Universe expansion

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I apologize if this has been asked before, but I have never seen a satisfactory or straightforward answer.
When we say the universe is expanding, do we mean it is "growing" or "stretching"?
In other words, if you considered a sheet of graph paper expanding, are we adding more squares or stretching the distance between squares?
 

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  • #2
phinds
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I apologize if this has been asked before, but I have never seen a satisfactory or straightforward answer.
When we say the universe is expanding, do we mean it is "growing" or "stretching"?
In other words, if you considered a sheet of graph paper expanding, are we adding more squares or stretching the distance between squares?
Space is not a "thing" that can grow or stretch like a sheet of rubber, despite what you see in pop-sci presentations. Things just get farther apart. I strongly suggest that you check out the link in my signature where it is discussed.
 
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Space is not a "thing" that can grow or stretch like a sheet of rubber, despite what you see in pop-sci presentations. Things just get farther apart. I strongly suggest that you check out the link in my signature where it is discussed.
So I read the balloon analogy "fix" in your sig - thanks for that, it does clear up some things.

However, I think the philosophical implications are still worth discussing - perhaps not here - and many obvious questions still remain unanswered, perhaps even become more difficult. For example, if the universe is not really expanding - things are just moving away from each other - then there must be an existing 'realm' of space they are moving into. In other words, the Big Bang didn't create the universe, it just created "stuff" to fill an existing space-time that happened to be (maybe) empty. Was that existing space-time eternal? Infinite? Is there other stuff already there that is being pushed away, or in some way affecting "our stuff"? IS our Big bang just one of many that happens all the time in this bigger space? What happens if another Big bang happens within our horizon? And so on...

So I would say your reference answers my question - space is neither growing nor stretching - but it nevertheless creates more questions than it answers.
I'm curious if you think I still don't get it. I just think after reading the balloon fixer article we still don't have the real answer.
 
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phinds
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... if the universe is not really expanding - things are just moving away from each other - then there must be an existing 'realm' of space they are moving into.
No there is no such thing.

In other words, the Big Bang didn't create the universe, it just created "stuff" to fill an existing space-time that happened to be (maybe) empty.
that is personal speculation on your part and not part of the Big Bang Theory (which is silent on the creation event)

Was that existing space-time eternal? Infinite?
Since it isn't even known to have existed, that's not a meaningful question.

Is there other stuff already there that is being pushed away, or in some way affecting "our stuff"?
No. Again, you are basing your statements on a false premise
IS our Big bang just one of many that happens all the time in this bigger space?
There are "multiverse" theories that you might want to look into.
What happens if another Big bang happens within our horizon?
Not believed to be possible

So I would say your reference answers my question - space is neither growing nor stretching - but it nevertheless creates more questions than it answers.
Then I would say you are not paying attention to current cosmology. There ARE open questions to be sure but most of your questions are based on misconceptions.
 
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CWatters
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PeterDonis
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if you considered a sheet of graph paper expanding, are we adding more squares or stretching the distance between squares?
These are not actually two different ways that the universe could be; they are just two different ways of describing the same universe, using two different coordinate systems. There is no physical difference between these two descriptions. I know it seems like there ought to be, but your ordinary intuitions that come from dealing with ordinary objects don't work in this case.
 
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These are not actually two different ways that the universe could be; they are just two different ways of describing the same universe, using two different coordinate systems. There is no physical difference between these two descriptions. I know it seems like there ought to be, but your ordinary intuitions that come from dealing with ordinary objects don't work in this case.
I appreciate you all bearing with me as I admit my mathematical understanding of the underlying principles is lacking.
Much of my question stems from my current thought that space must be quantized - I don't see it being possible as being continuous - which is where my graph paper analogy comes in. But I do get the point of expansion not 'creating space' rather spreading things out.

Yet - I still feel there is a minimal distance and time metric. Seems like a lot of fundamental concepts could be resolved with such a foundation.
This is just a thought - I'm trying to learn. If you can direct me to some good discussions on the topic (perhaps Quantum Loop Gravity?) to point me in the right direction.
 
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PeterDonis
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Much of my question stems from my current thought that space must be quantized
That is out of bounds for discussion here; there is no current theory in physics that has space (more correctly, spacetime) being quantized. There have been speculations along these lines, but they're just speculations. You might possibly find a peer-reviewed paper discussing some of those speculations; if so, you can start a new thread in the Beyond the Standard Model forum to ask questions about it (since that forum is where we discuss speculative ideas in the literature that go beyond our current confirmed theories).

perhaps Quantum Loop Gravity?
Loop quantum gravity isn't really a (speculative) theory of "quantized spacetime". It ends up saying that, at the Planck scale, spacetime is no longer the sort of continuum that appears in our current theories, but it doesn't just replace that with a simple discrete set of "cells" or something like that; it's more subtle.
 
  • #9
PeterDonis
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Since the OP question is based on a speculation that is outside the bounds of current confirmed theories, this thread is closed.
 

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