# Diameter of the observable Universe

• I
• Thecla
Thecla
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?

The universe does not have a center. The observable universe does - you, if you're the one doing the observing.

PeroK, russ_watters, berkeman and 2 others
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

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)

PeroK and jbriggs444
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.

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.

Bosko
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)

Bosko
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.

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.

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.

Bosko said:
Then the amount of matter is infinite.
The universe does not feel compelled to arrange itself to your liking, I'm afraid.

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?

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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