After about 13.8 billion years of non-stop expansion, new research has revealed that the universe could soon stop expanding, and then slowly begin to contract.
In a paper published in the Proceedings of the National Academy of Sciences in the United States, three scientists attempted to model the nature of dark energy – a mysterious force that appears to be causing the universe to expand faster than ever before – based on previous observations of cosmic expansion.
In this model developed by scientists, dark energy is not a constant force of nature, but an entity called substance, which can disintegrate over time.
The researchers found that although 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 quickly end within the next 65 million years. Then, within 100 million years, the universe could stop expanding entirely, and instead could enter an era of slow contraction that ends billions of years from now with the death – or perhaps rebirth – of time and space.
Study co-author Paul Steinhardt, director of the Princeton Center for Theoretical Science at Princeton University in New Jersey, explained that all of this can happen quickly and significantly.
“65 million years ago, that’s the time the Chicxulub asteroid hit Earth and wiped out the dinosaurs,” Steinhardt told Live Science. “On a cosmic scale, 65 million years are remarkably short.”
Gary Henchon, a professor of physics and astronomy at the University of British Columbia, told the magazine that “nothing about this theory is controversial or implausible.”
“Because the model is dependent on previous observations of expansion alone, and because the current nature of dark energy in the universe is a mystery, it is currently impossible to test the predictions in this research. At the moment, they can only remain theories.”
vacuum energy
Since the 1990s, scientists have understood that the expansion of the universe is accelerating, and that intergalactic space is now expanding faster than it was billions of years ago.
Scientists have dubbed the mysterious source of this acceleration dark energy, an invisible entity that appears to work against gravity, pushing the most massive objects in the universe apart rather than bunched together.
Although dark energy makes up about 70% of the total mass energy in the universe, its properties are still a complete mystery.
Albert Einstein put forward a popular theory that dark energy is a cosmological constant, a constant form of unchanging energy that is woven into the fabric of spacetime (a fusion of the concepts of time and space). And if that is the case, and the force exerted by dark energy can never change, then the universe must continue to expand (and accelerate) forever.
However, a competing theory (Steinhardt was one of three scientists who introduced the idea in a 1998 paper published in Physical Review Letters) suggests that dark energy does not need to be constant to accommodate previous cosmic expansion observations.
Alternatively, dark energy may be something called a core, which is a dynamic field that changes over time. About five billion years ago, the core became the dominant ingredient, and the effect of gravitational repulsion accelerated the expansion of the universe.
Dark energy death
In their study, Steinhardt and his colleagues, Anna Egas of New York University and Cosmin Andre of Princeton, predict how the properties of the substance might change over the next several billion years.
To do this, the team created a physical model of the core, showing its repulsive and attractive strengths over time to match previous 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 expanded their predictions for the future. The model showed that “dark energy can degrade over time.”
According to the team’s model, the driving force of dark energy could be in the midst of a rapid decline that likely began billions of years ago.
In this scenario, the accelerating expansion of the universe is already slowing today. And soon, perhaps in 65 million years, this acceleration could stop completely. Then, in 100 million years from now, dark energy could become attractive, causing the entire universe to start shrinking.
In other words, after about 14 billion years of growth, space can begin to shrink.
“This is going to 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 will be so slow that no hypothetical human still alive on Earth will notice a change,” he added.
According to the team’s model, it would take the universe a few billion years of slow contraction to reach half its size today.
From there, Steinhardt explained, “One of two things can happen: Either the universe shrinks until it collapses in on itself in a great crisis, and space-time as we know it ends – or the universe shrinks enough to return to a state similar to its original conditions, and another Big Bang occurs, or a bounce large, creating a new universe from the ashes of the old universe.”
In the second scenario, described by Steinhardt in 2019, the universe follows a cyclical pattern of expansion, contraction, contraction and reversion, which is constantly collapsing and reshaping.
“If this is true, our current universe may not be the first or only universe, but the latest in an endless chain of universes that expanded and contracted before ours. It all depends on the changing nature of dark energy.”
“Because all our observations of cosmic expansion come from objects millions to billions of light years away from Earth, current data can only inform scientists of the universe’s past, not its present or future,” Henshaw said. We have no way of knowing until long after the downturn begins.”