One of the biggest mysteries of the universe is why its expansion seems to be speeding up. Physicists have attributed it to a strange force called dark energy, but exactly how it works is largely speculative. Hypothetical "chameleon" particles could be behind dark energy, and physicists at CERN have been searching for these particles streaming from the Sun. Now, the team has reported the first results of their search.
It's been understood for decades that the universe is expanding, but it was only in 1998 that a team of physicists discovered that it's not happening at a constant rate, but is accelerating – a find that earned the researchers the 2011 Nobel Prize in Physics. "Dark energy" is the name scientists now use for the unknown force that's causing this acceleration, and it's been calculated to make up as much as 68 percent of everything in the universe.
But what dark energy actually is remains a mystery. One hypothesis suggests that the force is carried by particles with a mass that varies based on how much matter is around it. So when these changing "chameleon" particles are in areas with a high density of matter, such as near Earth, their mass would be large and they would would exert a force over a very small distance – less than 1 millimeter. But chameleon particles free-floating out in space would have much smaller masses and their influence would extend over much longer ranges.
That's a convenient explanation for dark energy, but the logical question is "do these chameleon particles even exist?" To find out, scientists have run experiments with the CERN Axion Solar Telescope (CAST), which was built to look for axions – hypothetical dark matter particles – streaming in from the Sun.
In this case, a detector called KWISP was connected to CAST to look for chameleon particles, which are also thought to be produced by the Sun. KWISP is made up of a thin membrane, with lasers trained on it. The idea is that this membrane is held extremely still, while an array of mirrors focuses any incoming stream of chameleon particles onto it. If these particles are indeed hitting the membrane, they should move it by a tiny but detectable amount – less than the width of a proton. This movement would then be picked up by the lasers.
KWISP was tested over a 10 day campaign, and the first results come from a 90-minute stretch in July 2017. Unfortunately, the team detected so sign of any solar chameleons. But that doesn't necessarily mean they don't exist – rather it allows scientists to put a constraint on the upper limit of the weight of these particles. That limit is roughly 44 piconewtons, give or take, or about the weight of a single human cell.
Of course, chameleon particles might not exist at all. There are other theories besides dark energy that could explain the acceleration of the expansion of the universe, after all. Dark energy and dark matter could both be explained away as a "dark fluid" with negative mass that fills the universe. Or we may have our math completely wrong, and dark energy may not exist at all.
Further KWISP experiments may help shed more light on the mystery.
The research was published online at ArXiv.
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