A Universe Full Of See-Through Stars
BBC Earth|May - June 2021
Mysterious discoveries around the globe have opened up a tantalising possibility: the cosmos could be full of ghostly stars that are invisible to our most sensitive detectors
Colin Stuart

Look up at the sky after sunset and the familiar quilt of night is punctured with bright stars. These blazing furnaces are so vivid that we can see their light, despite the fact that even the nearest are quadrillions of kilometres away. It’s a sight most of us have seen on countless occasions, so you’d be forgiven for thinking that all stars must behave this way. After all, isn’t shining just what a star does? Yet if a flurry of recent findings is to be believed, there’s an entirely different class of stars lurking out there – stellar ghosts cloaked under a veil of darkness. These transparent, invisible stars give out no light whatsoever, meaning they skulk unseen in the celestial shadows.

Astronomers already suspect that, unlike ordinary stars, most of the Universe is hidden from view. When they look at galaxies, such as our own Milky Way, they find stars on the outer edges moving far too fast. So fast, in fact, that they should fly off into space. For them to be kept in tow there has to be something reining them in. The most popular explanation is that there’s a lot of hidden material in the Galaxy providing a significant amount of extra gravity. Scientists call this material ‘dark matter’ and it’s thought to outnumber the ordinary matter that you and I are made of by a ratio of more than five to one.

The majority verdict over the last couple of decades has been that this celestial glue is made of Weakly Interacting Massive Particles (WIMPs). This had led physicists on an unprecedentedly intense hunt to snare them. They’ve built detectors under the ice in Antarctica, in abandoned gold mines and even aboard the International Space Station. So far all their searches have come up empty. It’s somewhat ironic, then, that one of our WIMP detectors may have just found evidence in favour of a rival theory of dark matter – one that opens the door to the possibility of invisible stars.

“If dark bosons are affected by gravity, then they should also clump together in the same way that ordinary matter does”

STUDYING THE SMALL

The XENON1T experiment is tucked away 3,600 metres beneath the Gran Sasso mountain in Italy and is the largest underground research facility in the world. A huge tank containing over three tonnes of liquid xenon was designed to act as a WIMP trap – if a WIMP hits an atom in the tank, then the atom will recoil and spit out electrons and photons (particles of light).

Yet in the summer of 2020, the XENON1T researchers announced that they’d seen something unexpected: an excess of electrons that didn’t fit with an influx of WIMPs. According to Dr Tongyan Lin, from the University of California, San Diego, there are three possible explanations. The first two explanations are particles from the Sun, or radioactive contaminants in the experiment. The third, and by far the most interesting, is the arrival of another proposed form of dark matter: dark bosons.

A boson is a subatomic particle that carries a force. The photon, for example, is a boson that carries the electromagnetic force. A dark boson, so the theory goes, could either be dark matter itself or at the very least be responsible for the way dark matter interacts with ordinary matter. If the XENON1T signal stands up to further scrutiny – and the other more mundane explanations can be excluded – it could be the first sign that dark bosons are indeed out there.

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