I was surprised to only recently notice that
An object of any density can be large enough to fall within its own
Schwarzschild radius.
Of course! It turns out that supermassive black holes at galactic centers can have an average density of less than water's. Somehow I always operated under the assumption that black holes of any size had to be superdense objects by everyday standards. Compare the Earth to collapsing into a mere 9mm marble retaining the same mass, in order for the escape velocity at the surface to finally reach that of light. Or Mt. Everest packed into one nanometer.
Reading on about this gravitational radius, it increases proportionally with total mass.
Assuming matter is accumulated at a steady density into a spherical volume, the volume's radius will only "grow" at a cube root of the total volume and be quickly outpaced by its own gravitational radius.
Question:
For an object the mass of the observable universe, what would have to be its diameter for it to qualify as a black hole (from an external point of view)?
Would this not imply by definition that:
- The Earth, Solar system and Milky Way are conceivably inside this black hole?
- Black holes can be nested/be contained within larger ones?
- Whether something is a black hole or not is actually a matter of perspective/where the observer is, inside or outside?
Best Answer
This is not exactly right, because the universe is expanding. You can't treat matter which is outside the cosmological horizon (if the concept is even meaningful, which I don't admit) as part of the matter which is gravitating, because it is not in causal or gravitational communication with matter here. The bounds you give are for matter sitting still.
The proper view is that the universe itself is an inside-out black hole, with a cosmological horizon that surrounds us. In this point of view, the matter inside the universe and the cosmological constant are, together, responsible for the shape of the enclosing horizon, or black hole.
But this is not a black hole, in that it isn't singular in the center, only (in certain energy models) singular in the past.