Was the Universe formerly a black hole?

This is a question that’s bugged me for a while.

First, here’s why I think this is a reasonable question to ask.

Suppose you cram a mass M into a spherical volume of radius R such that R is less than the Schwarzschild radius, i.e., R \leq 2GM/c^2. Then it’s a pretty well understood consequence of general relativity that the mass will collapse to form a black hole.

Current estimates of the mass of the (observable) Universe vary quite a bit. This webpage seems pretty representative, though, giving a value for the mass of 3 x 10^52 kg.

This gives a value for the corresponding Schwarzschild radius of about 6 billion light years.

The radius of the observable Universe is, of course, quite a bit bigger than this. But the Universe is also expanding, and at some point in the past its radius was quite a bit less than six billion light years.

If that was the case, why didn’t it collapse to form a singularity? In short, how come we’re still here?

Any cosmologists out there who can enlighten me?

Update: In comments, Dave Bacon points to an enlightening essay from John Baez, explaining some of what’s going on.

My interpretation of the essay is that the standard lore I learned as an undergraduate (namely, that if you take a mass M and compress it into a smaller radius than the Schwarzschild radius then a black hole must inevitably form) is wrong, and that the FRW cosmology provides a counterexample.

This begs the question of when, exactly, a black hole can be guaranteed to form.

Categorized as Physics

The Academic Reader and RSS readers

In comments, Yue Li writes of the Academic Reader:

This seems very similar with a production of Google, the google reader, a quantum specific google reader:)

In the existing site, the main difference between the Academic Reader and RSS readers like the Google Reader is that we have a variety of ways of searching and browsing older papers. This means the Academic Reader allows you to both (1) keep abreast of your current reading, and (2) look back into the past, discovering older papers and so on. RSS readers typically focus on just the first of these problems.

This functionality will be greatly extended in coming months!