Firstly, one needs to realize that the Big Bang is simple the time-reversal gravitational collapse, so understanding the Big Bang becomes much easier if one understands gravitational collapse.
Justforfun000 wrote:I appreciate the explanation but it's WAY over my head. You're talking to a layman who hasn't really studied serious science since high school. Ha. If you call THAT serious science. I have a good head on my shoulders, but your references are far too knowledge specific for me to follow you. I can tell by reading it that it makes sense, but I would need to learn things like Newton's shell theorem and Schwarzschild metric before I can really grasp what you're explaining to me.
The core idea is actually very simple: imagine a ball of neutral matter with spherically symmetric mass distrubution (or completely uniform density, if you are unsure of that means) initially at rest. If pressure effects are negligible compared to gravity, as it easily can be on the scale of the galaxies much less the universe [1], then this ball will collapse to a point-like mass in finite time. That's pretty much it; the hard part is extending this trivial observation that to the rest of the universe. [1: As an example, supermassive black holes have extremely low densities as measured by their event horizon volume, and once this critical density is reached, absolutely nothing will stop further collapse to singularity. This critical density can be made arbitrarily small by having a more massive but larger ball of matter, and less density means less pressure.]
Newton's shell theorem states that the a shell of matter with a spherically symmetric matter distibution produces no gravity inside of it, and the gravity induced by this shell to an external particle is that of a point-mass, as if all the mass was concentrated at its center. This enables us, for example, to treat the planets as point-masses to and still produce highly precise predictions in orbital mechanics. Now, imagine an infinite universe with a uniform matter distribution (again, this is actually a good approximation on the supergalactic scale), every point with the same mass density, and pick a small ball of matter about some center. One can partition the rest of the matter in the universe into concentric shells around this ball, and by Newton's shell theorem, the gravity induced by each shell on the center-point is zero, and hence the net gravity on it is zero. In one dimension, the picture is simple:
<-------(-+-)------->
The (-+-) represents the imaginary ball of matter about some center +, the gravity due to the matter located some particular distance to the right being cancelled out by the gravity due to the same distance to the left. Hence, there is no net gravity acting on the ball; it is left alone to collapse on its own. As above, it collapses to infinite density in finite time.
The key step is this: in an infinite universe with uniform (or nearly so) density of matter, any point whatsoever can be picked as the center of the imaginary ball. At every point, an observer would see the surrounding mass density increase to infinity in finite time. Since at every location, and observer would see it as the center, the whole concept of 'center of the universe' loses meaning. Another point of view to observe that the ball still attracts the rest of the matter of the universe, so leaving its boundaries alone, it will gain density because surrounding matter flows into it. That is the picture in my previous post, but the above it slightly simpler, even though the two views are completely equivalent.
The previously mentioned result of Birkhoff's theorem is like a general-relativisic analogue of Newton's shell theorem in that spherical distributions produce a metric that is externally conformal to the Schwarzschild metric and completely flat internally. One doesn't need to know the exact details except that the fact that under the Schwarzschild metric, the radial freefall that we are interested in (toward the center of the imaginary ball) is exactly Newtonian in proper time--in other words, as measured by a particle falling to the center of the ball, the time it takes to reach it is exactly what Newtonian gravity predicts. Big Bang cosmologies are usually studied using the Friedmann-Robertson-Walker metrics, but perhaps suprisingly to many, the mechanics of collapse are exactly the same as for the so-called 'Newtonian cosmology'. The only difference is that the formation of the all-pervasive singularity prevents us from talking about anything prior in any physically meaningful sense, and of course in the case of the closed (finite) universe there is a vast difference in interpretation.
Rye wrote:A singularity, that was fundamentally either unstable or had a virtual particle style property where it could borrow energy from the future in order to create things or otherwise somehow result in expansion. ... So my idea, though I'm not a physicist or anything, and I don't know if it's been proposed and shot down or whatever, is that the universe borrowed energy from the smallest amount of its own future which then had to exist in order for it to borrow said energy. Anf from there we get an endlessly expanding universe that presistantly borrows from its own future, which forces it to expand more as that future then has to exist.
The problem with describing the singularity as unstable is that the idea of being 'unstable' in the physical sense requires the concept of time, in as much as it is the tendency to become something else. The overall idea of the universe being a vacuum fluctuation has actually been explored, although prior to inflation theory, rather ignored. Understandably so, since vacuum fluctuation of such size are so unlikely as to be virtually meaningless. It is possible to interpet relativistic gravitational fields as having energy in the form of spacetime curvature, and this can be negative. There is a theorem somewhere (unfortunately, I cannot recall the details) that states that a closed universe has zero net energy. There is no real borrowing involved here; it's more separating a zero into the sum of positive and negative, so conservation of energy is preserved at every point in time. Inflation theory is really a runaway reaction for this kind of conversion. There are versions of inflation that produce open universes (er, negligibly open, since inflation forces flatness), but I'm not familiar with the details.
