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Originally Posted by Little Bang
After the first second the universe should be a sphere with a diameter of two light seconds that contains all the matter of our present universe. Then according to wiki gravity should now exist. If that is true why wouldn't the enormous gravitational well prevent any further expansion?
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First, there's no reason to think the universe had a diameter of two light seconds after 2 seconds. If you look at the
link that Craig gave, you'll see inflation is suspected to have happened well before 2 seconds. According to inflation, the universe expanded faster (perhaps much, much faster) than the speed of light.
Nevertheless, your point is—why didn't it immediately collapse back in on itself like a black hole.
The best answer (I think) is that general relativity doesn't demand it do that.
A black hole is a variation in gravitational potential due to a variation in mass density. Inside the black hole there is a lot of mass in a small volume. Outside the black hole there is no mass. This difference in density from one area to the next is what made the black hole. The early universe had the same density everywhere. No matter what area of the early universe you look at, it has the same density as the area next to it. So, nowhere does a black hole want to form.
In fact, black holes and the universe are two entirely different things that are described entirely differently (as they should be) by general relativity. The Schwarzschild solution to GR describes (among other things) black holes and the Friedmann solution describes an homogeneous and isotropic universe. These are both exact solutions to GR, but they are fundamentally different. A black hole (as described by Schwarzschild) is a static and vacuum solution. Spacetime is not moving and it is empty.
In the Friedmann universe, spacetime is moving and it is filled homogeneously. These differences are set out here:
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Why did the universe not collapse and form a black hole at the beginning?
Sometimes people find it hard to understand why the big bang is not a black hole. After all, the density of matter in the first fraction of a second was much higher than that found in any star, and dense matter is supposed to curve space-time strongly. At sufficient density there must be matter contained within a region smaller than the Schwarzschild radius for its mass. Nevertheless, the big bang manages to avoid being trapped inside a black hole of its own making and paradoxically the space near the singularity is actually flat rather than curving tightly. How can this be?
The short answer is that the big bang gets away with it because it is expanding rapidly near the beginning and the rate of expansion is slowing down. Space can be flat while space-time is not. The curvature can come from the temporal parts of the space-time metric which measures the deceleration of the expansion of the universe. So the total curvature of space-time is related to the density of matter but there is a contribution to curvature from the expansion as well as from any curvature of space. The Schwarzschild solution of the gravitational equations is static and demonstrates the limits placed on a static spherical body before it must collapse to a black hole. The Schwarzschild limit does not apply to rapidly expanding matter.
Is the big bang a black hole?
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Your question is also answered by some qualified people here:
According to the big bang theory, all the matter in the universe erupted from a singularity. Why didn't all this matter--cheek by jowl as it was--immediately collapse into a black hole?: Scientific American
and a snippet from that link:
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Second of all, the concept of a black hole is only one type of solution to Einstein's General Theory of Relativity, our best current theory of gravity. This reading of general relativity--known as the Schwarzschild solution--is thought to give an accurate description of the gravity near an isolated, nonrotating black hole, as well as the 'normal' gravity near the earth and throughout our solar system.
But other solutions to general relativity are known to exist, including ones that apply to a whole universe. These alternative solutions typically assume that the early universe was perfectly uniform so that there were no places for black holes to form, even if the density were so great that particles were "cheek by jowl." The most popular class of general relativity solutions applying to the entire cosmos are known as Friedmann-Robertson-Walker solutions. These formulations appear to describe correctly our expanding universe; that is, they demonstrate how objects not held together by local forces (such as the electromagnetism that bonds atoms in molecules or the gravity that keeps the earth intact) stream away from one another in a predictable manner.
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In other words, an area in our universe that has really high density and wants to collapse into a black hole is a different thing than the whole universe having really high density. In the former, spacetime is most certainly curved. In the latter, spacetime can be flat. As GR goes, that makes all the difference.
~modest
