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WE MUST be missing something. The universe is expanding 9 per cent faster than it should be. Either our best measurements are wrong, or a glimmer of new physics is peeking through the cracks of modern cosmology.

“We’ve given these young cosmologists a great toy, and they’re trying to break it. Maybe they have.“

If that’s the case, some lightweight, near-light-speed particles may be missing from our picture of the universe shortly after the big bang. But we might be in luck. Particle physicists have already spent over a decade chasing something that fits the bill: ghostly neutrinos unlike the three already known.

For a cosmological quandary, the issue isn’t that complicated: two ways of measuring how quickly the universe is flying apart are coming up with increasingly different numbers.

The first looks at dimples in the cosmic microwave background, a glow left behind by the hot, soupy universe just a few hundred thousand years after the big bang. The size of these fluctuations let us calculate how quickly the universe was expanding when it began some 13.7 billion years ago.

The other method measures how distant galaxies appear to recede from us as the universe expands – which led to the discovery of dark energy, a mysterious outward pressure pushing the universe apart.

The trouble comes when we compare the two estimates. “They don’t agree,” says Adam Riess of the Space Telescope Science Institute in Baltimore, Maryland, one of the recipients of the 2011 Nobel prize in physics for dark energy’s discovery and an author of a new paper pointing out the tension (arxiv.org/abs/1604.01424).

So what are we missing? Our picture of what the universe is made of can’t change much, since it agrees so well with observations. These show that the history of the universe has been a balancing act between just a few ingredients, which competed for dominance as the universe stretched and changed. This model of the cosmos has been the mainstream idea for years, but it’s showing signs of strain.

“We’ve given these really smart kids, the young cosmologists, what we thought was a pretty good toy, and now they’re trying to break it,” says Michael Turner at the University of Chicago. “Maybe they have.”

Would tweaking the ingredients themselves help make sense of the difference?

One possibility is that dark energy is a little stronger than we thought. Or it could have ramped up over time, giving expansion a bigger push. That’s not a very appealing theory, though, says Avi Loeb of Harvard University.

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