Astronomers puzzled over huge black hole in the middle of small galaxy | Hacker News Reader

Astronomers puzzled over huge black hole in the middle of small galaxy

arstechnica.com

33 pointsevo_915 comments

Wild speculation: our first evidence of advanced alien societies carrying out the transcension hypothesis?

http://accelerating.org/articles/transcensionhypothesis.html

http://www.youtube.com/watch?v=nQOyJUDTKdM

If the stars are all old and the black hole is surprisingly big, could it simply be that the black hole has swallowed most of the galaxy already?

It's generally hard to get stars to collide with black holes since even the very largest black holes are still very small relative to the distances between stars. There's just too much angular momentum that needs to be removed. (Which is not to say that it can't be done -- it can, and there are various mechanisms for doing so, but it's very difficult to do for a substantial fraction of a galaxy's mass.) Even if this occurred, it's nevertheless surprising that it happened in this particular galaxy, and didn't occur in pretty much every other one.

Fascinating.

I'm clearly guilty of believing left Black Holes are like plug-holes in the centre of galaxies and eventually everything will swirl down them.

So this isn't a valid picture? Do you have a more realistic one for me instead to think about?

Far from the event horizon, a black hole acts like a point mass just like every other star. If, for instance, the Sun were to immediately turn into a black hole, the Earth would continue to orbit it as if there had been no change.

Assuming it kept the same mass?

And even if it had a lot more mass?

Assuming it kept the same mass. If it had a lot more mass, the Earth's orbit would be identical to it's orbit around a star with that much more mass. The fact that an object is a black hole only becomes relevant when something passes the event horizon. Outside the event horizon, it acts like any other object with the same mass.

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?"

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?

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.

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.

Would we be able to differentiate a massive Dyson Sphere from a black hole?

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