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ortonomics
2008-Mar-23, 05:15 PM
After listening to episode 79 of astronomycast I am trying to wrap my head around the concept of an expanding universe, and am caught on a couple of things.

First, wouldn't something have to be finite in order to be expanding? Infinity... can't expand... if it's infinite. I know there is more to it that that, but I am hung up.

Second, when talking of expansion, is this something that is happening at a molecular level or just in the relationship (distance) between objects outside our planet? I can picture expansion happening, but at what diminishing level do objects cease to be moving away from one another? To put it a different way, is the distance between two objects here on earth growing porportionately to the expansion of the universe, and if not, why? And where does expansion start?

And for that matter, what force would cause current expansion, would it still be residual energy from the big bang?

Alot more questions... i will stop here for now... Thanks!
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Ken G
2008-Mar-23, 05:30 PM
After listening to episode 79 of astronomycast I am trying to wrap my head around the concept of an expanding universe, and am caught on a couple of things.
Welcome to the forum. It may comfort you that your questions are the normal ones to be asking at this point-- those are the questions you should be asking.

First, wouldn't something have to be finite in order to be expanding? Infinity... can't expand... if it's infinite. I know there is more to it that that, but I am hung up.
There are several ways that you can imagine answers to this. First of all, we do not know the universe is infinite, it just goes as far as we can observe or even infer. But that's not necessarily infinite. However, the idea that infinity could expand doesn't bother cosmologists, because you can imagine several ways of doing it. Imagine a number line of integers, that's infinite right? Now erase every odd number. Then take every even integer and overwrite half that number. You now have a copy of the original number line, but the numbers are twice as far apart, yes? You have just expanded infinity!

(An easier way to do it is just walk closer to the number line so that it appears to have stretched the distance between the numbers. The new appearance of the infinite thing is expanded relative to the previous appearance.)

Second, when talking of expansion, is this something that is happening at a molecular level or just in the relationship (distance) between objects outside our planet? It only happens on scales much larger than that, scales where there are not other effects that dominate the expansion. Typically the scales needed are many millions of light years!

I can picture expansion happening, but at what diminishing level do objects cease to be moving away from one another? To put it a different way, is the distance between two objects here on earth growing porportionately to the expansion of the universe, and if not, why?Not, because of the other forces on them. If everything expanded equally, we wouldn't notice anything at all.

And where does expansion start?At distances where other forces are negligible, that's many millions of light years. On those scales, the only forces that matter are those "of the universe as a whole".

And for that matter, what force would cause current expansion, would it still be residual energy from the big bang? Probably the best answer to that is simple inertia-- it was so it is. But then there's also the possibility of accelerated expansion due to "dark energy".

Noclevername
2008-Mar-24, 03:30 AM
Another example of an expanding infinite is: imagine an infinitely long ribbon, constantly getting wider, or better yet an infinite length of hollow tube constantly getting thicker in cross-section. The space inside it is increasing, yet it's no less infinite. These are two-dimensional and three-dimensional examples, and they can probably be extrapolated into a fourth-D shape as well, so that the "inner" space is, instead of an infinite cylinder as in the tube example, we get a "hypercylinder"-- instead of having a circular cross-section it has a spherical one.

ortonomics
2008-Mar-24, 03:59 AM
Another example of an expanding infinite is: imagine an infinitely long ribbon, constantly getting wider, or better yet an infinite length of hollow tube constantly getting thicker in cross-section. The space inside it is increasing, yet it's no less infinite. These are two-dimensional and three-dimensional examples, and they can probably be extrapolated into a fourth-D shape as well, so that the "inner" space is, instead of an infinite cylinder as in the tube example, we get a "hypercylinder"-- instead of having a circular cross-section it has a spherical one.

but... a tube that is growing presupposes that it was smaller to begin with, or that it has defined borders. How can something that starts off as "infinite" be smaller than what it is now, and what it will be later? How can borders be defined on an infinite mass?

steve000
2008-Mar-24, 12:25 PM
I'm not sure if this is a correct way of looking at it. (Though I am sure it will be pointed out to me if I'm attempting to display behavior beyond my intellectual means :)) I found this help full when trying to understand infinity.

(1 + (-1)) + (2 + (-2)) + (3 + (-3)) + (4 + (-4)) + (5 + (-5)) + (6 + (-6)) + (7 + (-7)) + (8 + (-8))... = 0 + 0 + 0 + 0 + 0 + 0 + 0 + 0 ... = 0

Zero always being the greater number...

(In a similar way)You could use Kens example to visualize expanding infinity

Imagine a number line of integers, that's infinite right? Now erase every odd number. Then take every even integer and overwrite half that number. You now have a copy of the original number line, but the numbers are twice as far apart, yes? You have just expanded infinity!
.1 2 3 4 5 6 7 8 9 .... (ect).
... 1...2... 3... 4 ...... (ect).

.1 + (-1)) + (2 + (-2)) + (3 + (-3)) + (4 + (-4)) + (5 + (-5)) + (6 + (-6)) + (7 + (-7)) + (8 + (-8))...(etc.) = 0
..................(1 + (-1))........ +......... (2 + (-2))......... +......... (3 + (-3)) ......... +........ (4 + (-4))...(etc.) = 0

(I'd better point out that this is only an example of infinity).