According to the team's model, dark energy's impetus 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 altogether – then, as soon as 100 million years from now, dark energy could become attractive and cause the entire universe to begin to contract. In other words, after about 14 billion years of growth, space may begin to shrink.

“That would be a very specific type of contraction that we call slow contraction,” Steinhardt said. “Instead of expanding, the field is shrinking very, very slowly.”

Steinhardt said that initially the contraction of the universe would be so slow that hypothetical people still alive on Earth would not notice a change. According to the team's model, it would take several billion years of slow contraction for the universe to reach about half its present size.

Is it the end of the universe?
Steinhardt said one of two things can happen from there. The universe itself contracts so much that it collapses in a very short time—or enough universe contracts to revert to a state similar to what we know—or another Big Bang, or a big leap in space—time—infinite, unending, old one. to create a new universe from the ashes.

In this second scenario (described by Steinhardt and another colleague in a 2019 article in Physics Letters B), the universe follows a cyclical pattern of expansion and contraction, exercise and leap, which is constantly collapsing and reshaping. If that's true, then our current universe may not be the first or the only universe, but the last in an endless series of universes that are expanding and contracting before us, Steinhardt said. And it all comes down to the volatile nature of dark energy.

How reasonable is all this? Hinshaw said the quintessence interpretation of the new paper is "a perfectly reasonable assumption for what dark energy is."

“Since all of our observations of cosmic expansion come from objects millions to billions of light-years from Earth, the available data can only inform scientists about the past of the universe, not its present or future,” he added.

So, the universe might very well be heading towards a squeeze, and we wouldn't have a way of knowing until long after the contraction phase begins.

“I think how compelling you find this theory, and more importantly, how testable do you find it?” added Hinshaw.

Unfortunately, Steinhardt admitted that there is no good way to test whether the essence is real or whether cosmic expansion is starting to slow down. For now, it's just a matter of fitting theory to past observations.

Source: LiveScience

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