Why doesn't dark matter fall into a black hole? (Advanced)

A previously answered question stated that our galaxy could not be "sucked in" to the Black Hole at it's center because of the great distance between it and the nearest matter. If 95% of the universe is composed of "Dark Matter" and ergo also our Milky Way, then why isn't it feeding the Black Hole thereby increasing the event horizon to "someday" include all of the galaxy?

You are right that the black hole at the center of the galaxy is increasing in mass as material falls onto it; this causes the Schwarzchild radius to increase a bit as well. However, dark matter very rarely falls into black holes for the same reasons that we don't see it: it interacts very little with itself and with ordinary matter. This is a problem because particles lose a quantity called angular momentum (which is proportional to the speed at which they circle around the galactic center and to their distance from it) by interacting with other particles. Now, the laws of physics say that in order for particles in an orbit around a black hole to fall into it, they must lose a large fraction of their angular momentum (this is essentially because black holes are very small; in order to fall in, then, the particles must get very, very close to the center of the galaxy, which means that they must go very, very fast if angular momentum is conserved). For ordinary particles this is not a problem: as they get closer to the black hole, they bump and rub against each other, losing rotational energy and angular momentum such that they spiral into the black hole (provided they are close enough to the galactic centre to begin with). But dark matter particles don't "bump and rub", by definition! This means that they can't lose angular momentum so they rarely get close enough to the black holes to be "sucked in". So, regardless of the amount of dark matter in the galaxy, it will not help fuel the black hole at its centre.

This page was last updated June 27, 2015.

About the Author

Kristine Spekkens

Kristine Spekkens

Kristine studies the dynamics of galaxies and what they can teach us about dark matter in the universe. She got her Ph.D from Cornell in August 2005, was a Jansky post-doctoral fellow at Rutgers University from 2005-2008, and is now a faculty member at the Royal Military College of Canada and at Queen's University.

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