What is the expansion of the universe

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Discussion Overview

The discussion revolves around the concept of the expansion of the universe, particularly focusing on Hubble's observations and the implications of such expansion. Participants explore theoretical aspects, practical interpretations, and the paradoxes associated with an expanding universe, while also addressing the role of dark energy and the nature of infinity.

Discussion Character

  • Exploratory
  • Debate/contested
  • Conceptual clarification
  • Technical explanation

Main Points Raised

  • Some participants question how the universe can be said to be expanding if everything is expanding, suggesting that it leads to a paradox where expansion becomes meaningless without a relative frame.
  • Others argue that the expansion refers specifically to the space between galaxies, not the galaxies themselves, and that this can be observed through redshift measurements.
  • A participant clarifies that it is the voids between galactic superclusters that are expanding, while regions dominated by gravity, such as clusters and solar systems, do not expand.
  • There is a discussion about the implications of an infinite universe and whether it can truly expand, with some suggesting that if the universe is infinite, it cannot increase in size.
  • Some participants propose that the mathematical representation of infinity does not necessarily reflect physical reality, raising questions about the nature of infinity in relation to the universe's expansion.
  • One participant emphasizes that while space itself is expanding, the objects within it are not increasing in size, and this expansion allows for galaxies to recede from each other at increasing rates.

Areas of Agreement / Disagreement

Participants express multiple competing views regarding the nature of the universe's expansion, the implications of infinity, and the role of gravity. There is no consensus on the interpretation of these concepts, and the discussion remains unresolved.

Contextual Notes

Participants highlight limitations in understanding the expansion, particularly regarding the definitions of infinity and the conditions under which expansion is observed. The discussion reflects a range of assumptions and interpretations that are not universally accepted.

  • #31
salvestrom said:
I was using the word beginning as in, wasn't there: then it was. I should also stress it was a reference to space, not the universe. I also accept that the universe may in reality have popped into existence already infinite in size, but like phinds, I have a personal preference (the opposite of his). Technically, reheat is a creation event. :P

Then I don't understand your argument. If an infinite amount of space expands or creates more space, it's still infinite. If it wasn't infinite and created more or expanded, it's still not infinite. The created volume of space that is created from another volume of space may be finite, but there are an infinite number of them.
 
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  • #32
Drakkith said:
Then I don't understand your argument. If an infinite amount of space expands or creates more space, it's still infinite. If it wasn't infinite and created more or expanded, it's still not infinite. The created volume of space that is created from another volume of space may be finite, but there are an infinite number of them.

Right... And I'm in favour of version two. Phinds has a preference for version one. Your last sentence just seems like an extention of the first.
 
  • #33
salvestrom said:
You also note that one potential explanation of space expansion is that new space is created. This presents a backtrapolatin of a point at which there was none. Anything that has a definite beginning cannot be infinite after a finite amount of time.

salvestrom said:
Right... And I'm in favour of version two. Phinds has a preference for version one. Your last sentence just seems like an extention of the first.

I'm sorry, I don't see how your statements are adding up correctly. If space is infinite then you cannot extrapolate backwards and get 0 space. Were you assuming that space was finite when you made that post? If so, I can see that being correct.
 
  • #34
Drakkith said:
I'm sorry, I don't see how your statements are adding up correctly. If space is infinite then you cannot extrapolate backwards and get 0 space. Were you assuming that space was finite when you made that post? If so, I can see that being correct.

Yes, that was the idea. The creation of new space might be worked back to a point where you simply end up with one unit of it, and then *poof*. Having an infinite universe pop into existence seems at odds with having it grow discretely - not because you can't add to infinity, just because its a dramatic switch from the creation of infinite space to just getting small packets pop in. As if one day another infinite amount of space will appear between us and Andromeda and that'll be the last we see of her. If space is infinite, it seems, on the surface, sensible to work from the point of view that it was always there. This tends to go along side the notion of spacetime. If space is infinite, time ought to be, too. And vice versa.

Not that they are inseperable, but if you're going to treat them that way, it creates automatic limits.
 
  • #35
Space doesn't grow discretely as far as we know. It is a smooth expansion or growth. And according to General Relativity time and space ARE inseparable. Both are described by the same metric.
 
  • #36
Drakkith said:
Space doesn't grow discretely as far as we know. It is a smooth expansion or growth. And according to General Relativity time and space ARE inseparable. Both are described by the same metric.

By discretely I meant that the moment to moment expansion would be much like watching your fingernails grow. And, yes, I know GR states they are inseperable. I was trying to acknowledge that it isn't absolute fact, or that I'm in a position to use a word like "are" to describe them. If at some point they are separated it would change many of the things I was saying. Is it the Wheeler equations that don't involve time??
 
  • #37
salvestrom said:
By discretely I meant that the moment to moment expansion would be much like watching your fingernails grow. And, yes, I know GR states they are inseperable. I was trying to acknowledge that it isn't absolute fact, or that I'm in a position to use a word like "are" to describe them. If at some point they are separated it would change many of the things I was saying. Is it the Wheeler equations that don't involve time??

The way that we perceive the universe is quantum in nature in terms of the information we receive and process. The way we conceptualize the universe tends to be necessarily alegorical. The mathematical allegories of GR , SR and QM are the most accurate-in terms of their corelation to the information we receive and process. SR is an inherent accepted element of QM.
GR is not a perfect fit with QM. To a degree the effective implemental domains of the two are at the two extremes of our perceptions of the universe.
Theoreticians coming from a QM background may meld QM with GR by going to an imeasurable domain beyond fundamental particles and geometries of today.
It is the Wheeler-DeWitt equation you are thinking of. It began with DeWitt.
"Beyond The Standard Model" here in these forums will provide you with months if not years of PDFs on models that follow lines of thinking similar to yours.
Happy reading
mathal
 
  • #38
This is off the original topic but I'm a startled bit by the discussion here (a thread split would be a good idea.)

I've always taken it as a given that all physical quantities are finite and moreover I'm not aware of a single counterexample.

I assert there is a general principle (indeed I thought all physicists assume this) that there are upper and lower bounds to all physical quantities. Saying that a quantity is unbounded in any given aspect means that it can't be measured. But that's math/philosophy. In physics we always run into a domain limitation. On the short end is the Planck volume. On the long end is the cosmological size of the universe.

Every physical observable will have a limit above which the physics changes. You can have any classical electric field magnitude. But in the real world there's an upper limit above which you are simply condensing matter. Classically you can have any density you want but in reality you will eventually form a black hole. And so on.

To say that the volume of the universe is without limit is to render it unphysical and give it a mathematical abstractness which nothing physical we know of possesses.
 
  • #39
Antiphon said:
Saying that a quantity is unbounded in any given aspect means that it can't be measured.

I think you somewhat misunderstand the concept of unbounded. The length of a straight line on a Mobius stip or on sphere are both unbounded. It is possible that the universe is unbounded in this sense.

I agree that we don't know for sure of anything that is infinite but it does seem possible that the universe is infinite. Unless you can prove that it is not, your logic cannot rule it out just because you don't like the idea.
 
  • #40
Antiphon said:
Saying that a quantity is unbounded in any given aspect means that it can't be measured.
No. The surface of the Earth is finite yet unbounded (i.e, it has no boundary, yet its area is quite finite and easily measurable).

This is a 2D example of one possible way that our 3D universe is finite yet unbounded.
 
Last edited:
  • #41
Antiphon said:
To say that the volume of the universe is without limit is to render it unphysical and give it a mathematical abstractness which nothing physical we know of possesses.

I disagree.
 

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