In the new work, three scientists try to model the nature of dark energy – a mysterious force that seems to cause the Universe to expand faster and faster – based on previous observations of cosmic expansion. In the group model, dark energy is not a constant force of nature, but an entity called quintessence, which can decompose over time. Researchers have found that although the expansion of the Universe has been accelerating for billions of years, the repulsive force of dark energy can be attenuated. 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 could enter a 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 could happen “extremely” 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 to 65 million years, that was when the asteroid Chicxulub hit Earth and wiped out the dinosaurs,” Steinhardt told Live Science. “On a cosmic scale, 65 million years is extremely short.” “Nothing about this theory is controversial or unlikely,” Gary Hinshaw, a professor of physics and astronomy at the University of British Columbia who did not participate in the study, told Live Science. However, because the model is based only on previous observations of expansion – and because 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, only theories can remain.
Vacuum energy
Since the 1990s, scientists have realized that the expansion of the Universe is accelerating. the space between galaxies is expanding faster now than it did billions of years ago. Scientists have named the mysterious source of this acceleration dark energy – an invisible entity that appears to work against gravity, pushing the massive objects of the Universe farther instead of dragging them together. Although dark energy accounts for about 70 percent of the total mass energy of the Universe, its properties remain an absolute mystery. A popular theory, introduced by Albert Einstein, is that dark energy is a cosmological constant – an unchanging form of energy woven into the web of spacetime. If this is the case, and the force exerted by the dark energy can never be changed, then the Universe must continue to expand (and accelerate) forever. However, a competing theory suggests that dark energy does not have to be constant to match observations of previous cosmic expansion. In contrast, 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 paper in Physical Review Letters.) In contrast to the cosmological constant, quintessence can be either repulsive or attractive, depending on the ratio of its kinetic and dynamic energy at a given moment. For the last 14 billion years, quintessence has been disgusting. For most of this period, however, it contributed insignificantly to the expansion of the Universe compared to radiation and matter. That changed about five billion years ago, when quintessence became the dominant ingredient and its gravitational repulsion accelerated the expansion of the universe. “The question we ask in this article is, ‘Should this acceleration last forever?’ said Steinhardt. “And if not, what are the alternatives and how soon could things change?”
The death of dark energy
In their study, Steinhardt and his colleagues, Anna Ijjas of New York University and Cosmin Andrei of Princeton, predicted how the properties of quintessence could change over the next billions of years. To do this, the team created a natural quintessential model, showing its repulsive and attractive power over time to match previous observations of the expansion of the Universe. Once the team model could reliably reproduce the history of the expansion of the Universe, it extended their predictions to the future. “To their surprise, the dark energy in their model can decompose over time,” Hinshaw said. “Its power can be weakened. And if it does in a certain way, then eventually the antigravity property of dark energy disappears and it goes back to something that looks more like ordinary matter.” According to the group model, the repulsive force of dark energy could be in the middle of a rapid decline that potentially began billions of years ago. In this scenario, the accelerating expansion of the Universe is already slowing down today. Soon, perhaps in about 65 million years, this acceleration could stop altogether – then, in just 100 million years from now, dark energy could become attractive, causing the entire universe to begin to contract. In other words, after nearly 14 billion years of growth, space could begin to shrink. “This would be a very special kind of contraction that we call slow contraction,” Steinhardt said. “Instead of expanding, space is shrinking very, very slowly.” At first, the contraction of the Universe would be so slow that any hypothetical human still living on Earth would not even notice a change, Steinhardt said. According to the team model, it will take several billion years of slow contraction for the Universe to reach about half its size today.
The end of the Universe?
From there, one of two things could happen, Steinhardt said. Either the Universe shrinks until it collapses on its own at a great “critical moment”, ending spacetime 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 great “bounce” – it happens, creating a new Universe from the ashes of the old. In this second scenario (which Steinhardt and another colleague described in a 2019 paper in Physics Letters B), the Universe follows a circular pattern of expansion and contraction, creasing and bouncing, constantly collapsing and rebuilding. If this is true, then our Universe today may not be the first or the only Universe, but simply the last in an infinite series of Universes that have expanded and contracted before ours, Steinhardt said. And it all depends on the volatile nature of dark energy. How realistic is all this? Hinshaw said the interpretation of the quintessence of the new newspaper was a “perfectly reasonable assumption about what dark energy is”. “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 about the Universe’s past, not its present or future,” he added. Thus, the Universe could well be moving towards a critical moment, and we would have no way of knowing it long after the onset of the contraction phase. “I think it really sums up how exciting you think this theory is and, most importantly, how controlled you think it is?” Hinshaw added. “Unfortunately, there is no good way to check whether quintessence is real or whether cosmic expansion has begun to slow,” Steinhardt admitted. For now, it’s just a matter of combining theory with observations of the past – and the authors do so with skill in their new work. Whether a future of endless growth or rapid decay awaits our Universe, only time will tell. This article was originally published by Live Science. Read the original article here.
