Does the Universe Have a Circular Structure?

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Discussion Overview

The discussion revolves around the structure of the universe, specifically whether it has a circular or finite nature, and the implications of this on the behavior of massless particles like photons. Participants explore concepts related to the balloon analogy, the nature of infinity, and the universe's expansion, touching on theoretical and conceptual aspects without reaching a consensus.

Discussion Character

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

Main Points Raised

  • Some participants question whether the term 'circular' accurately describes the universe, suggesting it may be finite or infinite without a definitive boundary.
  • There is speculation about whether photons could eventually return to their starting point, with references to the balloon analogy and its limitations.
  • Participants discuss the implications of the universe being finite or infinite, with some arguing that an infinite universe could have originated from a singularity.
  • One participant suggests that in a closed universe without dark energy, photons could meet again at the universe's end, while others challenge this interpretation.
  • Concerns are raised about the concept of nothingness having a metric, questioning the nature of the universe's origin.
  • Some participants express uncertainty about the physics before the Planck Time, indicating a lack of consensus on the universe's size and structure.

Areas of Agreement / Disagreement

Participants express multiple competing views regarding the universe's structure, whether it is finite or infinite, and the behavior of photons. The discussion remains unresolved, with no consensus reached on these complex topics.

Contextual Notes

Participants acknowledge limitations in their understanding, particularly regarding the assumptions underlying the balloon analogy and the nature of the universe's expansion. There are unresolved questions about the universe's state before the Planck Time and the implications of its potential finiteness or infiniteness.

geordief
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I am assuming the answer to this is no but having view to the balloon analogy in the neighbouring thread (that I don't want to dilute with this probably trivial question by joining)
I just thought I might ask whether a photon (or any other massless particle which seem to move forever at a continuous speed unless they meet an obstacle) might eventually end up where it had started from?

Otherwise stated would we see the backs of our own neck if we looked far and well enough?

I thought of this when I looked at the balloon diagram but I assume that this is just one of the misperceptions that the balloon analogy can give rise to (I know we have been told repeatedly to view it with a pinch of salt).
 
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I don't know if 'circular' is the proper term to use.

Yes, there is a good possibility the universe is finite. The same chance that it is infinite, there is no reason to suppose either is correct. However, it most definitely does NOT have a boundary if it is finite.

It's actually the whole purpose of the balloon analogy - to show that the universe can expand without having any edge or center.
 
Mark M said:
I don't know if 'circular' is the proper term to use.

Yes, there is a good possibility the universe is finite. The same chance that it is infinite, there is no reason to suppose either is correct. However, it most definitely does NOT have a boundary if it is finite.

It's actually the whole purpose of the balloon analogy - to show that the universe can expand without having any edge or center.

How can it be infinite if it had a start ~13.7 billion years ago?
 
geordief said:
I am assuming the answer to this is no but having view to the balloon analogy in the neighbouring thread (that I don't want to dilute with this probably trivial question by joining)
I just thought I might ask whether a photon (or any other massless particle which seem to move forever at a continuous speed unless they meet an obstacle) might eventually end up where it had started from?

Otherwise stated would we see the backs of our own neck if we looked far and well enough?

I thought of this when I looked at the balloon diagram but I assume that this is just one of the misperceptions that the balloon analogy can give rise to (I know we have been told repeatedly to view it with a pinch of salt).

Yes, I think here you are trying to take the balloon analogy beyond its scope, the balloon analogy is mostly to show how something with no edge or center can expand causing arbitrary points on the balloon to move away from any other arbitrary point.

Gytax said:
How can it be infinite if it had a start ~13.7 billion years ago?

This one is easy, there is no proof either way, but to be infinite the universe simply had to be infinite at the start.
 
Gytax said:
How can it be infinite if it had a start ~13.7 billion years ago?

As Some Slacker pointed out, it could have began infinite. Remember that the expansion of the universe is a metric expansion - the universe expands from every point.
 
An infinite universe coming out from nothing simultaneously all at once ?
(simultaneously = in some billion years)

If the universe came out from nothing with a "diameter" other than zero, then even before there was a metric in the nothingness.
But how can nothingness have a metric ?

(Scary to imagine)
 
Quinzio said:
An infinite universe coming out from nothing simultaneously all at once ?
(simultaneously = in some billion years)

If the universe came out from nothing with a "diameter" other than zero, then even before there was a metric in the nothingness.
But how can nothingness have a metric ?

(Scary to imagine)

Considering we don't know what kind of physics was obeyed at the Planck Time and before, the most we can, as of now, speak about factually, is the universe after that time. So whatever went on then, it is possible (and preferred by a large number of cosmologists) that the universe was infinite in size.
 
geordief said:
I just thought I might ask whether a photon (or any other massless particle which seem to move forever at a continuous speed unless they meet an obstacle) might eventually end up where it had started from?

In the absence of dark energy, in a closed universe, I believe the photon would get exactly half way round in the time between the big bang and the big crunch. Two photons launched in opposite directions would just meet up as the universe ended. :-(
 
Gytax said:
How can it be infinite if it had a start ~13.7 billion years ago?

If the universe is infinite in spatial extent, aka size, that does not require that it is also exist for an infinite amount of time.

Quinzio said:
An infinite universe coming out from nothing simultaneously all at once ?
(simultaneously = in some billion years)

If the universe came out from nothing with a "diameter" other than zero, then even before there was a metric in the nothingness.
But how can nothingness have a metric ?

(Scary to imagine)

We don't know how the universe was formed or whether it is infinite or finite. We do know that the OBSERVABLE universe was packed into a very very tiny volume, but this is ok because the observable universe is definitely not infinite.
 
  • #10
GeorgeDishman said:
In the absence of dark energy, in a closed universe, I believe the photon would get exactly half way round in the time between the big bang and the big crunch. Two photons launched in opposite directions would just meet up as the universe ended. :-(
Is anyone else of that opinion? Since my question was based on an apparently erroneous interpretation of the balloon analogy would you be coming to that supposition on a different basis?
Maybe it wasn't meant seriously (it wouldn't be hard to go over my head)
 
  • #11
geordief said:
Is anyone else of that opinion? Since my question was based on an apparently erroneous interpretation of the balloon analogy would you be coming to that supposition on a different basis?
Maybe it wasn't meant seriously (it wouldn't be hard to go over my head)

It is something I remember from many years ago but there may be a subtlety I missed, see the post from pervect in this old thread:

https://www.physicsforums.com/showthread.php?t=42599
 
  • #12

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