Though it’s still going strong after 13.7 billion years, the universe may be headed for the big sleep, according to a group of theoretical physicists. A recently published study provocatively suggests that time will end—and for our galaxy, the end will come within the next five billion years.
In keeping with popular theory, most physicists assume that the universe is expanding and will continue to do so infinitely in time and space. But researchers led by Raphael Bousso at University of California, Berkeley are confronting this idea, called eternal inflation, because it can result in some unsettling conclusions.If there is no end to the universe, suggests the new paper, submitted September 23 to Physics arXiv, then every event, even the most improbable, will occur an infinite number of times. When that happens, our concept of probability collapses.
“The kind of probabilities we’re talking about are not some obscure thing that only philosophers are concerned with,” said theoretical physicist Ken Olum of Tufts University, who studies cosmological inflation but was not involved in the new paper. For example, he asked, “What’s the chance that you’ll walk out your front door and get hit by a truck?” In an infinite universe, we have no sense of how likely this or any other event would be on any given day.
Physicists deal with this uncertainty by making a mathematical assumption that the universe won’t expand forever. Specifically, they apply geometric cutoffs to the expansion, allowing them to impose probabilities on the universe and do their calculations. But the new paper argues that if physicists choose to use cutoffs in their work, they should take them seriously: “We should really imagine that the universe comes to an end, that cutoffs are not just a calculational device,” said Olum. If this is true, then every galaxy in the universe must eventually meet the cutoff, and different objects in the universe will encounter the cutoff at different times.
Publication of the article, which averages several of the most successful geometric cutoffs to get the five billion-year endpoint for our galaxy, prompted a torrent of blog posts—an unusual response to research in theoretical physics. But Bousso wasn’t surprised that the ideas received such wide coverage. “Whenever you write a paper that points out that some theory leads to a crazy-sounding conclusion,” he said, “people tend to read only the conclusion: ‘Physicists predict that crazy thing X happens!’”
And that is exactly what happened: headlines proclaimed that the end of time is nigh. While this doomsday hook isn’t solid science yet, Bousso is convinced that his team’s work will inspire significant research. Physicist Adam Brown of Princeton said that the paper reflects “a gathering consensus that there is a problem” with the current understanding of eternal inflation—this is one possible resolution of that problem.
Said Bousso: “Unlike in the economy, crises in physics are an extremely good thing. They tend to precede major upheavals or breakthroughs” in the field, which may lead to drastic changes in the way we conceive of the universe.
There is plenty of discussion and disagreement within the small community of scientists investigating this problem. Two physicists, Vitaly Vanchurin of Stanford University and Alan Guth of the Massachusetts Institute of Technology, are working on a potential resolution of the problem of infinite expansion that doesn’t require the end of time. And Bousso acknowledges that the biggest question—whether there is actually a mechanism that could make time stop—remains unanswered. “If it’s really the end of time,” Olum said, “there ought to be a reason that time comes to an end then and not some other time.”
Only time—or maybe the end of it—will tell how these questions will be resolved.