Kinda dumb cosmic expansion question

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Sweeney_Todd
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Kinda dumb cosmic expansion question

Postby Sweeney_Todd » Mon May 30, 2011 6:38 pm UTC

If the universe is curved in such a way that anything at one end is moved to the opposite end, then how can the universe be expanding? Wouldn't space expand in upon itself?

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Re: Kinda dumb cosmic expansion question

Postby ++$_ » Mon May 30, 2011 8:10 pm UTC

In the same way that you can blow up a balloon, even though it wraps around on itself.

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Re: Kinda dumb cosmic expansion question

Postby eSOANEM » Mon May 30, 2011 8:40 pm UTC

++$_ wrote:In the same way that you can blow up a balloon, even though it wraps around on itself.


QFT. It's important to note that with the balloon analogy, the universe is drawn on the surface of the balloon rather than filling it. This means that, as you blow it up things get stretched and the space in between them expands.
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Aiwendil
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Re: Kinda dumb cosmic expansion question

Postby Aiwendil » Tue May 31, 2011 12:31 am UTC

QFT


But GR is a classical field theory!

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Re: Kinda dumb cosmic expansion question

Postby poxic » Tue May 31, 2011 3:06 am UTC

Aiwendil wrote:
QFT


But GR is a classical field theory!

It's quantum all the way down up.
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Tass
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Re: Kinda dumb cosmic expansion question

Postby Tass » Tue May 31, 2011 9:05 am UTC

Sweeney_Todd wrote:If the universe is curved in such a way that anything at one end is moved to the opposite end, then how can the universe be expanding? Wouldn't space expand in upon itself?


You are thinking that there is a finite amount of "space" available for "space", this feels intuitive but is flawed thinking. "Space" is all the space there is (well maybe) and it is expanding.

Even the balloon analogy has limits. With this you can understand how a two dimensional surface can be closed yet keep getting bigger. But only because you understand that it expands in the third dimension. In reality the extra dimension is not necessary. Think of a playing field in a computer game, that wraps around on it self (this would tend to be torroidal not spherical, but never mind), now make it twice as big. Did it expand in upon itself? No, it just got bigger.

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Re: Kinda dumb cosmic expansion question

Postby Moose Hole » Tue May 31, 2011 4:59 pm UTC

I have a related question. Is the speed of light constant as the universe expands? That is, seconds after the big bang, could light travel from one extreme to the other (or loopback to the origin as the case may be)?

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Re: Kinda dumb cosmic expansion question

Postby Tass » Tue May 31, 2011 5:40 pm UTC

Moose Hole wrote:I have a related question. Is the speed of light constant as the universe expands? That is, seconds after the big bang, could light travel from one extreme to the other (or loopback to the origin as the case may be)?


That is a brilliant question.

With regular expansion the universe would always have been bigger than it is old, so to speak. So back in time the universe would have been smaller but there would also have been less time for light to move. So an even smaller part of the universe would have been observable, gradually as time goes by more of the universe becomes observable from a given point, but light has not made it round yet.

This poses a problem. Why is the universe so uniform at large scale? No information can travel faster than light, so how could one part of the universe "know" how the other looked? A theory that explains it (and much other) is inflation. In the very early universe there was an epoch where the universe expanded like crazy, before that the universe was so small that light could travel what is now great distances and bring big parts (if not the entire universe (if it is indeed closed and finite)) into equilibrium.

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Re: Kinda dumb cosmic expansion question

Postby Moose Hole » Tue May 31, 2011 7:00 pm UTC

Ok, I was under the impression that c changes as the size of the universe changes. Because the universe is made of timespace, I figured the time component would be stretching along with the space components, and therefore c, being a speed, would change along with it. This may be the case anyway, as far as I know, but I'm not sure I have my head around it enough to figure it out.

I can understand now that if the universe expands faster than c, then light can not traverse the entire universe. This concept seems strange to me given that the universe was once the size of a basketball, but then if I add in the caveat that it was a basketball growing faster than c, it makes sense.

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Re: Kinda dumb cosmic expansion question

Postby Tass » Tue May 31, 2011 8:06 pm UTC

Moose Hole wrote:Ok, I was under the impression that c changes as the size of the universe changes. Because the universe is made of timespace, I figured the time component would be stretching along with the space components, and therefore c, being a speed, would change along with it.


Given that the meter is defined from the speed of light that would actually mean that the universe couldn't change size...

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Re: Kinda dumb cosmic expansion question

Postby ++$_ » Wed Jun 01, 2011 12:22 am UTC

Is there any way to tell the difference between an expanding universe and a universe where the speed of light is decreasing?

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Re: Kinda dumb cosmic expansion question

Postby Tass » Wed Jun 01, 2011 5:42 am UTC

++$_ wrote:Is there any way to tell the difference between an expanding universe and a universe where the speed of light is decreasing?


Yes and no. If we changed the speed of light and nothing else it would be a very different universe. However we could change other constants as well in such a way that it would be the same.

Physical constants that have units can always be redefined by redefining the units. So does the speed of light change, or does the meter change? That is a question of definition. But there are constants without units, the most known one is the fine structure constant. Unitless constants are the true constants, they cannot be redefined, and if they were to change we would know, no matter our definitions.

The distance between two distant galaxies divided by the Bohr radius would be dimensionless, and it is surely increasing. We are pretty sure that the meter measured in Bohr radii is constant.

So yes, it is much easier to say that the universe is expanding, rather than saying that atoms are shrinking with all the changing physical constants that follows from that. But really there is no way to tell the difference.


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