# Center of Universe?

Discussion in 'Astronomy, Exobiology, & Cosmology' started by eram, Sep 7, 2012.

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1. ### eramSciengineerValued Senior Member

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Mechanical Universe 15 - Conservation of Momentum - YouTube

Watch the above video, I skipped to the crucial part.

Provided no external forces act on a system, the system's center of mass is always in an inertial frame.

If we consider the entire universe as a system, and assume that no external forces can act on the universe, then the universe has a center of mass. In other words, a center!

We may even measure absolute values of relative velocities and positions from the center of mass.

I am either a genius, or have become completely retarded.

Last edited: Sep 8, 2012

3. ### AlphaNumericFully ionizedRegistered Senior Member

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-> Cosmology, Astronomy and something or other.

5. ### originIn a democracy you deserve the leaders you elect.Valued Senior Member

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I think the problem here is trying to consider the universe as a system in the calssic sense.

7. ### GorlitzIron ManRegistered Senior Member

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Would you be able to calculate the centre of the universe from the fact that the furthest points in the universe would appear to be receeding fastest away from that spot as the distances to the furthest points must always be the greatest from the centre, if indeed the 'wrap around' effect is correct?

8. ### eramSciengineerValued Senior Member

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Sorry, but i really can't understand what you're saying.

And what is a universe in a non-classical sense. GR sense?

9. ### GorlitzIron ManRegistered Senior Member

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Well General Relativity tells thats us that the universe shouldn't remain at it's current size, that would suggest it has to be either expanding or contracting, presently we believe it to be expanding.

I would say that if the 'wrap around' effect of the universe turns out to be proved, meaning that if you travel far enough in one direction you'll eventually end up at the beginning of the opposite direction, then the furthest distance you can ever travel before encountering the 'wrap around' effect must also always be the furthest point from the centre.

Now given that we believe the universe to be expanding and expanding from everywhere, that means the more space between points the more expansion can take place thus the faster those points seems to be receeding away. That means the centre of universe must always be the point from which has the greatest potential for expansion to place and the point at which the farest points in the universe seem to be receeding away the fastest.

Last edited: Sep 7, 2012

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11. ### GorlitzIron ManRegistered Senior Member

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Well I think if consider the idea of no external influences being exerted on the universe then we have to consider the the idea that everything is eventually moving to maximum entropy whilst ever expansion is still taking place. The reason of this being that there is only a finite amount of energy being streched over ever greater distances. Until finally we get to a point where the distances become so vast as to virtually nullify any motion whatso ever. If the size of the universe remained the same then so should the motion but this is not the case.

12. ### originIn a democracy you deserve the leaders you elect.Valued Senior Member

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Based on this analysis we are at the center of the universe!

The other option (and a fine option it is) is that ALL points (excluding the effects of local gravity) are moving away from each other - due to and overall expansion of the universe.

13. ### originIn a democracy you deserve the leaders you elect.Valued Senior Member

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I do not want to watch the whole video, why don't you tell us the 'crucial part'.

I can agree that is true for systems in our universe.

A system is something that exists in the universe. The universe is necessary to be able to define the system. The universe cannot be a system unless it is a part of a larger whole. The universe however is the universe it is the whole enchilada.

Of course there are no external forces acting on the universe there is nothing outside of the universe.

No, for there to be a center of mass the system has to be compared relative to something. For a system (like the Sol system) it is the rest of the universe.

There can be no line drawn because there is no way to detect a center of mass.

Nah you lie somewhere in the continuum between these 2 extremes.

14. ### GorlitzIron ManRegistered Senior Member

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I don't see the two as mutually exclusive, ALL points can be, and I believe actually are, expanding away from each other. However that does not preclude the possibility of a point which exists within the centre of the universe. Imagine drawing a line from the furthest expanded part of the universe to the opposite furthest expanded part. The point along that line which is exactly equidistant from both furthest points would be the centre and you can do this for all three spatial dimensions. Then the point where all 3 intersect should be your universal centre.

You can do a little 2D experiment to see how this works in practice. Simply take a piece of paper and draw a squared grid using 1cm squares, now mark a spot in the exact centre of your grid. Now redraw your grid but this time use 2cm squares. Your centre spot stays in exactly the same place but all the space around it has expanded.

15. ### originIn a democracy you deserve the leaders you elect.Valued Senior Member

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Here is the point. We see that every distant galaxy is receding from us. The farther the galaxy the faster it is receding. Lets assume galaxy XYZ is 12 billion ly from us and receding like a bat out of hell from us. It looks like we are at the 'center' of the universe and galaxy XYZ is on the 'edge' of the universe. An alien on a planet in galaxy XYZ sees exactly the same effect as we see. The farther the galaxy is from galaxy XYZ the faster it is receding. He would say it looks like he at the 'center' of the universe and the milkyway galaxy is on the 'edge' of the universe.

There is no edge. There is no center. There is absolutely no way to tell where any sort of center could possibley be!

16. ### hansdaValued Senior Member

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Provided further, there is no inertial frame-dragging.

17. ### GorlitzIron ManRegistered Senior Member

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Surely though there must be a point, call it a centre if you will, at which you are equidistant from either point in either direction in which anything beyond would have you travelling back towards your start point from the opposite direction. I mean take for example the inside of a sphere you could say that has no centre because you could just keep going round ad infinitum, but if you were to divide it into hemi-spheres, one division for each of the 3 spatial dimensions and say the points where they intersect is exactly equidistant then you could say it has a centre an plot it exactly.

18. ### originIn a democracy you deserve the leaders you elect.Valued Senior Member

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It is the same problem as trying to determine absolute motion. There may be some rock somewhere that God can look at from outside the universe and he can say, "Yep, that rock is not moving and everything else in the universe is moving relative to that stationary rock". But you can't ever under any circumstace determine that. The center of the universe is essentially the same situation - God probably knows but he's not telling and you cannot ever detect it.

The only caveat to this is, when my daughter was about 13 she was pretty sure she was the center of the universe.

19. ### GrumpyCurmudgeon of LucidityValued Senior Member

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To find the "center" of the Universe, take your finger and touch your nose. At the beginning that point you are touching was at the exact center. That is also true of every other point. We see all non-gravity-bound galaxies receding from us, but even the very distant galaxy dwellers see exactly the same thing. Of course, we do not see the Universe as it is today, we see the Universe as it was then, the further we look the further back in time what we are seeing existed. "Finite but unbounded" describes our Universe, it is of a finite size, but it does not have an edge. Our mundane understanding cannot easily conceptualize this, but it's true. And that means there is no place that could be called a center, no center of gravity and no "furthest" point, each point in the Universe "sees" itself in the center but none of them are. There are four dimensions, not just the three we can grock, time is the additional factor.

Grumpy

20. ### GorlitzIron ManRegistered Senior Member

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aaarr sweet.

If we made a model of the universe do think that also wouldn't have a centre?

21. ### RJBeeryNatural PhilosopherValued Senior Member

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Eram, if the universe is finite then it would have a center of mass although some might dispute this point. See here

22. ### wlminexBannedBanned

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The universe is the all-inclusive system (be it finite or infinite). The mass portion is a subsystem. The energy portion is a subsystem. Both of these latter (subsytems) interact (at least down to the planck scale) within the larger system (universe) according to testable physical "laws". Since we have problems detecting subplanck scale stuff, other, or different "laws" may apply.

23. ### kaduseusmelencolia IRegistered Senior Member

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Why not both at the same time?
Both expanding and contracting at the same time?
Before you quoting redshift, it is an assumption based on observation open to interpretation.

Also if the universe has a center of mass, every point that constitutes part of the universe is the center of mass of the universe, assuming a finite mass.....

Problem with the expansion model is the edges, if we take a galaxy on the edge of the known universe, then to it, we are at the edge of the universe, except we can 'see' for 13.2 billion light years all the way around, so in relation to the galaxy we on the edge, we 'see' an extra 13.7 billion years into the past, we do that for another galaxy on the opposite edge of the universe, in relation to that galaxy we 'see' an addition 13.7 billion years into the past. Either light can only travel for 13.7 billion years, or we screwed up.
According to one cosmologist the universe is flat.