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.
> it's possible for a black hole to have a lower density than water
But that density is not homogeneous. It's much denser in the middle. As you cross the event horizon you wouldn't notice anything happening (as long as the BH is big enough that the tidal forces were small so you didnt get spaghettified). But things would feel very weird as you got closer to the middle.
Probably. Nobody has proved any of this but it seems likely.
Therefore... what? The universe is a black hole? (If so, what if anything does that change? Is there a singularity that we hit within a finite proper time?)
Or the universe is not a black hole? If that's the case, why isn't it?
The universe is not a black hole because the "Schwarzschild radius equation" only applies in a very specific/narrow circumstance, namely a perfectly spherical, non-rotating mass surrounded by vacuum in flat spacetime.
Applying that equation to the observable mass of the universe violates basically every single requirement needed to apply the equation.
Often it's taken out of context by pop sci channels to propose the universe "could be" or even "is" in a black hole. But from my understanding, as a non-physicist, it's not surprising or interesting to most physicists that the math works out like this, and there's nothing else to suggest that we're in a black hole.
I don't think it means anything, it's probably just a curiosity. Simple equations like that probably don't work correctly when applied universe-scale.