You're missing the point of the analogy. The point is that galaxies are *NOT* moving through space any more than the raisins are moving through the dough. The point is that spacetime itself is expanding and that expansion causes the galaxies to *look like* they're moving farther apart. Nobody ever said that the universe was made out of bread dough. Space expands like the dough expands. Whatever is driving this expansion sure as heck ain't yeast. All we're saying is that whatever

Which would look the same as space expanding. Do you have any evidence - either from observation or from theory - that provides any reason why this model is preferable?

I don't buy this analogy at all. For starters, there doesn't seem to be any any plausible explanation for what caused the pre-Big Bang singularity to explode. More importantly, the geometry is different. The fireworks model requires the universe to be a flat four-dimensional space. General relativity predicts that the universe is a curved space - and subsequent observations (perihelion advance of Mercury, gravitational lensing, etc.) have confirmed this. Einstein may have said that this curvature can't be measured directly, but he did predict - and Eddington proved - that we can see its effects. The other problem is that this requires the universe to have a center and to have an edge. To date, no such features have been observed. It also doesn't explain why galaxies are moving away from the Earth at roughly equal speeds in all directions. Put another way, it doesn't explain why the Earth, from our point of view, is at the exact center of an expanding sphere. That we're sitting near the proposed epicenter of the Big Bang seems ludicrously improbable. It would be far more likely to see galaxies moving away from us at higher relative velocities as we looked away from the epicenter than we would see as we look towards the epicenter. No such discrepency has been observed. Even if the universe was extremely large, we should still see some difference here, however small. The evidence just doesn't back this up. It's also worth pointing out here that the fireworks theory requires a finite universe whereas spacetime expansion allows for an infinite universe, and as far as I know the only way to prove this would be to find either the edge or center.

Also, how does the fireworks theory hold up from a quantum mechanical point of view? I'm no expert here, but as I understand it, particle physics does predict the existence of a cosmological constant as a necessary consequence of spontaneous pair creation and that, if true, this constant should be *much* greater than predicted by general relativity alone. Correct me if I'm wrong, but essentially the instability of the vacuum forces empty space to expand. Given that quantum mechanics has been repeatedly proven, I see no reason to discard its effects here. It also seems like this effect should have caused the universe to expand more slowly at the beginning, as expanding spacetime slowed the ballistic expansion of matter, and then increased the speeds at which galaxies moved away from the epicenter. I was under the impression that it's becoming increasingly evident that the universe underwent a period of inflation shortly after the Big Bang. It seems that the rate of expansion started fast, then slowed down, and has been gradually expanding ever since.

Lastly, the fireworks model doesn't offer any explanation for the formation of galaxies. A spherical object exploding in otherwise empty should have produced a hollow, spherical shell of gas - much like a supernova bubble. Astronomers have mapped the distribution of galactic clusters across a significant portion of the sky in both hemispheres and found no evidence that matter is distributed in any sort of spherical pattern. Any other distribution of matter is impossible with an explosive force. Taken a step further, that you have objects like galaxies *at all* is evidence for an inflationary universe - which is only possible with spacetime expansion.

The galaxies have sufficient gravity to counter any spacetime expansion taking place inside them.

Bad example - we live in a four-dimensional universe, not a two-dimensional one.

Not from the point of view of someone actually on the surface of the balloon. From there, the more distant galaxies will appear to recede more quickly than closer galaxies.

I was under the impression that this was more or less observed to be true. Anyone know of any evidence to the contrary?

Only for a closed finite universe. Spacetime expansion *is* possible for a universe that is both open and infinite. Admittedly, this is where the balloon analogy breaks down, but that doesn't necessarily mean that the balloon analogy doesn't show how an expanding surface works.

You're mixing apples and Volkswagons by mixing reference frames. In the real universe, we're stuck on a four-dimensional surface - whatever that is. The equivilent reference frame for the balloon analogy would be the point of view of a two-dimensional creature (or even a tiny insect) on the surface of the balloon.

The balloon analogy doesn't claim to be anything other than a two-dimensional model.

Again, you're mixing reference frames and getting a non-sensical result. Ignoring time for a moment, our eyes are three-dimensional because we live in three dimensions of space. If we lived in a two-dimensional universe, like the hypothetical observer on the surface of the balloon, we would have two-dimensional eyes.

For the purposes of the balloon analogy, what goes on inside the balloon is irrelevant. Remember, a two-dimensional observer *on the surface of the balloon* would be totally unaware of the inside of the balloon.

On second thought, as four-dimensional creatures we may very well be in the same situation, living our lives blissfully unaware of what's going on in the other seven dimensions.

Of course not. Our universe is four dimensional.

The universe is curved in four dimensions. As such, our three-dimensional eyes can't perceive the curvature directly. We can, however, observe the effects of curvature. For instance, gravitational lensing has been observed on multiple occasions.

Galaxies don't expand in size because their gravity counteracts the expansion of spacetime on smaller scales.

All of which boils down to an accident of evolution more than anything. Again, we see the universe in as having three spatial dimensions because biology has only given us the equipment necessary to see three dimensions. We don't know why this is, but it's true. However, our lowly three-dimensional eyes can observe things that can't be explained by the universe having a flat, three-dimensional geometry. The only theory that I know of that successfully predicts an orbit's perihelion advance is general relativity, and this prediction depends on the universe having a curved, four-dimensional geometry. Given that general relativity successfully predicts this phenomenon - and several others - I'm not ready to junk it until I see an alternative.

Actually, this has been directly observed. Eddington's famous observations during a solar eclipse, for one.

Nobody said they are. The balloon analogy is a model. It in no way implies that the universe is rubber with spots drawn on.

Ok, let me try to explain my reasoning about this.

Let’s compare our universe to our own galaxy, which does have a center.

In the 19th Century, astronomers could not see the center or the outer edge of the galaxy.

In the late 18th Century, Herschel thought the Milky Way represented the entire universe, and since he saw a “ring” of stars circling the earth (the Milky Way), he thought we were in the center of the ring.

Early in the 20th Century astronomers began to notice a slight movement of a lot of stars in our vicinity. They began calculating and discovered there was a mass movement of stars in our area of the sky. This eventually led to the discovery that we are inside a disk-shaped galaxy, with the center in one direction of our view, and the outer edge in the opposite direction. No one got upset when they learned that our galaxy had a center. They also eventually learned that the center of the galaxy is different from the rest of the galaxy, because it has a “bulge”, and that helps indicate it is not only different, but that it is the center.

Now, if our expanding universe is spherical and if has a current actual diameter of let’s say 100 billion light years, and if our viewing radius is currently is only about 4 billion light years (a diameter of 8 billion light years), we could be 30 billion light years away from the center and 20 billion light years away from the outer edge. This means we could not see either the center or the outer edge, and we might never be able to see the center or the outer edge.

If we were in that location, what we would observe from our position is most of the galaxies moving away from us, from inside our “sphere of visibility” which has a radius of 4 billion light years. All the galaxies would be moving away from the center, including our own, but we would not see the center, or the edge. All we would see is the galaxies in our area of the universe separating from us, i.e. moving away from us, and they would see us moving away from them.

Here’s the thing, the very high speed of the most distant galaxies and the slow speed of light causes the universe to expand more while the light from those galaxies is moving toward us.

A few nights ago, Spiff said something about a galaxy being 2.8 light years away from us when it emitted the light we now see. I think he said it took something like 13.2 billion years for that light to reach us, because that galaxy was moving away from us at high speed, while the light from it was traveling toward us. Someone said the light actually traveled a total distance of 13.2 light years before it reached us. This is because the light, which is moving at “c” while the galaxy is moving away from us at several times “c”, actually moved away from us for a long time before it started moving toward us.

And I think Spiff said that that galaxy is probably now about 30 billion light years away from us. Ok, we won’t see the light that the galaxy is emitting right now, for many billions of more years. So by the time that light reaches the earth, that galaxy might be 60-80 billion light years away from us, and the earth might no longer exist by then. So, our universe is apparently much larger than we usually think it is. What we are seeing now is light that started out when the galaxy was only 2.8 light years away from us. We are seeing something like a 13.2 billion year old movie of the galaxy, as it was 13.2 billion years ago, when it was only 2.8 billion light years away from us.

So, our actual current sphere of visibility might be only 3 or 4 billion light years. Even though the astronomers say we are looking back 13.2 billion years in time, we, according to what some of the guys said about this the other night, are seeing the equivalent of only about 2.8 billion light years along our “sphere of visibility” radius line, since, when we see the light from the galaxy, it was only 2.8 billion light years away when that light was emitted. So we are not seeing 13.2 billion light years away from us, we are seeing the equivalent of 2.8 billion light years away from us. This is the phenomenon that that we have to deal with, since the distant galaxies are moving away from us at many times the speed of light, so it takes longer for their light to reach us.

We probably could see as much as, let’s say, 14 billion light years away from us, if the distant galaxies weren’t moving. But they are moving and at very high speeds. So, by the time their light reaches us, we see the light from some of them when they were 2.8 billion light years away, and that is about the limit of our visual “sphere of visibility” right now. Whereas in reality, they’ve physically moved on to 20, 30, or even 40 billion light years distance away from us.

So, the “center” of the universe could be crowded with galaxies, like the center of our own galaxy is crowded with many stars, or the center could be hollow, with an absence of galaxies. But we can't see it, because it is too far away, and we might never see it, because of the problem of the “lookback time” and the “lookback distance”, since the distant galaxies we can see are moving so rapidly.

So there is nothing wrong with the universe having a 3-D Euclidean “center”, just as there is nothing wrong with our galaxy having a 3-D center or our solar system having a 3-D center or our earth having a 3-D center.

I'd just like some feedback here. I propose that the redshift which is readily observable from our galaxy is not caused by galactic motion (for the most part)... instead, light 'waves' reaching earth are subject to the force of gravity 'emited' by the earth, our sun, and most importantly, the quasar in the center of the milky way. This being true, and gravity being equivalent to ACCELERATION, isn't true that ANY light reaching our galaxy from a distant object would accelerate toward the center of our galaxy. This acceleration of ALL incoming light would undoubtably stretch the light and we would observe "redshift". Being use to earth-centric doppler effects, we incorrectly conclude that this shift is caused by motion... everyone but me

1) There is no quasar at the center of the Milky Way (There is, however, a massive black hole).

2) In most cases, gravitational redshift is negligible except near a black hole (30,000 light years - our distance from the center of the Milky Way - is not close) or neutron star. When necessary, it is easy to factor it into the equations.

__________________
Any day you wake up on "the right side of the dirt" is a good day.

T. Anderson

Okay, my bad. There is something massively gravitionaly at the center of our galaxy. I'm not convinced that anyone has a clue what it is, but for this argument, I'll accept that.

I find it hard to believe that we know that "gravitational redshift is negligible" when we do not fully comprehend gravity to begin with. Do you have evidence? (I'm not just being difficult, I'd just like something to wrap my greedy little mind around)

While we do not "know" exactly how and why gravity works the way it does, that does not mean that we cannot calculate its effects to a high degree of precision and accuracy. If the equations we use did not work, we could not predict eclipses accurately, much less send probes to other planets with the precision we do.

__________________
Any day you wake up on "the right side of the dirt" is a good day.

T. Anderson

Of coarse our calculations work for inter-solar-system equations. Probably even for anything within our galaxy. Our model is bias for exactly that reason. Such a small scale (probes to planets) when we are considering light traveling between galaxies. Maybe my proposal seems a bit close minded, but what I'm driving at is that the "expansion" is an illusion. I'd be much more convinced if we could take measurements from well outside our galaxy.

There is tremendous evidence to support the big-bang theory, which predicts the "outward" motion of all bodies. It seems we have started with a conclusion and are building evidence to suit. In the same way that the wave and particle models can be interchanged to suit our needs, I tend to think that an entire model has been developed to accomidate our observations. Let us never forget that any model is by nature an incomplete and overly simplified picture.

This "conversation" looks hauntingly familiar.

I am now virtually convinced that Sam5 is the banned poster, "JR", resurrected.

Another regular poster that goes back that far has mentioned this also.

__________________
Some try to tell me, thoughts they cannot defend,... - Moody Blues.

Neptune- The original Dark Matter.

The author feels that this technique of deliberately lying will actually make it easier for you to learn the ideas. - Donald Knuth

More than one.

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SeanF

"Ask to understand, but don't challenge unless you have the knowledge."--NEOWatcher

The contents of this post are ©2009 by SeanF and may not be copied or retransmitted in any form without the express written consent of SeanF

Shoot, musta missed the others. ops:

__________________
Some try to tell me, thoughts they cannot defend,... - Moody Blues.

Neptune- The original Dark Matter.

The author feels that this technique of deliberately lying will actually make it easier for you to learn the ideas. - Donald Knuth

He wrote that she wrote that he wrote that she wrote that "JR" and "Sam5" were caught making out in home room.

tehe... just curious, what was (s)he banned for?

Actually, the banned poster was JW and IIRC he was banned for general beligerence and ad hominem attacks.

__________________
Any day you wake up on "the right side of the dirt" is a good day.

T. Anderson

Yes, that is who I meant. Thank you.