Is Every Galaxy the Center of the Universe?

In summary, after reading about the center of our Universe, it is important to understand that the Universe is homogeneous and isotropic, meaning that it looks the same in all directions and there is no special position. This is evidenced by Hubble's relationship and is compatible with a Copernican view of the Universe. Therefore, there is no specific center of the Universe, as it is constantly expanding and there is no point from which it started expanding. It is also important to note that our understanding of space-time in the context of general relativity is limited to cosmological scales, and it is difficult to define a center in this context. Furthermore, the assumption that the Universe is infinite is based on observation, but cannot be confirmed. Therefore,
  • #36
VikingF said:
Ok, that was my point. :smile:
But all matter doesn't necessary come from the same Big Bang, does it?
In "my" model it does. It's homogenous so it all has the same density at all times.
 
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  • #37
EL said:
In "my" model it does. It's homogenous so it all has the same density at all times.

How can something that is spread all over in an infinity come from one singularity/one point? If it has expanded at the speed of light (or so) in 13.7 billion years, it must have been expanded to a certain (finite) area? If I start walking today, then in 3 month, I have left a finite length behind me, right?
 
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  • #38
VikingF said:
How can something that is spread all over in an infinity come from one singularity/one point? If it has expanded at the speed of light (or so) in 13.7 billion years, it must have been expanded to a certain (finite) area?
But it didn't come from one point. It came from a state of very high (infinite) density, i.e. the Big Bang singularity. The matter was all over the universe already right after Big Bang.
 
  • #39
EL said:
But it didn't come from one point. It came from a state of very high (infinite) density, i.e. the Big Bang singularity. The matter was all over the universe already right after Big Bang.


Ok. I see now. I had missed that point. (Norwegians, you know...) :rolleyes: :wink:
 
  • #40
VikingF said:
Ok. I see now. I had missed that point. (Norwegians, you know...) :rolleyes: :wink:
Hehe, yeah, stick to your crosscountry skiing instead... :-p :biggrin:
(just kidding)
 
  • #41
EL said:
But it didn't come from one point. It came from a state of very high (infinite) density, i.e. the Big Bang singularity. The matter was all over the universe already right after Big Bang.

Are you saying the Big Bang singularity, is the same as the moment right after?
 
  • #42
Eric England said:
Are you saying the Big Bang singularity, is the same as the moment right after?
If EL will pardon me, I'll try and give you my spin in this.:wink:

As EL stated before, the [postulated] singularity of the BB model was a state of infinite density. It could have been of any size, even infinitely large, whatever that may mean.

So..., right after the BB the universe could have been infinite in size, but no longer infinite in density, because space was expanding. Granted, in the inflationary epoch, the expansion rate could have been very slow initially, but it had to be there. Does this make any sense?

Jorrie
 
  • #43
Jorrie said:
Does this make any sense?

Thank you, but it doesn't seem to speak to the question I asked.

Let me add one thing EL said earlier – "Comment: The today observable universe, which is finite in size, of course expanded from a single point.

Then on to the statement I originally quoted – which needs to be explained as to why it is consistent with the above and consistent within itself.

His statement (part one) – "But it didn't come from one point. It came from a state of very high (infinite) density, i.e. the Big Bang singularity."

His statement (part two) "The matter was all over the universe already right after Big Bang."

First of all, is in one point or isn't it?

Secondly, both parts of the latter statement are meant to describe the same thing (in context to the question he was answering) – but in part two he is describing no earlier than Plank time and in part one he is describing prior to Plank time.

Someone, please zero in on this question for me. Thank you.
 
  • #44
Ok, let me try som more:
Speaking about the "size" of the Universe at the Big Bang singularity is really a bit confusing. At t=0 the distance between any two specific points in space is zero and yet the theory (GR) allows that right after this moment space is infinite. Hence it's a bit ambigous to say that the size of the universe was infinite at the singularity. What's important is that GR allows for an infinite homogenous universe. The discrepancy really steams from how we look at the limit t --> 0.
Important to note is though that this "Big Bang singularity" is something which is predicted by GR, but at the same time we know GR can't describe the universe well at the earliest times. Hence all the Big Bang theory can say at the moment is that once the universe was in a state of really high density (although not infinite) from which it has expanded into what we see today. If the universe is infite now, it sure was infinite at this "really high density"-moment too. What happened before this we can only speculate in, since we've still not found a good quantum theory of gravity. (But when we find such, it will hopefully remove the GR singularity.)
So, speaking about the size of the universe "at the Big Bang singularity" is not really relevant, since the singularity is predicted by GR, which we know do not hold at the earliest times...
 
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  • #45
I just want to add something to what EL wrote.

A spacetime singularity doesn't have to be "like" a point, it can be more "like" an edge.

Going back in (cosmological) time, the finite/inifinite nature of space doesn't change as long as t > 0, no matter how small t. In other words, infinite space could have been "born" out of an edge-like singularity.

As, EL says, physics doesn't say anything about the "birthing" at t = 0, and we expect general relativity to break down for very small t. Maybe a new theory will address t = 0, and the transition to non-zero t in a scientific way.
 
  • #46
Thanks... I was using "point" figuratively.

Isn't the "big bang" another way of saying time, space, and mass = 0?

So doesn't the big bang = 0?

Doesn't the "before" the big bang have nothing to do with time, space, or mass? Isn't it a question of what the 0 is "inside" of?

As for the transition to non-zero – isn't it a question that involves time, space, and mass – but has nothing to do with any of them?
 
  • #47
It would, I think, be fair to say that empty space arose as a consequence of matter. Matter had issues and needed time to think. So it created space to separate itself from its brethren - then it inflated space to buy some additional time. This impish idea created entropy, which can never be decreased so long as time, another poorly thought out idea, continues to exist.
 
  • #48
Eric England said:
Isn't the "big bang" another way of saying time, space, and mass = 0?
No.
"Big Bang is the scientific theory of how the universe emerged from a tremendously dense and hot state about 13.7 billion years ago." See more at http://en.wikipedia.org/wiki/Big_bang .
 
  • #49
Chronos said:
It would, I think, be fair to say that empty space arose as a consequence of matter. Matter had issues and needed time to think. So it created space to separate itself from its brethren - then it inflated space to buy some additional time. This impish idea created entropy, which can never be decreased so long as time, another poorly thought out idea, continues to exist.
Risking another warning from a moderator for being "not helpful" or "antagonistic" here, but what on Earth are you talking about? :confused:
 
  • #50
EL said:
"Big Bang is the scientific theory of how the universe emerged from a tremendously dense and hot state about 13.7 billion years ago." See more at http://en.wikipedia.org/wiki/Big_bang .

Plank time says space, time, and mass is > 0... doesn't it?

So what = 0?
 
  • #51
Chronos said:
It would, I think, be fair to say that empty space arose as a consequence of matter. Matter had issues and needed time to think. So it created space to separate itself from its brethren - then it inflated space to buy some additional time. This impish idea created entropy, which can never be decreased so long as time, another poorly thought out idea, continues to exist.

Actually, given the scientific bent of the author, I find this extremely clever. However, he and I both know he is being a naughty boy.
 
  • #52
Eric England said:
Plank time says space, time, and mass is > 0... doesn't it?

So what = 0?
I do not understand what you are trying to say. Could you state your question more clearly please?
 
  • #53
EL said:
I do not understand what you are trying to say. Could you state your question more clearly please?

Which one are you unclear about?
 
  • #54
So doesn't the big bang = 0?
0 of what?

Doesn't the "before" the big bang have nothing to do with time, space, or mass? Isn't it a question of what the 0 is "inside" of?
What is "the 0"?

Eric England said:
Plank time says space, time, and mass is > 0... doesn't it?
Planck time is a certain amount of time. How could it say anything?

So what = 0?
I simply do not understand anything about this question.
 
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  • #55
"0 of what?"... time, space, and mass.

"What is the 0?"... a dimensionless point at the center of 1.

"Planck time is a certain amount of time. How could it say anything?"... is is a "knowable" condition of space and mass being >0 at the "time" of time being >0
 
  • #56
0 is the dimensionless point at the center of 1?

Come on, Eric... please respect our guidelines, as I explained. Thread locked.

- Warren
 

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