Can dark matter be pulled into a black hole?

Yes (dark matter may not interact otherwise, but it does interact through gravity), but it is a lot harder for dark matter to fall into a black hole. Let me explain why.




Take a cloud of normal matter, with a source of gravitation within (a planet, a star, a black hole, whatever.) As normal matter falls towards that source of gravity, it becomes denser and hotter. Some of its kinetic energy is converted into pressure waves and, ultimately, heat that is radiated away to infinity. So normal matter loses kinetic energy as it forms an ever more compact cloud around that source of gravity.

In contrast, dark matter does not self-interact. It has no pressure. Even if it becomes denser, its particles do not collide with each other. There is no dissipation mechanism that would convert its kinetic energy into heat.

Now falling into a black hole is not easy. A black hole’s gravity is not magic. At a given distance, it is not any stronger than the gravity of a star of the same mass. You can get a lot closer to a black hole, of course, because it is a lot smaller in size, and that close, its gravity is strong. But to get that close, you need to aim for the black hole very precisely. Most particles of matter, even when they fall towards a black hole, miss that tiny target and instead, escape to infinity on the opposite side, as they follow a hyperbolic orbit.

Or rather, they would escape if they didn’t lose kinetic energy through the aforementioned dissipation process. So some particles of matter lose enough kinetic energy to become stuck in orbit around the black hole. Once enough particles are present, they form a noticeable structure, the so-called accretion disk. This “disk” is just a flat pancake of swirling matter that, because of its internal friction, continuously loses kinetic energy and thus falls towards the black hole.

Dark matter cannot do this. It will not form an accretion disk because there is no dissipative mechanism. So unless a dark matter particle happens to hit the bullseye, it will not fall into the black hole as it passes by. So a black hole will capture dark matter particles far less efficiently than normal particles.

Leave a Reply

Your email address will not be published. Required fields are marked *