Must the universe have a geometric center if it's finite?

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Jim Hasty said:
But what if the expansion of space was accelerating with distance from the singularity, i.e. the epicenter of the universe, like we see today? Then if you looked in the direction of the expansion those galaxies would be accelerating away from you; and looking in the direction of the center those galaxies would appear to be accelerating away but it is actually you moving away from them. Would there be a way to discern the true direction of a center? I am not convinced that this gives the same picture as the balloon surface scenario; but I hope I am conveying my idea.
That's exactly the scenario I was just describing. Yes, you could tell which direction the center was. If the center was to your left, the galaxies to your right would be moving away from you faster than the galaxies to your left (and with speed differences that depend on distance from the center). That's not what is observed.
 
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OCR said:
If you head in any direction on the surface of a sphere, you will arrive back at your starting point.
Then, how could you know this, if...?
Any point on the surface of a sphere looks the same as any other point.
I should have been more precise to say 'observing from any point on the surface looks the same as observing from any other point on the surface'
i.e no matter where you are, you will head out in any direction and arrive back at your starting point.
So, no point is special, denoting any kind of centre.
OCR said:
Even in a wraparound universe, doesn't the spacetime mathematical model still apply... "time" has changed (passed), does your starting point actually still exist?

In any given spacetime, an event is a unique position at a unique time.
Yes, I have not suggested otherwise.
But if every point in the universe can lay claim to the same form of 'uniqueness' then none of them are preferred i.e. no centre.

Notice that this actually has nothing to do with the type of curvature. It applies equally to the flat and saddle shapes universe Bandy mentions.
 
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russ_watters said:
That's exactly the scenario I was just describing. Yes, you could tell which direction the center was. If the center was to your left, the galaxies to your right would be moving away from you faster than the galaxies to your left (and with speed differences that depend on distance from the center). That's not what is observed.

Yes, I see that now. I did a little geometry and math and the fit with observations is much better using the 'surface of a balloon' analogy. In the google research I found some discrepancies with the cosmology principle [2011-2012] but a recent article [2014] validates symmetrical expansion. Thanks for everyone's patience.
 
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phinds said:
Your concept is clear, and clearly wrong. You are not thinking through the geometry of your proposed situation. Think about how you would see a different speed for objects not directly on a line with this magical center you want to invoke than you would for ones on that line. That implies a preferred frame of reference and that's a no-no.

Update on my recent reply. I see what you mean now by 'preferred frame' in the context of the cosmological principle. There are several googled articles [2011-2012] which mention 'preferred frames' in the context of 'asymmetrical expansion'. So yes, I agree with you in that context.
 
Jim Hasty said:
But what if the expansion of space was accelerating with distance from the singularity, i.e. the epicenter of the universe, like we see today? Then if you looked in the direction of the expansion those galaxies would be accelerating away from you; and looking in the direction of the center those galaxies would appear to be accelerating away but it is actually you moving away from them. Would there be a way to discern the true direction of a center?

As long as the expansion follows Hubble's law, there is no way to find the "true direction of a center". The universe looks identical in all directions from any position.
 
Arthur Edington's 1933 balloon analogy of big bang is the most confusing one. We should remember that nothing exists outside the balloon surface. The balloon surface is increasing because it is expanding but you cannot decrease the surface area, go back in time and then reach to a singularity. That provision is there in principle but do not do it, because, then the analogy breaks. The surface area of the balloon is not known, but it has to be infinite because curvature has found to be nill. The balloon expands but not the objects that make the balloon, means expand everything but not the galaxies, observations show no expansion of galaxies. So, balloon and even Big Bang lives in a highly protected physical scenario.

Several people in this forum tried to create a center of the universe, those are not scientific nonsense. "It is less well known that Lemaître found a more general class of solutions that describe a spherically symmetric expanding universe. These solutions, now known as Lemaître-Tolman-Bondi (LTB) models, describe possible forms for a universe that could have a centre. Since the FLWR models are actually a special limiting case of the LTB models, we have no sure way of knowing that the LTB models are not correct. The FLWR models may just be good approximations that work well within the limits of the observable universe but not beyond." Therefore, many of the proposals and comments for finding the center of the universe is not nonsense. These were the early days of Big Bang theory development.

Science is not blind it accepts all views, if you are aware about scientific literatures, wild ideas do come, very scientifically. Several proposals which I found have been discarded as "nonsense", are not that nonsense at all, scientists thought and those thought led to enormous protections.
 
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Infintelycuriou said:
Several proposals which I found have been discarded as "nonsense", are not that nonsense at all.
The aim of science is to discover good explanations for things which are observed, explanations which make predictions that are testable.
A proposal which does not explain something or is not testable is not in the domain of science,
so yes it would likely be discarded as being invalid scientifically, it would not even deserve the status of being 'wrong'.
What kind of proposal did you have in mind though?
 
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rootone said:
What kind of proposal did you have in mind though?
danger.jpg
 
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OCR said:
Thanks for responding, Dave...
Tongue only half-in-cheek. Rootone is in danger of giving carte blanche to the posting of a personal theory of a new member who may not be aware of PF's policy on personal theories.
 
DaveC426913 said:
T... Rootone is in danger of giving carte blanche to the posting of a personal theory of a new member who may not be aware of PF's policy on personal theories.
No!, it may have seemed that way I can see your point.
I was not inviting personal theories, just asking what Infinitelycurio meant by proposals being rejected,
and trying to convey that it shouldn't be surprising if untestable ideas are not given much credibility as science.
 
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PeroK said:
Instead of considering the universe, let's just consider the question of shapes and geometry.

In two dimensions: a disc (the area within a circle) has a centre that is part of the disc, but the centre of a circle is not part of the circle. If you were compelled to live on a circle, there would be no centre that you could visit.

In three dimensions, you have the same situation with a solid sphere (the centre is part of the sphere) and a spherical surface (its centre is not part of itself).

In higher dimensions, you have similarly the concept of a hypersurface.

To begin with, you could expand your knowledge of shapes and geometry. Then use this knowledge to understand what people are saying about the universe.

Are you saying the universe is the surface of a 4-D sphere?
 
AgentSmith said:
Are you saying the universe is the surface of a 4-D sphere?

No. Why would you think that?
 
PeroK said:
No. Why would you think that?
It's a logical deduction when projecting 2D geometry to 3D geometry.

AgentSmith said:
Are you saying the universe is the surface of a 4-D sphere?
It turns out that the geometry of curvature works perfectly well without having to invoke a 4th dimension. This has been demonstrated mathematically.
 
Jim Hasty said:
Update on my recent reply. I see what you mean now by 'preferred frame' in the context of the cosmological principle. There are several googled articles [2011-2012] which mention 'preferred frames' in the context of 'asymmetrical expansion'. So yes, I agree with you in that context.
You are not being clear here as to whether or not you still argue that the universe has a center. Are you now convinced that it does not?
 
OCR said:
Thanks for responding, Dave...
That was a "thanks for responding" to my post... post # 90.

I wasn't making a response about our good friend, Rootone... honest ! ... :angel:
Dave said:
Rootone is in danger...
https://www.physicsforums.com/threa...er-of-the-universe.845032/page-5#post-5308128I know, I know... I should shut up now, but ... lol

Because, you see, I still don't get... if
..."time" has changed (passed), does your starting point actually still exist?

I will shut up now... interesting thread, though...

Carry on... :oldwink:
 
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The OP has been answered several times over: No, expansion does not require a center of the universe and no this is not a matter of opinion.

Thread closed.