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

[xiphias.livejournal.com]

You DO have a Schwartzchild radius. It's just very, very big.

[brooksmoses]

No, you've got that backwards. His Schwartzchild radius is teeny-tiny, much smaller than he is, and since he's not all within that radius, he's not a black hole.

However, there is an amount of mass which, if packed into a sphere with the same density he has, would be a black hole. It has a radius about the distance of Mars from the sun, IIRC from the article. Which, if you packed it with stuff at that density, is still a startling lot of mass.