- #1
Gerinski
- 323
- 15
I have read that the Schwarzschild radius of a black hole with the mass-energy of the observable universe is roughly equal to the actual Hubble radius of 13.8 billion light years. And I have read that contrary to some popular esoteric interpretations such as "the universe is a black hole", "we are inside a black hole" etc, this simply means that the universe is spatially flat, or nearly so, that the equivalence of Hubble radius and Schwarzschild radius for a flat universe is derived from the Friedmann equations.
So far so good, but there are things I do not understand.
As I understand, this means that was the universe not expanding, it would collapse into a black hole, it has already the total average density of a black hole with an event horizon the size of it. But this seems highly counterintuitive, the density of the observable universe seems incredibly thin, it is by far mostly empty space. How can it have the same density as a black hole?
Alright, a big share of its energy contents is dark energy, but even so, how can we then make the equivalence to a black hole? Dark energy may contribute to the total energy density of the universe but it causes it to expand, so it goes against the tendency to collapse gravitationally. It can not be right to include dark energy in the mass-energy computation to say that the mass-energy of the observable universe is equal to that of a black hole the same size, is it?
Related question: in a black hole, the density at the singularity is infinite, whatever mass divided by zero volume. But if we express the density as the mass of the black hole divided by the volume of its Schwarzschild sphere, what would the density of a black hole be like?
Thanks
So far so good, but there are things I do not understand.
As I understand, this means that was the universe not expanding, it would collapse into a black hole, it has already the total average density of a black hole with an event horizon the size of it. But this seems highly counterintuitive, the density of the observable universe seems incredibly thin, it is by far mostly empty space. How can it have the same density as a black hole?
Alright, a big share of its energy contents is dark energy, but even so, how can we then make the equivalence to a black hole? Dark energy may contribute to the total energy density of the universe but it causes it to expand, so it goes against the tendency to collapse gravitationally. It can not be right to include dark energy in the mass-energy computation to say that the mass-energy of the observable universe is equal to that of a black hole the same size, is it?
Related question: in a black hole, the density at the singularity is infinite, whatever mass divided by zero volume. But if we express the density as the mass of the black hole divided by the volume of its Schwarzschild sphere, what would the density of a black hole be like?
Thanks