Diameter of the observable Universe

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Discussion Overview

The discussion revolves around the concept of the observable universe's diameter of 93 billion light years and whether this implies the existence of a center in the universe. Participants explore various cosmological models, geometries, and implications of the universe's expansion.

Discussion Character

  • Debate/contested
  • Conceptual clarification
  • Exploratory

Main Points Raised

  • Some participants assert that the observable universe has a center based on its diameter, while others argue that the universe itself does not have a center.
  • There are claims that if the universe has an infinite geometry, it should have an edge and thus a center, but this is contested by others who state that a uniform infinite expanse has no center.
  • Participants discuss the implications of different geometries: flat, open, and closed (spherical) universes, and how these relate to the distribution of matter.
  • Some argue that the matter density in the universe is uniform and infinite in flat and open geometries, while others find this contradictory.
  • There is a discussion about the nature of the Big Bang and whether it represents a center or point of origin for the universe.
  • A question is raised about whether observers in different galaxies would agree on the size of the observable universe, with a later reply suggesting that they would, given the same elapsed time since the Big Bang.

Areas of Agreement / Disagreement

Participants express multiple competing views regarding the existence of a center in the universe and the implications of different cosmological models. The discussion remains unresolved with no consensus reached.

Contextual Notes

Participants reference assumptions about homogeneity and isotropy in cosmological models, but these assumptions are not universally accepted or clarified, leading to differing interpretations of the implications for the universe's structure.

Thecla
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TL;DR
If the universe has a diameter does it have a center?
Cosmologists say that the observable universe has a diameter of 93 billion light years. Does this mean the universe has a center?
 
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The universe does not have a center. The observable universe does - you, if you're the one doing the observing.
 
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Ibix said:
The universe does not have a center. The observable universe does - you, if you're the one doing the observing.
The spatial universe is believed to have one of three possible geometries
universe_geometry.gif

The matter in the universe began to expand before the final (Big Bang) time at some final high speed.
There are two possibilities:

If the universe has an infinite geometry (either flat or negative curvature), then it has an edge that matter has reached in its expansion. In this case, it should have a center.

Another possibility is that the universe has a spherical (positively curved) geometry and is expanding along with the matter in it.In this case there is no center. (The center is actually back in the time of the Big Bang)
 
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Bosko said:
If the universe has an infinite geometry (either flat or negative curvature), then it has an edge that matter has reached in its expansion. In this case, it should have a center.
No. All FLRW solutions, including the flat and open ones, are spatially homogeneous. There is matter everywhere and always was.
 
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Bosko said:
If the universe has an infinite geometry (either flat or negative curvature), then it has an edge that matter has reached in its expansion.
Wrong. The matter density in the universe is the same everywhere, even in spatially infinite universes.

Bosko said:
In this case, it should have a center.
No, it doesn't. A uniform infinite expanse has no center.

Bosko said:
(The center is actually back in the time of the Big Bang)
No, it isn't. A moment in time is not a center of anything.
 
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Thecla said:
TL;DR Summary: If the universe has a diameter does it have a center?

Cosmologists say that the observable universe has a diameter of 93 billion light years. Does this mean the universe has a center?
Now that this has been answered, the real mind blowing question you will find is how the observable universe can be 93 billion ly across when the universe is less than 14 billion years old. 😉

(The answer being the expansion of the universe)
 
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PeterDonis said:
Wrong. The matter density in the universe is the same everywhere, even in spatially infinite universes.
Then the amount of matter is infinite.
That seems contradictory to me for a flat and open universe.
Only a closed (spherical) universe does not seem contradictory to me.
 
Bosko said:
Then the amount of matter is infinite.
That seems contradictory to me for a flat and open universe.
Only a closed (spherical) universe does not seem contradictory to me.
Why would it be contradictory? It is very consistent with the assumptions of homogeneity and isotropy.

Remember that the matter does not originate in a pointlike explosion within the universe. It originates everywhere in the same fashion. A pointlike explosion would directly contradict the cosmological assumptions.
 
Orodruin said:
Why would it be contradictory? It is very consistent with the assumptions of homogeneity and isotropy.
A flat and open universe has infinite volume, if I'm not mistaken.
Homogeneously distributed mass should be infinite.
 
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Bosko said:
Then the amount of matter is infinite.
Yes.

Bosko said:
That seems contradictory to me for a flat and open universe.
It's a valid solution of the relevant equations. What more do you want?

Bosko said:
Only a closed (spherical) universe does not seem contradictory to me.
Nature doesn't care what does or does not seem contradictory to you.

You have now been banned from further posting in this thread as it is not your thread and you are not helping the OP.
 
  • #11
Bosko said:
A flat and open universe has infinite volume, if I'm not mistaken.
It does.

Bosko said:
Homogeneously distributed mass should be infinite.
It is, as has already been said.
 
  • #12
Bosko said:
Then the amount of matter is infinite.
That seems contradictory to me
The universe does not feel compelled to arrange itself to your liking, I'm afraid.
 
  • #13
If you could travel infinitely fast to all the billions of galaxies in the universe at the same time would all the observers in those galaxies say : yes the observable universe is 93 billion light years?
 
  • #14
Thecla said:
If you could travel infinitely fast to all the billions of galaxies in the universe at the same time would all the observers in those galaxies say : yes the observable universe is 93 billion light years?
You can't travel infinitely fast, but if we rephrase your question as, will observers in all the other galaxies, at the same elapsed time by their clocks from the Big Bang, say that their own observable universe is 93 billion light years wide, the answer is yes.
 
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