Physicists hunt for dark matter dragging on black holes
With elusive dark matter continuing to evade detection, scientists are having to search in stranger and stranger places. In a new study, physicists at MIT have studied the spins of black holes for signs of drag from dark matter slowing them down.
Of all the matter in the universe, the regular stuff that we interact with every day only accounts for around 15 percent. The vast majority is tied up in what we call dark matter, which seems to only interact with normal matter through its gravitation.
Direct detection of dark matter particles has eluded scientists for decades, but it’s not from lack of trying. Experiments on Earth have been searching for the strange stuff using the Large Hadron Collider, “axion radios,” arrays of billions of tiny pendulums, huge underground tanks of exotic fluids, or superconducting cavities.
But the cosmos seems to be running its own experiments, allowing us to spot the signature of dark matter in space – if we know where to look. That could include unusual X-ray emissions from galaxies as dark matter particles decay, or perhaps odd flashes of light or X-rays near neutron stars, as dark matter particles convert to photons in their powerful magnetic fields.
Now, physicists at MIT’s LIGO Laboratory have searched for signs of the mysterious matter in a new environment, around black holes. Their dark matter target was a type of hypothetical particle called ultralight bosons, which, as the name suggests, have an extremely small mass – less than a billionth the mass of an electron.
If these ultralight bosons were to exist, quantum theory predicts that black holes of a certain mass would draw in huge clouds of them. But they wouldn’t be simply sucked in as you might expect – instead, these particles would gather around the black hole and actually drag on it, slowing down its spin. Therefore, if you find that black holes of a certain mass are spinning more slowly than they otherwise “should” be, it could be evidence of dark matter’s influence.
“If bosons exist, we would expect that old black holes of the appropriate mass don’t have large spins, since the boson clouds would have extracted most of it,” says Kwan Yeung Ng, lead author of the study. “This implies that the discovery of a black hole with large spins can rule out the existence of bosons with certain masses.”
This bizarre effect is the result of some quantum quirks. Basically, because of their incredibly small mass, sub-atomic particles can’t be described as being in one particular place at a time. Instead they’re described with a wave of probable locations, and the smaller the particle, the longer that wave becomes (ie. the more possible places it could be at any given time).
So, if ultralight bosons exist within a particular mass range, their wavelength would be comparable to the radius of a black hole of a certain mass. Because you can never pin down exactly where one of these tiny particles is, if it’s near a black hole then you can never be totally sure that it’s “fallen in.” In effect, the cloud pops in and out of the black hole, sapping its angular momentum in the process.
“If you jump onto and then down from a carousel, you can steal energy from the carousel,” says Salvatore Vitale, co-author of the study. “These bosons do the same thing to a black hole.”
To detect whether such clouds of dark matter exist, the astronomers studied the spins of 45 black hole binaries. These were taken from data from the LIGO and Virgo collaboration’s studies of gravitational waves, which are produced when black holes collide.
The team calculated how fast each of these black holes would be spinning if they’d interacted with ultralight bosons within a certain mass range – between 1.3x10^-13 electronvolts and 2.7x10^-13.
They found that two black holes in particular were spinning much too fast to have had any interaction with ultralight bosons. In fact, one of them was spinning at close to the absolute top speed possible.
This result, the team says, is conclusive enough to rule out ultralight bosons within that mass range as dark matter. It’s not to say that dark matter itself doesn’t exist – instead, like the many other null results received in other experiments, it simply means that we’re narrowing down the field of possible particles that it could be made of.
The research was published in the journal Physical Review Letters.