There might be a big clump of dark matter in our galactic neighbourhood
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There seems to be an enormous cloud of dark matter next to our solar system. We have never found one of these clouds in the Milky Way before, but precise cosmic clocks called pulsars finally made it possible.
Our best cosmological models hold that galaxies are embedded in tenuous clouds of dark matter called halos, with smaller sub-halos studded throughout. But dark matter doesn’t emit, absorb or reflect light, so halos and especially sub-halos are extraordinarily difficult to find.
To spot this one, Sukanya Chakrabarti at the University of Alabama in Huntsville and her colleagues used pairs of rapidly spinning neutron stars called pulsars. Pulsars spin at an astonishingly regular rate, sweeping beams of light across the sky as they do so, which makes them crucial cosmic clocks. With a pair of them, we can use changes in their orbits around one another to measure the acceleration that occurs when a massive object is nearby.
Dark matter interacts with regular matter via gravity, so if a dark matter sub-halo is near a pair of pulsars, it should stretch their orbits slightly. That is exactly what Chakrabarti and her colleagues found a little more than 3000 light years from our solar system. “There’s one pair of pulsars and the [individual] pulsars around it – there’s something in this part of the sky that’s pulling all of these pulsars in this weird direction that we didn’t expect,” says team member Philip Chang at the University of Wisconsin-Milwaukee.
The researchers analysed the extent of the pull and found that it must come from an object about 60 million times the mass of the sun, measuring several hundred light years across. They compared the location of the mysterious, massive object with maps of stars, gas and other objects made of regular matter, finding no matches. If the object is truly there, which the researchers say isn’t definitively confirmed yet, it must be dark matter.
If so, it could be the only sub-halo of its size in our corner of the galaxy. “There might only be one or two locally, but it depends on the model of dark matter,” says Alice Quillen at the University of Rochester in New York. “Different models predict different distributions of these clumps.”
That is what set Chakrabarti on the path of hunting sub-halos in the first place. “Our goal is to map out as many of these sub-halos as we can across the galaxy, and we’ve just started being able to do that. Then the ultimate goal is to understand the nature of dark matter,” she says.
However, pulsar binaries are relatively rare: there are only 27 for which we have observations precise enough to measure their acceleration due to gravity, which is part of why this sub-halo wasn’t found before now. “The amount of pulsars is finite, so we’re trying to come up with other ways to trace this with objects that are more numerous,” says Chang. If they are able to do so, we may finally have a crucial tool for probing what dark matter really is.
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