What I find really interesting is that black holes don't really "require" infinite density; the mass density inside the event horizon never exceeds neutron star levels, and becomes in fact lower the heavier the black hole gets (=> event horizon enclosed volume grows faster than mass).
The event horizon radius grows linearly, so it's possible for a black hole to have a lower density than water (around 7 billion solar masses). Another curiosity is that if you plug in the mass of the regular matter in the observable universe into the schwarzchild radius equation (R = 2GM/(c^2)) you get a black hole event horizon larger than the the observable universe.
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> the mass density inside the event horizon
Is not a well-defined number. The number you are calculating assumes that the horizon is an ordinary 2-sphere enclosing an ordinary 3-ball [edited, see grandchild post] in Euclidean space, and divides the hole's mass by the calculated volume of that 3-ball. But the spacetime of a black hole doesn't work that way. The "space" inside the hole's horizon is not like any kind of ordinary space that your intuitions will work for.
Also, the interior of the hole is vacuum, so its mass is not something that's distributed throughout any kind of volume; it's just a geometric property of the spacetime.
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The event horizon isn't the black hole. It's just the point of no return. As far as we know the black hole is a singularity with infinite density. But that probably isn't true. It's just a model and the model isn't perfect
I can't think of a way of getting a singularity. As soon as the mass in a region of space becomes high enough to start a black hole, time would freeze and it would take several millions universe lifetimes for the mass to fully collapse into itself, no matter how close of far the observer is to the mass. The way I see it, things would go on as usual inside the black hole, but there's a universal curtain preventing anyone from accessing the interior.
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I never thought about this, but I guess it makes sense. What an insane thing to consider - one more reason for to love neutron stars.
Some of.the biggest ones woild float on water... which tells you its not very useful to compare density based on Schwarzschild radius
For large blackholes of course. And this actually makes other interesting property. Our universe is in fact giant blackhole with our own event horizon outside it. This also support cyclic big bang theory.
Once entire universe will collapse to singularity, the smallest possible particles shatters, leaving pure energy. Pure energy doesnt have "static" mass so no gravity => kaboom, we have new iteration. Theory of multiverse (multiple black holes aka universes) is out of scope here because we will never be able to escape ours.
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