A hypothetical particle known as the ultralight boson could be responsible for our universe's dark matter.
While the ultralight boson isn’t directlyobservable, it might clump up around black holes, triggering an exotic mechanism that causes it to explode — in a massive burst of gravitational waves. Even better: these gravitational waves may be detectable with the next generation of detectors.
A little light reading
We don't know what 85% of the mass in the universe is made of (thoughwe wish we did). We call it "dark matter," but it might as well be "invisible matter," because it doesn't interact with light in any way, shape or form. In fact, dark matter doesn't scatter, reflect, absorb, refract or really have anything at all to do with radiation.
But what dark matter does have is gravity. Through its gravitational pull, we can see it affect the behavior, movement and evolution of galaxies.
But what could this mysterious, invisible dark matter be? Astronomers and physicists have been puzzling over the question for decades and are slowly narrowing in on some potential answers.
Diving into dark matter candidates
Among the candidates is a hypothetical particle known as an axion. The axion was first proposed to existback in 1977, before we even knew that dark matter was a thing, and it has some properties that make it attractive and alluring as a dark matter candidate.
For one, axions can be light — very light — which makes it easy for them to flood the universe. This is exactly what we expect dark matter to be like, as it is after all the most dominant form of matter in the cosmos.
Second, the axion (and theoretical particles related to the axion, like the so-called "dark photons," which are like axions but they can carry a hypothetical fifth force of nature) doesn't really interact with radiation or normal matter, which is yet another criteria that would align with dark matter.
Black hole bomb
Dark matter candidate in hand, we can start looking around for reasons to think it might actually exist. Does the axion, or any of its friends, make some sort of noise or commotion that allows us to detect it?
Well, according to a paper recently appearing in the preprint journal arXiv, the axion can turn into a bomb.
And, if you ever wanted to make an axion bomb (or a "black hole bomb"), you're in luck, because I'm about to tell you how.
First, you start with a black hole. Next, make sure the black hole is rotating. Spinning black holes can drag spacetime around them, like trying to spin a heavy coffee table on top of a rug. That rotation can transfer energy from the rotation of the black hole to any surrounding material. This can be a pretty useful energy source: just get near a black hole and use its spin to power whatever you want! (... in theory at least.)
This applies to everything — regular matter and dark matter alike. And if the dark matter is made of axions, something extra special could happen because of that rotation.
Depending on the mass of the axion particle, when they come close to a black hole (which is not a hard thing to do, because of the gravitational attraction of the black hole), it can trigger an instability.
The axions swirl around, stealing some energy from the black hole and that extra energy causes them to swirl around even faster, coming even closer to the black hole. That then pulls even more energy to the axions, causing them to swirl faster and faster.
This process is called the "superradiant instability," but I prefer the term "black hole bomb."
Read more: "Searching for Dark Matter with Paleo-Detectors"
Movement in the dark
When it comes to axions (and theoretical particles like the axions), this bomb doesn't produce a flash of light. Instead, the axions cluster around black holes in a specific configuration, arranging themselves in peaks and valleys that look almost like standing waves.
Those waves rotate with the black hole, becoming more and more energetic. The rotations release a tremendous amount of gravitational waves — the subtle ripples of gravity that constantly wash through the universe.
We've detected gravitational waves with instruments like LIGO and VIRGO for years now, but those instruments are tuned to the biggest energetic events, like two black holes or neutron stars colliding. But behind those super-loud events sits a general background murmur of gravitational waves. Like listening to the hubbub of a busy restaurant, that background is too faint to pick out the individual sources generating all the waves — you just have to listen to the noise.
Depending on the exact mass of an axion (the theoretical models behind axions don't really predict a firm mass for the particle), black hole bombs could be going off all the time. While powerful, each individual event would be too faint for us to detect directly with LIGO or LISA, but it would contribute to the general background.
As of yet, there's no evidence in the gravitational wave background for these black hole bombs — and hence no evidence linking them to the dark matter behind them. But that non-detection helps us understand these models — if the axion was heavier than a certain mass (and we're right about how black hole bombs work), then they would've shown up in the background by now.
The next generation of gravitational wave detectors will be even more sensitive, and we just might see our first black hole bomb. And, along with it, our first conclusive evidence of the identity of dark matter.
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