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Comment by pc86 | Hacker News Reader

I know next to nothing about black holes, is there any relation between the size of a black hole and the amount of matter it's "swallowed?"

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Yes. The radius of the "event horizon," which is the spherical boundary that serves as the point of no return, grows as it swallows more mass. For a Schwarzschild black hole (which doesn't rotate and has no charge), the event horizon radius is 2GM/(c^2), where c is the speed of light, M is the mass of the black hole, and G is the gravitational constant.

The singularity at the center doesn't grow. But a black hole's "size" generally refers to the radius of its event horizon.

Wolfram Alpha computes it for you: http://www.wolframalpha.com/input/?i=schwarzschild+radius+fo...

I learned this interesting factoid in an astrophysics class. All data is from Wolfram Alpha:

Mass of universe: 3 * 10^52 kg

Schwarzschild radius for that mass: 4.455 * 10^25 meters

Volume of sphere with that radius: 3.704 * 10^77 m^3

Density = mass / volume = 8.099 * 10^-29 g/cm^3, or roughly 8 times the approximate universe density

I think it's interesting that a universe-sized black hole would be a mass within an order of magnitude of the universe's mass.

Isn't the ratio between dark matter and 'normal' matter something on the order of 8:1 as well?

I've heard a black hole's singularity defined as 'infinitely dense and infinitely small'; I don't think such a thing can be said to 'grow'; however, as the density increases I think that the Event Horizon expands outward increasing the diameter of 'black' so the answer to your question may be yes.

Yes. Formally, a singularity is just a "hole" in spacetime. As you increase the mass of the black hole, the singularity itself doesn't change, but the curvature of the spacetime around it becomes stronger.

My (poor) understanding is that the "size" of a black hole refers to the size of its event horizon, which is directly related to its mass (more mass -> larger event horizon). That mass is still compacted at the center in a single point of zero (?) size.

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