MadSci Network: Astronomy
Query:

Re: Are conditions at the center of a black hole simalar to pre-big bang cond?

Date: Mon Jun 28 14:45:09 1999
Posted By: Stephen Murray, Physicist
Area of science: Astronomy
ID: 922906813.As
Message:

Hi Duane,

There *is* an analogy between the universe and a black hole, if the universe is closed. By closed, I mean that the universe contains enough matter that its gravity is sufficiently strong for it to eventually collapse back on itself. In that case, the universe is a finite entity. I believe that Einstein referred to it as "finite, but unbounded." It would be rather like the surface of a beach ball. The ball has a definite, finite surface area, but there is no edge to the two-dimensional surface. A closed universe would be like a three-dimensional analogy to that. Most importantly for the black hole analogy, light could never leave a closed universe.

There are fundamental physical differences between a black hole and the universe as a whole, though, even if the universe is closed. In a closed universe, light cannot leave, simply because there is no place for it to go. Spacetime does not exist outside of the universe.

A black hole, on the other hand, is a piece of spacetime that has, in a sense, "pinched off" from the rest of the universe. This is because gravity in its vicinity is so strong that light cannot escape. The critical distance from the black where this happens (the event horizon) becomes a sort of one-way surface.

A black hole, however, is not completely isolated from the surrounding universe. For example, it continues to warp spacetime locally, that is, we feel its gravity. And spacetime does continue across the event horizon, so, unlike the universe, it does make sense to talk of the inside and outside of the black hole. Mathematically, as well, a black hole is described *very* differently from the universe. The upshot is that a black hole will stay a black hole, and if we were to "cram" more matter into it, it would just become a bigger black hole. It will never explode into another big bang.

For the universe, however, the big bang obviously could occur, and carried all of spacetime out with it. Exactly why, we don't know yet, because we don't have the physical understanding needed to explain how things behaved at the incredibly high temperatures that existed at the moment of the big bang.


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