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

[maggiebloome.livejournal.com]

But does the density have to be uniform? Because the universe certainly isn't... and if it DOESN'T have to be, that opens up the possibility that actual black holes that we're aware of have other black holes inside them. Which is a bit of a headache inducing thought. Damn recursion.

[brooksmoses]

It doesn't have to be uniform, no. And in general it's not going to be. The article this is from was just noting that, if you put X much stuff within a sphere of size Y or less, then it will be a black hole -- regardless of how you distribute it. And that equation relating Y to X is such that the required average density (computed by taking the volume of Y) goes down -- so that, no matter how low a density you want to talk about, if you fill a large enough sphere Y with it, there will be enough mass of it to be a black hole.

[maggiebloome.livejournal.com]

Right, so you COULD have a black hole within a black hole within a black hole... whoah...