# Can we identify the centre of the Universe?

• B
Hi

I have two novice question

If the universe is expanding and it is all moving apart, how is it that galaxies are colliding?

If our galaxy and all others are moving away from some singularity central point. And as we see other galaxies moving away from us at a speed relative to there distance from us.

Shouldn't we be able to find the general direction of that singularity central point of the universe by identifying galaxies that seem to be heading in the same direction as our own galaxy but more slowly instead of fast according to there distance from our home galaxy because they are closer to that point?

jefsART

Chronos
Gold Member
The putative 'center' of the universe is a common misperception. It is widely accepted that the universe has no center, or if you prefer, the center is wherever you happen to be in the universe. No matter where or when you happen to be in the universe, you are always exactly one Hubble radius from the edge of your observable universe in every direction. The simplest conclusion is the universe has no center. Part of the blame for this confusion has to do with popular science descriptions of the big bang, where the universe originated from a tiny primordial singularity. The term singularity, however, does not imply anything tiny nor confine it to any particulaqr location. It merely means our mathematical models fail to yield sensible solutions when pushed to extreme limits - like t=0.

Algr and ProfuselyQuarky
mathman
In addition to expansion (significant only in large scale activity), galaxies have their own proper motion, so they can collide.

The universe has no center.

phinds
Gold Member
2021 Award
I have two novice question

unusually_wrong and ellipsis
Thanks for the replies. I shall look up the link. So this means (the universe has no centre) that if we rewind the video it would not contract to a singularity?

phinds
Gold Member
2021 Award
Thanks for the replies. I shall look up the link. So this means (the universe has no centre) that if we rewind the video it would not contract to a singularity?
No, it WOULD contract to a singularity. What has already been explained to you is that "singularity" does not mean "point", it just means a place where the math model breaks down and we don't know what is going on.

ComplexVar89
In addition to expansion (significant only in large scale activity), galaxies have their own proper motion, so they can collide.

The universe has no centre.
True but they are moving away from each other at tremendous speeds. Moving away and colliding at the same time creates a problem for me. The mode has problems. No?

phinds
Gold Member
2021 Award
True but they are moving away from each other at tremendous speeds. Moving away and colliding at the same time creates a problem for me. The mode has problems. No?
No. There are two different modes involved. Read the link I provided.

No, it WOULD contract to a singularity. What has already been explained to you is that "singularity" does not mean "point", it just means a place where the math model breaks down and we don't know what is going on.
It hasn't been explain its been stated. In the sub atomic things dont make sense, ok. But on the large scale if it dont make sense then you must resolve the contradiction. Dont take offence but your words "we don't know what is going on" seem to apply.

phinds
Gold Member
2021 Award
It hasn't been explain its been stated. In the sub atomic things dont make sense, ok
What do you think the sub atomic scale has to do with the expansion of the universe?

But on the large scale if it dont make sense then you must resolve the contradiction.

Dont take offence but your words "we don't know what is going on" seem to apply.
Only because you haven't studied the stuff yet.

Gravitationally bound systems such as galactic clusters, solar systems, you, atoms, etc do not expand. Galactic clusters and larger objects recede from each other.

ProfuselyQuarky
micromass
Staff Emeritus
Homework Helper
Imagine beings like ants or bacteria walking on a sphere and having only knowledge of the sphere. Then some ant asks where the center of their universe is. You can't really show them since while there is a center, it is not part of their universe. In our case it's even harder since there is nothing outside our universe, so there is no center at all.

Drakkith
Staff Emeritus
Thanks for the replies. I shall look up the link. So this means (the universe has no centre) that if we rewind the video it would not contract to a singularity?

Singularity, in this context, does not mean a single location in space. It means that the density everywhere in the universe goes to infinity as you look further back in time.

phinds
Gold Member
2021 Award
Singularity, in this context, does not mean a single location in space. It means that the density everywhere in the universe goes to infinity as you look further back in time.
Would it not be better to say "... approaches infinity" ? We don't know what the singularity is but saying " ... goes to infinity" seems to imply that it is infinitely dense and we don't know that.

Drakkith
Staff Emeritus
Would it not be better to say "... approaches infinity" ? We don't know what the singularity is but saying " ... goes to infinity" seems to imply that it is infinitely dense and we don't know that.

Not sure.

Chronos
Gold Member
It may help to think in terms of the observable universe, which has always been the only portion of the universe that is observationally accessible to us. The size of the observable universe has, and will always be limited by the age of the universe due to the finite speed of light and basically works out to cT, where T = the age of the universe and c = the speed of light. As is evident the size of the observable universe approaches zero as T approaches zero. Which, of course, wreaks havoc when you try to make any mathematical sense out of the size of the universe at T=0. Cantors work on set theory is still regarded as a good place to start unraveling this mystery.

"Only because you haven't studied the stuff yet."
True, but I know a contradiction when I see one.
"Galactic clusters and larger objects recede from each other."
If they recede from "each" other then they can collide. Study that!
It may help to think in terms of the observable universe, which has always been the only portion of the universe that is observationally accessible to us. The size of the observable universe has, and will always be limited by the age of the universe due to the finite speed of light and basically works out to cT, where T = the age of the universe and c = the speed of light. As is evident the size of the observable universe approaches zero as T approaches zero. Which, of course, wreaks havoc when you try to make any mathematical sense out of the size of the universe at T=0. Cantors work on set theory is still regarded as a good place to start unraveling this mystery.

Hi Gold Member. Thanks for giving an explanation.

I will rephrase my question.

Seeing as although we are limited by the “observable universe, which has always been the only portion of the universe that is observationally accessible to us. “

Yet we can draw all sorts of conclusion such as we are expanding from a singularity (can we still use that term?).

question:
“Shouldn't we be able to find the general direction of that singularity 'central point?' of “observable universe" by identifying galaxies in the “observable universe” that seem to be heading in the same direction as our own galaxy but more slowly instead of fast according to there distance from our home galaxy because they are closer to that point of creation (although not observable to us) from which they are expanding (receding)?

Is there a difference between recede and expand? Is expand out now? If everything is moving away from everything else isn't that an expansion?

We draw a lot of conclusions even though we have this limitation of the “observable universe“.

Isn't it very relevant to our present model that we see more distant galleys 'receding' faster than closer galaxies?

"Galactic clusters and larger objects recede from each other." And bump into each other.

That may be a local thing but still it may be a hint of something more.

Are the more distant galaxies at the edge of “the observable universe” bumping into each other???

It seems to me we have much more to understand. When ever we have some understanding wrapped up nice and neat some trouble maker like Einstein comes along and changes every thing. He ended the billiard ball concept of the universe which was a happy explanation of things at the time. Just asking.

davenn
Gold Member
2021 Award
Yet we can draw all sorts of conclusion such as we are expanding from a singularity (can we still use that term?).

question:
“Shouldn't we be able to find the general direction of that singularity 'central point?' of “observable universe" by identifying galaxies in the “observable universe”

You didn't heed what you were told above, aye

you were told that the singularity DOESNT reference a single point source from which everything exploded out of
Until you come to that understanding/realisation, you will continue to search for a non existent centre of the universe

Reread the posts by Phinds and Drakkith above

Dave

ComplexVar89
mathman
True but they are moving away from each other at tremendous speeds. Moving away and colliding at the same time creates a problem for me. The mode has problems. No?
Distant galaxies are moving away from us at high speeds, due to expansion of the universe. Nearby galaxies may be moving toward us - Andromeda galaxy for example.

Chronos
Gold Member
You have drawn inferences unsupported by facts. Galactic clusters, like individual galaxies, are also known to collide. This is possible because clusters have their own peculiar motion, also just like individual galaxies. Such collisions were more common in the early universe because the average density of the universe was greater than it is at present. For discussion see http://arxiv.org/abs/1502.01339, The merger rate of galaxies in the Illustris Simulation: a comparison with observations and semi-empirical models. As far as the OP question is concerned, welcome to the apparent center of the universe! Given that distant galaxies are receeding at velocities proportionate to their distance from earth. It is a fairly simple matter to deduce earth is located at the center of the observable universe. If that sounds like a highly improbable coincidence, welcome to the non-intuitive science of cosmology.

ProfuselyQuarky
You didn't heed what you were told above, aye

you were told that the singularity DOESNT reference a single point source from which everything exploded out of
Until you come to that understanding/realisation, you will continue to search for a non existent centre of the universe

Reread the posts by Phinds and Drakkith above

Dave
Hi Dave. That must mean. If you rewind the tape.The universe does not contract to a "single" point of infinite mass etc from whence it came "you were told that the singularity DOESNT reference a single point". I can live with that but you cant have it both ways. I'm not searching for the centre of the universe. Are you saying Hubble was wrong and out dated now?

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You have drawn inferences unsupported by facts. Galactic clusters, like individual galaxies, are also known to collide. This is possible because clusters have their own peculiar motion, also just like individual galaxies. Such collisions were more common in the early universe because the average density of the universe was greater than it is at present. For discussion see http://arxiv.org/abs/1502.01339, The merger rate of galaxies in the Illustris Simulation: a comparison with observations and semi-empirical models. As far as the OP question is concerned, welcome to the apparent center of the universe! Given that distant galaxies are receeding at velocities proportionate to their distance from earth. It is a fairly simple matter to deduce earth is located at the center of the observable universe. If that sounds like a highly improbable coincidence, welcome to the non-intuitive science of cosmology.

Hi Chronos. Thanks again for your patience and an honest attempt to answer me. (no capitals of frustration)
Mathman (Gold Member) say the universe is expanding. 'due to expansion of the universe"
So if you play the tape back, the observable universe to us, would contract all the way back to our milky way. But because there is a limit to how far we can see, and we assume there is much much more we cant say what would happen?
Am I getting close?

davenn
Gold Member
2021 Award
Hi Dave. That must mean. If you rewind the tape.The universe does not contract to a "single" point of infinite mass etc from whence it came

that's correct
Have you yet read the link at the bottom phinds' posts on balloon analogy ?
be did suggest early in this thread that you do so

but then you contradict that with this .....
So if you play the tape back, the observable universe to us, would contract all the way back to our milky way.

as that would still infer everything coming back to a fixed known point

I'm not searching for the centre of the universe.

you topic title suggests you are ....
Can we identify the centre of the Universe?

so to use your words ... you cannot have it both ways

Dave

davenn
Gold Member
2021 Award
If the universe is expanding and it is all moving apart, how is it that galaxies are colliding?

If our galaxy and all others are moving away from some singularity central point. And as we see other galaxies moving away from us at a speed relative to there distance from us.

cant remember if these were sufficiently answered for you

Not all galaxies are moving away from each other. There are clusters of, even super clusters of galaxies that are gravitationally bound

our Milky Way galaxy belongs to a cluster called the Local Group
https://en.wikipedia.org/wiki/Local_Group
They are gravitationally bound and indeed some of those galaxies are moving closer to us. Eg M32, the Andromeda Galaxy

But between any cluster of galaxies the distances are expanding

Dave

Chronos
Gold Member
Yes, you are on the right track. The observable universe appears to shrink as you rewind the movie. At some point in the distant past it would appear to be smaller than our own galaxy. Of course the universe is older than our galaxy so nothing paradoxical would occur. In fact neutral hydrogen, the stuff from which the first stars were formed, did not even exist until a few hundred thousand years after the big bang - the universe was simply too hot for atomic nuclei to capture and retain electrons any earlier than that. The observable universe, in theory, was already over 3 times the present size of our galaxy back when hydrogen atoms emerged. See here http://www.haystack.mit.edu/edu/pcr/Astrochemistry/3 - MATTER/nuclear synthesis.pdf, for discussion.

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I don't have any degree in Astronomy, neither in Physics, but since my humble point of view, I had see the questions and the arguments that jefsART had made, and for me are very reasonable. And I still had not read any clear answer to their questions.

I don't have an answer for the question of jefsART, but the only way, in which I can conceive an answer to his questions, accepting what science tell us about a Big Bang, could be based in the following points:

1. The singularity from which our universe started was not a 3D singularity, but an upper dimensions singularity, and it may had bang in 3D, or maybe not, But we only can see it as 3D.

2. Some original forces in the original bang, may had appear in the upper dimensions, that twist our universe, in dimensions that we can't perceive.

3. Our universe is 3D, but it's deformed in higher dimensions. We can't see a center in the 3D Universe, but there is center in a upper dimension, that we can´t perceive.

4. Gravity influence in local space, for make able the collisions. This point, had been presented, as an answer, and I believe is okay.

I may be totally wrong, but please, try to give a clear answer, to what appear as very valid questions.

Chronos
Gold Member
The explanation is readily apparent once you remove the blinders [common intuition]. The universe has no center, or if you prefer, it is centered everywhere an observer happens to make an observation. This is simply a consequence of residing in a universe with a finite age and finite invariant speed of light. All measurements to date strongly support the idea the speed of light is finite and invariant, but, to a lesser extent, that the universe has a finite age.

Fervent Freyja
Drakkith
Staff Emeritus
Hi Dave. That must mean. If you rewind the tape.The universe does not contract to a "single" point of infinite mass etc from whence it came "you were told that the singularity DOESNT reference a single point". I can live with that but you cant have it both ways. I'm not searching for the centre of the universe. Are you saying Hubble was wrong and out dated now?

If you rewind the tape, you will see everything appear to contract towards you here in the Milky way. But this would also happen if you were standing on a planet in the Andromeda Galaxy. Or in a galaxy 10 billion light-years away. The expansion causes galaxies to recede from each other like raisins in a loaf of bread as it rises. If you measure the distance between any two raisins as the bread rises you will find that the distance increases proportionally. If the bread increases its dimensions by 2x then the distance between any 2 raisins doubles. This means that raisins further away from each other initially also recede the greatest amount.

Of course that's only an analogy. The expansion process, the part about how distances behave over time, holds true, but the universe is obviously not a loaf of bread. There is no known edge and no known center. In addition, galaxies are free to drift about within the universe, unlike the raisins, which are held fast within the dough.

Hi Chronos. Thanks again for your patience and an honest attempt to answer me. (no capitals of frustration)
Mathman (Gold Member) say the universe is expanding. 'due to expansion of the universe"
So if you play the tape back, the observable universe to us, would contract all the way back to our milky way. But because there is a limit to how far we can see, and we assume there is much much more we cant say what would happen?
Am I getting close?
@jefsART: I have no idea how far you are in your studies, but if still at the very beginning you might like to have a look at this free online course which starts next Monday, 9 May 2016: https://www.futurelearn.com/courses/gravity/2.

The center is where ever you are at that time.

phinds
Gold Member
2021 Award
The center is where ever you are at that time.
Which is exactly what Chronos said in post #28. My point it that it's a good idea to read a whole thread before responding, so that you don't just give the same answer that someone else has already given.

This discussion also begs the question of whether or not the universe is bounded in the sense of space (as opposed to time). I.e., does it go on forever, in all directions?
-Scott V.

phinds