Dark Matter Around Black Holes: How to Hear the Invisible express
The universe is full of invisible substance that governs the movement of galaxies. In the 1970s, astronomer Vera Rubin showed: without dark matter, stars on the outskirts would have long since flown apart. But what it's made of remains unknown.
One hypothesis says that dark matter is clouds of ultra-light particles enveloping black holes. Such a cloud is like a sound-absorbing fog. When a smaller black hole falls into a larger one, space trembles, emitting a 'hum' – gravitational waves. The fog of dark matter changes this sound, muffling and distorting it. The LISA detector, set to launch in the 2030s, like a sensitive ear, will hear these changes. From the distorted 'ringing', scientists will determine the mass and properties of the particles, even if they shun interactions with particles from the Standard Model – the full set of known particles.
A surprising fact: these particles can be billions of times lighter than an electron, yet their collective pressure in the cloud is capable of altering the orbit of an entire black hole.
Dark matter particles in such clouds can be billions of times lighter than the electron, but their collective pressure can change the trajectory of an entire black hole.