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Originally Posted by Sam5
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Originally Posted by Squink
If this big bang as exploding firework model is to have any validity, it needs to explain a couple of things.
•Why don't we see a central void? When something explodes, the debris cloud starts out spherical, but after a little time has passed, you always get an expanding shell of material around a central void, rather like a planetary nebula. If the universe contains such a void, it should be pretty easy to find.
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There are different kinds of explosions. An atomic bomb explosion tends to send out a “bubble” or a “thin shell” of a shock wave. But there is another kind that is like an explosion in a pile of dirt or sand. That kind sends out particles in all directions, with the central stuff not moving too much at all and the outer stuff moving the fastest. But this is only speculation, which originally was based on the idea that if the galaxies are moving apart now, they must have been closer together in the past and maybe very close together in the distant past. |
True, but 'very close' is a relative term. What kind of proximity are you talking about and how would that affect galactic evolution? Whether we're talking about an explosive event or spacetime expansion, we still have to account for the early universe being very hot and very dense - too hot and dense to allow gravity to form galaxies. Taken a step further, that also implies that the universe cooled unevenly, with the outermost regions cooling before the innermost regions.
If the Big Bang was an explosive event, how do you account for these facts? It would imply that the outermost galaxies formed well before before the innermost galaxies. No such disparity has been observed. It would also imply that the cosmic microwave background should be noticeably uneven, with the innermost regions of the universe having a higher temperature and, consequently, a higher-frequency background radiation than the older, cooler outer regions. Again, no such disparity has been observed - in fact the cosmic microwave background is one of the most consistently uniform features of the universe.
Arguing that an explosive Big Bang would produce a larger universe than the currently held belief in spacetime expansion doesn't work since the generally accepted theory proposes an infinitely large universe where as an explosive Big Bang implies a finite universe. That would seem to bring the question back to the age of the universe. Most cosmologists would agree that the age of the universe is known to within half a billion years to be about 12.6 billion years old. The chief argument against this is the age of certain globular clusters, but that's only known to an error of +/- 3.5 billion years (11 - 18 billion years old). That's a sevenfold increase in uncertainty making this number less reliable by far. It would seem that for an explosive Big Bang to produce a universe that was large enough to render these predictions unobservable would require the universe to be far *older* than the current theory allows.
How do you account for this? Is our understanding of the age of the universe flawed? If so, how do you propose we recalculate its age? Keep in mind that to do this you would need to predict the cosmic microwave background, the average temperature of the universe, the distribution of galaxies, the motion and speed - apparent or otherwise - with which galaxies are receeding from us, *and* the rate or rates at which the universe cooled allowing everything from atoms to galaxies to form. It would also need to do this within the restrictions of lookback time.
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But no one really knows what happened or why the galaxies are moving (or appear to be moving). If our universe is bigger than suspected, the “center” could be so far away from us we might not be able to see it.
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A couple of points here:
I disagree that we don't know the 'why.' An explosive event and spacetime expansion would explain why they're moving. They differ on *how* they're moving.
An explosive Big Bang requires a finite universe. Spacetime expansion allows for an infinite universe. Since there's no way that a finite universe could be bigger than an infinite universe, you can't argue that the current theory predicts a size for the universe that is too small to account for the observational evidence.
You can't speek of a metaphorical center to the universe and argue for the finite universe. An explosive Big Bang requires a finite, spherical universe which, for obvious reasons, *must* have a center. You only avoid the issue of the universe not having a center with an infinite universe.
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If the “explosion” was really uniform and the universe bigger than we think, then we would see what we see, but we wouldn’t have to be near the center at all.
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Again, a poor argument. We think the universe is infinitely large. Literally.
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One reason astronomers were able to detect our position in our own galaxy was because of the “sideways” motion of some of the distant stars (I think that’s the way to explain it). But, the problem with detecting any “sideways” motion of any of the distant redshifted galaxies is because they are so far away, a slight “sideways” motion is just too small to be noticed.
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Actually, as I understand this, the problem isn't detecting 'slight' tangential (sideways) motions. The problem is detecting *any* tangential motion. Remember that although the objects in question are moving at large speeds compared to, say, the orbital speed of Jupiter, the distances involved are so large that it would take millions of human lifetimes to observe any change in relative position. As far as I know, the only way we have to measure the motion of distant galaxies is by calculating their redshift. However, redshift is completely useless for measuring tangential motion. The only reason it works for figuring out the rotation of the Milky Way is that stars in different *parts* of the Milky Way have different redshifts relative to the Earth (i.e., stars nearer the center of the galaxy have a lower redshift than stars nearer the edge). Since galaxies essentially move as a whole relative to the Earth, this technique won't work.
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Newton offered a third alternative to the 1) collapsing or 2) expanding list of possibilities. He said the entire universe might be revolving, something like the way our solar system works. Well, if it’s revolving, it would have to be somewhat overall “flat” shaped similar to a galaxy
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Not necessarily. Planets and stars revolve, but they're essentially spherical. Globular clusters can revolve, but they're also, as their name implies, essentially spherical. Therefore, it seems reasonable that the universe could be revolving at a low enough speed to keep its mass distribution essentially spherically symmetric.
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but from where we are I don’t think we see anything but a visual “sphere”. So I suppose it would have to be much larger than we think it is, in order for it to be revolving without our noticing it yet. But, actually, that’s pretty much the way the 19th Century astronomers thought of our own galaxy, non-rotating and made up of “fixed” stars and “nebula”.
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A spherical observable universe is an unavoidable consequence of lookback time. Light travels at a finite speed. Light has only been travelling through the universe for a finite length of time (since the universe cooled to become opaque). Therefore, we should not be able to see any object whose light would take longer to reach us than the age of the universe. Since this is true with both an infinite universe and a large enough finite universe, the issue is not the size of the universe, but its age.
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Personally, I don’t know what’s going on with the overall universe. I don't even know if it's really "expanding". I’m just basing some of my comments on what I’ve read about the different theories over the years.
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All the evidence suggests that the universe is expanding. Everything not gravitationally bound to the Milky Way is moving away from it. Either the universe is a finite cloud of matter with a center and edge, as predicted by an explosive Big Bang, or spacetime itself is expanding causing the distance between galaxies to increase, as predicted by the generally accepted theory that the Big Bang was the beginning of this expansion. There is no evidence that the universe is contracting (no unbound galaxies moving towards the Milky Way), nor that the universe *will* contract (the expansion is accelerating).
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What Eddington did in his book in 1933 was just avoid the “center” problem by saying that the universe probably existed as a small sphere (many light-years across) for a long time and then started to expand. He just avoided discussing the very beginning.
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I haven't read the book myself so I can't check this argument (Anyone have a citation?). However, if he said this he was wrong. If the universe was the same as it is now, only smaller, there is no reason why it would suddenly start to expand. There are only two possible scenarios with this model. If the universe was not rotating, it could not start expanding without violating the law of momentum conservation. Objects at rest *stay* at rest until acted upon by an outside force. This scenario does not propose any method of generating that force. On the other hand, if the universe was rotating stably (centripital force balanced gravitation), some torque would have been needed to speed up the rotation and make the centripital force greater than the binding gravity. This violates the law of angular momentum conservation. Spinning objects remain spinning in a constant speed and direction unless acted upon by an outside force. The only way around this would be for the universe to have never been stable as a sphere, which this model doesn't allow.
In addition, spheres have centers. Period.
(Edited once for grammar)