But two new studies by Stanford University cosmologists suggest that it may be time to rethink this popular view of a "runaway universe." Instead of expanding exponentially, our cosmos may be in danger of collapsing in a "mere" 10 to 20 billion years, according to the Stanford team.
"The standard vision at the moment is that the universe is speeding up," said physics Professor Andrei Linde, "so we were surprised to find that a collapse could happen within such a short amount of time."
Linde and his wife, Renata Kallosh – also a professor of physics at Stanford – have authored two companion studies that raise the possibility of a cosmic "big crunch." Both papers are available on the physics research website, www.arXiv.org. "We tried our best to come up with a good theory that explains the acceleration of the universe, but ours is just a model," Linde noted. "It's just part of the answer."
If the Linde-Kallosh model is correct, then the universe, which appears to accelerating now, will begin to slow down and contract. "The universe may be doomed to collapse and disappear," Linde said. "Everything we see now, and at a much larger distance that we cannot see, will collapse into a point smaller than a proton. Locally, it will be the same as if you were inside a black hole. You will just discontinue your existence."
The fate of the cosmos has been hotly debated for decades.
In the early 20th century, Albert Einstein, along with most physicists, believed that the universe was static – even though the equations he developed for his general theory of relativity in 1917 suggested that space itself was either contracting or expanding. To ensure that his new theory was consistent with nature, Einstein invented the "cosmological constant": an arbitrary mathematical term he inserted into his equations to guarantee a static universe – at least on paper.
To Einstein, the cosmological constant may have represented some kind of invisible energy that exists in the vacuum of empty space – a force strong enough to repel the gravitational force exerted by matter. Without this mysterious vacuum energy opposing gravity, the universe eventually would crash in on itself, according to general relativity theory.
But observations by astronomer Edwin Hubble and others in the 1920s proved that distant galaxies are not stationary but are, in fact, moving away from one another. Since the universe was expanding, Einstein no longer needed an antigravity factor in his equations, so he rejected the cosmological constant as irrelevant.
"First Einstein introduced the cosmological constant in his equations, then he said that this was the biggest blunder of his life," Linde observed. "But I recently heard that, apparently, he still liked the idea and discussed it many years later – and continued writing equations that included it."
Fast-forward to 1998, when two independent teams of astronomers discovered that not only is the universe expanding, it is doing so at an ever-faster pace. Their findings were based on observations of supernovae – exploding stars that emit extraordinarily bright light. A supernova is a rare event, but new telescopes equipped with sophisticated electronic sensors allowed the research teams to track dozens of stellar explosions in the sky. What they saw astonished the world of astronomy: The supernovae, it turned out, actually were speeding up at a rate that outpaced the predicted gravitational pull of matter.
What force could be strong enough to overcome gravity and cause the universe to accelerate? Perhaps Einstein was right all along – maybe there is some kind of vacuum energy in space. Einstein called it the cosmological constant, and 80 years later, astronomers would give this invisible force a new name – dark energy.
"The supernova experiments four years ago confirmed a simple picture of the universe where approximately 30 percent of it is made of matter and 70 percent is made of dark energy – whatever it is," Linde observed.
Overnight, a concept that Einstein had rejected was now considered the dominant force in the universe. "The cosmological constant remains one of the biggest mysteries of modern physics," Linde pointed out.
Current predictions that dark energy will continue to overwhelm gravity and produce a runaway universe are based on the assumption that the total density of dark energy in the universe is greater than zero and will remain so forever.
This seems obvious at first glance, since logic dictates that the density of dark energy has to be a positive number. After all, how could the universe be filled with "negative energy"?
But in the strange world of quantum physics and elementary particle theory, everyday logic doesn't always apply.
"During the last year, physicists came to the realization that it is very difficult to understand the origin of positive dark energy in the most advanced versions of elementary particle theory – such as string theory and extended supergravity," Linde said.
"We have found that some of the best attempts to describe dark energy predict that it will gradually become negative, which will cause the universe to become unstable, then collapse," he added. "People who studied general relativity many years ago were aware of this, but to them, this was an academic possibility. It was weird to think about negative vacuum energy seriously. Now we have some reasons to believe it."
The Linde-Kallosh model produced another surprising result: The cosmos will collapse in 10 to 20 billion years – a timeframe comparable with the age of the universe, which is estimated to be about 14 billion years old.
"This was really strange," Linde recalled. "Physicists have known that dark energy could become negative and the universe could collapse sometime in the very distant future, perhaps in a trillion years, but now we see that we might be, not in the beginning, but in the middle of the life cycle of our universe."
The good news, wrote Linde and Kallosh, is that "we still have a lot of time to find out whether this is going to happen."
Linde is quick to acknowledge that the collapsing universe scenario is not the final word on the fate of the cosmos.
"Astronomy is a science once known for its continuous errors," he quipped."There was even a joke: 'Astrophysicists are always in error but never in doubt.' We are just in the very beginning of our investigation of this issue, and it would be incorrect to interpret our results as a reliable doomsday prediction. In any case, our model teaches us an interesting lesson: Even the most abstract theories of elementary particles may end up having great importance in helping us understand the fate of the universe and the fate of humanity."
Direct observation of space with state-of-the-art telescopes, satellites and other instruments will answer many unresolved questions, he added. "We're entering the era of precision cosmology, where we really can get a lot of data, and these data become more precise. Perhaps 10 years, 20 years, 30 years, I don't know, but this is the timescale in which we will get a map of the universe with all its observable parts. So things that were a matter of speculation will gradually become better and better established."
Linde helped pioneer inflationary cosmology – the theory that the universe began not with a fiery big bang but with an extraordinarily rapid expansion (inflation) of space in a vacuum-like state. According to inflationary theory, what we call the universe is just a minute fraction of a much larger cosmos.
"The universe actually looks, not like a bubble, but like a bubble producing new bubbles," Linde explained. "We live in a tiny part of one bubble, and we look around and say, 'This is our universe.'"
If our bubble collapses into a point, a new bubble is likely to inflate somewhere else – possibly giving rise to an entirely new form of life, Linde said.
"Our part of the universe may die, but the universe as a whole, in a sense, is immortal – it just changes its properties," he concluded. "People want to understand their place in the universe, how it was created and how it all will end – if at all. That is something that I would be happy to know the answer to and would pay my taxpayer money for. After all, it was never easy to look into the future, but it is possible to do so, and we should not miss our chance."
Graduate student Sergey Prokushkin and Marina Shmakova, a research associate at the Stanford Linear Accelerator Center, also contributed to the studies. Research was supported with grants from the National Science Foundation, the Templeton Foundation, the U.S. Department of Energy and the Stanford Graduate Fellowships program.
FOR COMMENT CONTACT:
Andrei Linde, Physics
Renata Kallosh, Physics
EDITORS: A photo of Professors Linde and Kallosh is available at http://newsphotos.stanford.edu (slug: "linde/kallosh"). Supernova images are available at www.nasa.gov. The study, "Supergravity, Dark Energy and the Fate of the Universe," can be downloaded at www.arxiv.org/abs/hep-th/0208156. "M-theory, Cosmological Constant and Anthropic Principle" can be downloaded at www.arxiv.org/abs/hep-th/0208157.