Talking to my father tonight, he brought up an interesting concept

[xiphias]

Okay. So, apparently, a black hole does not actually need to be super-dense. It's just, the less dense it is, the bigger its radius has to be in order for it to be a black hole.

A solar-system-sized black hole would only need to be about as dense as air. If you made a big sphere the size of the orbit of Neptune, and filled it with air -- that'd be a black hole.

Here's the weird thought. Um. Not that that previous thing ISN'T a weird thought. But here's a weirder one:

The density of the intergalactic medium is probably something like one hydrogen atom per cubic meter. Not very dense.

But nonetheless, a density.

That means that there exists a radius such that the entire universe is a black hole. And it's calculable.

Which puts an upper bound on the size of the universe. And leaves the possibility that our entire universe is, in fact, on the black hole side of an event horizon.

Comments

and before you have a black hole...

[(Anonymous)]

The black hole is like the final curtain, which means there is interesting stuff going on before you get to that point. What I find interesting is that "clocks" at the center of a sphere of non-zero density run slower than clocks outside. Postulatin a transparent sphere, then light from the center of the sphere get shifted towards the red. So, if you look at a star a billion light years away, it may be thought of as being in a transparent sphere of non-zero average density. We are at the edge of this large shere and should see a red shift of about 10% of the Hubble red shift.
Dark matter is postulated in part to explain the increasing red shift at extreme distances. Maybe things need to be reexamined?
Duzzy