Does the Universe Have a Circular Structure?

In summary, the conversation discusses the possibility of the universe being infinite or finite, and whether objects such as photons can eventually end up where they started. The balloon analogy is also brought up, with some participants cautioning against taking it too literally. The conversation also touches on the idea of the universe expanding without an edge or center, and how this can be difficult to imagine. Ultimately, the conversation concludes that we do not have enough information to definitively answer these questions about the universe.
  • #1
geordief
214
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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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  • #2
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.
 
  • #3
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?
 
  • #4
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.
 
  • #5
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.
 
  • #6
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)
 
  • #7
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.
 
  • #8
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. :-(
 
  • #9
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

1. What is the shape of the Universe?

The shape of the Universe is a subject of ongoing debate among scientists. While some theories suggest that the Universe is flat, others propose that it may be curved or even have a shape that we cannot comprehend. However, the most widely accepted theory is that the Universe is flat, meaning that it has a shape that is similar to a sheet of paper.

2. Is the Universe circular?

The answer to this question is not clear-cut. While the Universe may appear to be circular from our perspective, it is actually much more complex. According to current theories, the Universe may be infinite and have no boundaries, making it impossible to define a specific shape like a circle.

3. How do scientists study the shape of the Universe?

Scientists study the shape of the Universe using various methods, such as studying the cosmic microwave background radiation, measuring the curvature of space, and observing the distribution of galaxies. These methods provide valuable insights into the overall structure and shape of the Universe.

4. What is the significance of the Universe's shape?

The shape of the Universe has significant implications for our understanding of its origins and evolution. For example, a flat Universe would suggest that it will continue to expand forever, while a curved Universe may eventually collapse in on itself. Additionally, studying the shape of the Universe can help us understand the fundamental laws of physics that govern its behavior.

5. Can the shape of the Universe change?

According to the current understanding, the shape of the Universe is not expected to change. However, this is still an area of ongoing research, and new discoveries may challenge our current understanding. For now, it is believed that the shape of the Universe is constant and will remain so for the foreseeable future.

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