How could the universe possibly expand??

cliowa

My understanding concerning black holes is this: If there is enough mass (which means quite alot, in this case) with only very little extension, i.e. lots of mass in a tiny little region of spacetime, spacetime will curve according to this and eventually (the event being: enough mass) form a black hole, from which there is no way out. Absolutely no way out, not even for light. (I'm not talking about quantum fluctuations at the "border" of a black hole, here.) My point is: once you're inside, you can't get out.

Now the standard Big Bang theory, as far as I understand, says that at the very beginning all mass of the entire universe was at one point, from which it expanded: it made bang. But: How could that possibly happen? All the mass of the entire universe is certainly enough for a black hole to form, the spacetime region certainly small enough. Why didn't the whole thing collapse into a black hole instead of expanding all the way?

Thanks alot for any explanations. Best regards...Cliowa

mathman

Big bang theory has space itself expanding. This is different from something trying to escape from a large gravitational field (e.g. black hole).

marcus

there are several rival approaches to quantizing gravity. Several of them get the same result of repairing the big bang singularity so the problem you are talking about does not arise. the new theories await testing. Some tests are possible near-term (next year or two).
To sample current ideas, try Ashtekar's articles
"Quantum Nature of the Big Bang"
or "The Issue of the Beginning in Quantum Gravity"
or the talk which he gave at the Einstein Centennial in Paris last year,
which was called "Gravity, Geometry, and the Quantum"

I will get some links.
Here is one written for general audience:
http://www.physorg.com/news66660003.html

In the loop gravity model which Ashtekar helped to develop
gravity becomes repulsive at very high densities

in situations where the 1915 Einstein theory gave a "singularity" the loop gravity model predicts a bounce

"singularities" do not, as far as is known, exist in nature. they are places where a human-constructed theory breaks down. the 1915 theory had a failure at the big bang moment when expansion began----it broke down and failed to compute meaningful numbers

so for many years popularizers of physics gave the public some song and dance about "infinite density" and talked as if the singularity was a real thing----but meanwhile researchers like Ashtekar were working on it trying to fix it.

since 2001 the problem seems to have been fixed, and to have stayed fixed, at least provisionally until observations can be made to TEST the new theory

http://arxiv.org/abs/physics/0605078
The Issue of the Beginning in Quantum Gravity

http://arxiv.org/abs/gr-qc/0602086
Quantum Nature of the Big Bang

http://arxiv.org/abs/gr-qc/0605011
Gravity, Geometry and the Quantum
"After a brief introduction, basic ideas of the quantum Riemannian geometry underlying loop quantum gravity are summarized. To illustrate physical ramifications of quantum geometry, the framework is then applied to homogeneous isotropic cosmology. Quantum geometry effects are shown to replace the big bang by a big bounce. Thus, quantum physics does not stop at the big-bang singularity. Rather there is a pre-big-bang branch joined to the current post-big-bang branch by a `quantum bridge'. Furthermore, thanks to the background independence of loop quantum gravity, evolution is deterministic across the bridge."

there is a BBC news article about this. maybe I can find the link