Evidence suggests the universe’s expansion may be slowing

To track the universe’s expansion, scientists used the Desi to map millions of galaxies and then measured the distances between them at different points throughout cosmic history. The results they are getting suggest the universe was expanding between 1% and 3% faster a few billion years ago than it is now, and that the dark energy physicists believe drives the expansion is decreasing.

This is at odds with the mainstream “standard model of cosmology”, which explains how the universe has evolved from the big bang. While the standard model includes the existence of dark matter and dark energy, it assumes that dark energy is constant, meaning the cosmos’s expansion should continue indefinitely rather than slow down. Seshadri Nadathur, a Desi collaborator and cosmologist at Portsmouth University, was one of the first scientists to see the result. “The data looked pristine,” he says. “I was just amazed. I had goosebumps when I first saw the plot.”

That said, more data will be needed to confirm the result; it is still below the “5-sigma” statistical threshold that is typically needed to hail a new discovery in physics. The result, which has been combined with data from other experiments, including maps of the cosmic microwave background radiation – the light left over from the big bang – and supernovas, or exploding stars, stands at 4.2 sigma at the most, which means there is a small chance the new results may be a statistical or measurement error. As Nadathur says, 5-sigma suggests there is a one in 1.7 million chance of the result being a fluke, whereas 4.2-sigma is more like a one in 37,000 chance.

Many people would take those odds, though. Cosmologist John Peacock of Edinburgh University has said he would bet £1,000 on the result being correct.

Not everyone is quite as confident. Peacock’s university colleague Prof Neil Turok is concerned about the need to combine the measurement with other experiments to get a significant result. But if the result is confirmed by data in the next couple of years, it would be a huge deal, he admits.

Prof Katie Mack, a theoretical astrophysicist at the Perimeter Institute for theoretical physics in Ontario, says a confirmation that dark energy is weakening would be “the first significant change” in the standard model of cosmology since the discovery of dark energy in the 1990s. It would ultimately point at some unknown force, process, field or particle lurking in the universe that is not part of our current theories. If scientists can find out what it is, they may be able to update their theories in a way that makes it easier to unite them.

“It would be a deep advance,” says Peebles, who in the 1980s incorporated the concepts of dark matter and dark energy – thought to be a “cosmological constant” – into the standard model of cosmology. But “it doesn’t change everything”, he adds. The universe would still be expanding.

Peebles has always been open to the possibility that dark energy may not be constant. “What I threw into cosmology in the 1980s was just the simplest thing I could get away with. I never thought it would work as well as it did. And I’m pretty sure it’s not complete – it’s got to be something more interesting than what we have now.”

Perhaps we are about to unravel the interesting bit, which is the true nature of the dark energy that makes up about 68% of the universe. Nobody actually knows what dark energy is. In the standard model, it is assumed to be the natural energy of empty space . We may think of empty space as a sort of nothingness, but that does not mean it is. After all, a fish may think of water as nothingness, but water has energy due to its movement, surface tension and so forth.

Similarly, empty space also has energy, according to quantum mechanics, the theory that rules the microworld of atoms and particles. Quantum theory suggests that empty space is never completely empty – there are particles popping in and out of existence constantly. But when you use quantum mechanics to calculate what energy empty space should have, you get a “ridiculously large” value, according to Peebles – and one that contradicts real-world measurements. Indeed, if dark energy were that large, it would likely rip all matter apart before galaxies and planets could form.

But if dark energy varies over time, as is now suggested, it may be something different from the energy of empty space described by quantum mechanics. It may instead be an enigmatic substance that physicists call “quintessence”. This would be a type of field in space that changes over time. The trouble is, if such a field existed, it would exert a “fifth force” in the universe, potentially influencing the orbits of planets, which we have not seen any evidence of. What’s more, none of the simplest models for a quintessence field seem to fit the Desi data. “If it is a new physical field, it can’t be the simplest kind. It’s got to be something more complicated,” says Nadathur.

Physicists are therefore rushing to come up with new ideas. Some have suggested that a sort of interaction between dark energy and dark matter may be driving the effect rather than dark energy itself changing. Perhaps dark matter and dark energy are simply different sides of the same coin. But there are other ideas too. A quintessence field interacting with gravity in some way may also fit the data. Or there could be more than one field, or brand new particles that we have not yet discovered.

Some of these models are so complex that they may require us to make changes to Einstein’s well-tested theory of general relativity, which is another cornerstone of the standard model of cosmology. “Einstein invented a cosmological constant because it is obviously consistent with the law,” says Turok. “It may well be that we have to update general relativity.”

Whatever model is right, “it would be new physics” beyond the standard model of particle physics and the standard model of cosmology, says Mack. “That’s exciting. That might give us insights into where we should go next with our theories.”

For a long time now, string theory has proposed a set of models that attempt to unite the clashing branches of general relativity and quantum mechanics. But no matter how excited physicists get about string theory, they have so far failed to verify it with actual experiments – meaning there is no way of knowing whether it is a truthful description of nature or just clever maths.

A weakening dark energy, however, may be interpreted as support for some models of string theory, as recent research has argued. “There have been growing arguments that a constant dark energy is not compatible with string theory,” says Turok. “Many string theorists have argued that if there’s a cosmological constant, it’s only temporary.” And that is what the Desi seems to indicate.

Desi may also force us to reconsider the fate of the universe. The most likely scenario for a universe with constant dark energy is a “big freeze” – a chilly, empty and uneventful cosmos. But if dark energy can suddenly change, there is no way of knowing what will happen. Dark energy may weaken so much in the future that the universe starts to contract, ending in a “big crunch”. Or it may suddenly start to accelerate so much that it tears itself apart, ending in a “big rip”.

And these possibilities have some pretty serious consequences for our understanding of the universe. If the universe could indeed contract, “cyclical models” in which the universe expands, contracts and is reborn, over and over, perhaps through consecutive big bangs, may be plausible. Ultimately, such models suggest that something may have existed before the big bang – a speculation that has long been unpopular among physicists.

Turok created one of the most well-known models of a cyclical universe in 2002 with his colleague Paul Steinhardt from Princeton University. But he has since given up on the idea, as he thinks it relies on too many complicated assumptions. However, one of these assumptions is a varying dark energy. “While the Desi data doesn’t seem to fit with our specific model, it might well fit with more complex cyclical models,” he says. “That would be exciting.”

For now, it is impossible to know if the universe is cyclic. We ultimately need better and more fundamental theories of nature that are informed and verified by observations. And if dark energy really turns out to be weakening, it may help physics break its impasse and finally uncover that much coveted theory of everything.

Photographs by PR, Portsmouth University, Getty