A new study suggests that the universe may begin to shrink in "just" 100 million years The study, published in the proceedings of the National Academy of Sciences (PNAs), points out that after nearly 13.8 billion years of uninterrupted expansion, the universe may soon stop expanding and then begin to contract slowly.
Hundreds of millions of years after the big bang, stars formed in the early universe (image)
In this new paper, three scientists attempt to model the nature of dark energy based on past observations of the expansion of the universe. Dark energy is a mysterious force that may lead to the accelerated expansion of the universe. In the team's model, dark energy is not a continuous natural force, but a hypothetical form called the "quintessence", which can decay over time.
The researchers found that although the accelerated expansion of the universe has lasted for billions of years, the repulsive force of dark energy may be weakening. According to their model, the accelerated expansion of the universe may end quickly in the next 65 million years, and then the universe may stop expanding completely in 100 million years. At that time, the universe may enter a period of slow contraction and usher in the "death" - or "rebirth" of time and space in billions of years.
Paul Steinhart, one of the study's authors and director of the Princeton Center for theoretical science at Princeton University, said this could happen "very quickly". "Back 65 million years ago, when the Chicxulub asteroid hit the earth and the dinosaurs died out," Steinhart said. "On a cosmic scale, 65 million years is very short."
Gary Hinshaw, a professor of physics and astronomy at the University of British Columbia in Canada, was not involved in the study, but said the theory was not too controversial or unreliable. However, because the model only relies on the observation of the expansion of the universe in the past, and scientists still lack understanding of the nature of dark energy in the universe, the prediction in this paper can not be verified and can only stay at the theoretical level.
Energy in the void
Since the 1990s, researchers have learned that the expansion of the universe is accelerating; The between galaxies is expanding at a faster rate than billions of years ago. Scientists have named the mysterious force that causes this accelerated expansion dark energy - an invisible entity that seems to be opposite to gravity, which can push the largest objects in the universe farther away from each other rather than pull them together.
Although dark energy accounts for about 70% of the total mass energy of the universe, its nature is still a mystery. The general theory of relativity proposed by Albert Einstein holds that dark energy is actually a cosmological constant and an eternal energy form woven in the structure of time and space. If so, the force exerted by dark energy will never change, and the expansion of the universe should continue (and accelerate) forever.
However, another theory competing with general relativity holds that dark energy is not necessarily constant in order to comply with previous observations of the expansion of the universe. On the contrary, dark energy may be the so-called "Fifth Element", a dynamic field that changes with time. In 1998, three scientists published a paper on this concept in Physical Review Letters, and Steinhart was one of them.
Different from the cosmological constant, "the fifth element" can be either repulsive or attractive, depending on the ratio of kinetic energy and potential energy at a specific time. In the past 14 billion years, the "Fifth Element" has been exclusive. Steinhart said that for most of the 14 billion years, "the fifth element" did not contribute much to the expansion of the universe compared with radiation and matter. This situation changed about 5 billion years ago, when the "Fifth Element" became an important part of the universe, and its gravitational repulsion led to the accelerated expansion of the universe.
"The question we ask in this paper is: must this acceleration last forever?" "If not, what are the other possibilities and how quickly will things change?" Steinhart said
"Death" of dark energy
In the study, Steinhart and two colleagues - Anna iyosh of New York University and kosmin Andre of Princeton University - predicted how the attributes of the "Fifth Element" would change over the next few billion years. To do this, the team created a physical model of the "Fifth Element", showing its repulsion and attraction over time, in line with past observations of the expansion of the universe. If this model can reliably reproduce the expansion history of the universe, they can extend the prediction results to the future.
"It's surprising that the dark energy in their model can decay over time," Hinshaw said. "The power of dark energy will weaken. If so, eventually the antigravity of dark energy will disappear and it will change back to something more like ordinary matter."
According to the team's model, the repulsive force of dark energy may have begun to decay rapidly billions of years ago. In this case, the accelerated expansion of the universe has slowed down. Soon, perhaps within about 65 million years, this acceleration may stop completely; Then, from now on, in less than 100 million years, dark energy will turn into gravity, causing the whole universe to shrink. In other words, after nearly 14 billion years of expansion, space may begin to shrink.
"This will be a very special contraction, which we can call slow contraction," Steinhart said. "Space no longer expands, but contracts at a very slow rate."
Steinhart said that at first the universe would contract so slowly that humans on earth (if they still exist) would not notice any changes. According to the team's model, the universe needs billions of years of slow contraction to reach half its size today.
The end of the universe
Stanhart said that when the universe shrinks to half its size today, two situations may occur. In one case, the universe shrinks until it collapses in a "big squeeze", ending what we know as space-time. In another case, the contraction of the universe will just bring it back to a state similar to the original conditions, and then another "Big Bang" - or "big rebound" - will produce a new universe from the ashes of the old universe.
Stanhart and another colleague described the second situation in a paper published in Physics Letters B in 2019, pointing out that the universe will follow a periodic pattern of expansion, contraction, compression and rebound, in which it will continue to collapse and reshape. If so, our present universe may not be the first or only universe, but just the latest one in the infinite universe series that expanded or contracted before. Stanhart said that all this depends on the changeable nature of dark energy.
So how credible is stanhart's description? Hinshaw believes that the explanation of "what is dark energy" in the new paper is a very reasonable assumption. He added that since all our observations of the expansion of the universe come from objects millions to billions of light-years away from the earth, the current data can only tell us the past of the universe, not its present or future. The universe is likely to be shrinking rapidly, but we won't know until long after the contraction phase begins.
Unfortunately, stanhart also admitted that there is no way to test whether the "Fifth Element" really exists or whether the expansion of the universe has begun to slow down. This is just an attempt to match the theory with previous observations, and the research author has done this well in the new paper. Only time can tell us whether what awaits us in the future is the endless expansion of the universe or the rapid contraction (on the cosmic scale).