# Thread: Center of Universe?

1. 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!

And there can even be an imaginary straight line that represents the center of mass's inertial frame. We may even measure absolute position from this straight line.

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

2.

3. 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!
Are you familiar with the Balloon analogy? Have a look: Where is the centre of the universe?

4. but what about the points made in the video? Unless there is a completely isotropic distribution of an infinite amt. of mass, the universe should have a center of mass.

5. The difference is that with the system in the video, there is something "outside" of the system. This gives the system a boundary by which the center of mass can be measured with reference to. The universe has nothing external to it and no such boundaries.

6. Originally Posted by Janus
The difference is that with the system in the video, there is something "outside" of the system. This gives the system a boundary by which the center of mass can be measured with reference to. The universe has nothing external to it and no such boundaries.
The center of mass, or barycenter, is the weighted average location of all mass in a body or system of bodies. As far as I know, no knowledge of anything outside of the system (boundary) is necessary in order to determine it's location (relative to the bodies within the system).

7. If there's no boundary, there's no 'in', so there's no center.

8. Yes, if the universe is infinite (without boundary) then the center is unknowable. Naggy Doggy has already mentioned that exception.
Originally Posted by Naggy Doggy
Unless there is a completely isotropic distribution of an infinite amt. of mass, the universe should have a center of mass.

9. so the question is, is the amt. of mass in the universe finite?

10. If the universe is finite, it can still be of a geometry that does not admit of anything but an arbitrarily chosen center.

11. The center of the universe is a picture we can see billions of years ago, but does not function as a center of mass any longer. The reason for this is because the universe stretched out from the center in every direction, settled, exploded, and then stretched out again, settled, exploded, and again, many times, like tree branches. There is no longer a center, but there is a point of origin.

12. Originally Posted by Bud
The center of the universe is a picture we can see billions of years ago, but does not function as a center of mass any longer. The reason for this is because the universe stretched out from the center in every direction, settled, exploded, and then stretched out again, settled, exploded, and again, many times, like tree branches. There is no longer a center, but there is a point of origin.
There is neither a centre nor a point of origin in our 4-dimensional universe, unless it is embedded in some higher dimensional manifold, in which case it would depend on the global topology whether such a point can be defined or not.
This issue has been exhaustively discussed countless times before.

13. The point of origin is the same distance away from here in all directions.

When we look out into the universe we are looking back in time, in whichever direction we look. In all directions, we are looking back towards the Big-Bang.

When we look at all the distant galaxies, they are all receding directly away from here. If we run time backwards, everything gets closer and closer to here. The Big-Bang happened here.

And the same would be true, wherever you are in the universe.

14. Originally Posted by SpeedFreek
And the same would be true, wherever you are in the universe.
Precisely. That is why there is not one, unique point of origin.

15. Well I suppose that stops the headache of the recurring question whats beyond that, seems we are all trapped in a giant universal hamster ball.

16. There a trillions and trillions of origination points. But there was a unique one indeed, the first of which, and had about 10 stars formed from this big bang. Not so big when you think about it now eh? After these 10 stars were formed, they die and explode, now there are 10 different origination points for hundreds of others, so on and so forth.

17. Originally Posted by Bud
There a trillions and trillions of origination points. But there was a unique one indeed, the first of which, and had about 10 stars formed from this big bang. Not so big when you think about it now eh? After these 10 stars were formed, they die and explode, now there are 10 different origination points for hundreds of others, so on and so forth.
What nonsense. There never was any unique point of origin, and there were no stars until about 150 million years after the Big Bang ( the "Reionization Period" ).

18. Those trillions and trillions of points all end up as the same point in the beginning. No point can claim theirs is unique any more than any other.

19. Originally Posted by Markus Hanke
Originally Posted by Bud
There a trillions and trillions of origination points. But there was a unique one indeed, the first of which, and had about 10 stars formed from this big bang. Not so big when you think about it now eh? After these 10 stars were formed, they die and explode, now there are 10 different origination points for hundreds of others, so on and so forth.
What nonsense. There never was any unique point of origin, and there were no stars until about 150 million years after the Big Bang ( the "Reionization Period" ).
That does not disprove me.

20. Originally Posted by Bud
That does not disprove me.
There is nothing to be disproven. Your assertion about one unique point of origin with ten stars in it is simply nonsense.

21. Your perception of what I'm saying is your own, and you are contorting it in your brain. You are smart enough to know and discuss when the first stars were formed, and then you turn around and say they werent formed? where did these 10 stars come from? Try taking the two puzzle pieces, and next time, put them together.

22. Originally Posted by Bud
You are smart enough to know and discuss when the first stars were formed, and then you turn around and say they werent formed?
Where did I say stars weren't formed ? What I said was that stars did not start to form until the Reionization Period, at which time the universe was already of substantial size, and no unique point of origin existed. Refer to post 16. At the time immediately after the Big Bang itself there weren't any stars.

After these 10 stars were formed, they die and explode
Just a note here - stars don't always explode at the end of their lifecycle.

23. There was a small bang before the "big" bang, the first of which forming the first dozen or however many super blue giants. The BIG bang was the 10 stars, exploding in a chain reaction due to proximity, a supernatrually large explosion.

24. Originally Posted by Bud
There was a small bang before the "big" bang, the first of wich forming the first dozen or however many super blue giants. The BIG bang was the 10 stars, exploding in a chain reaction due to proximity, a supernatrually large explosion.
This is getting more and more ridiculous - there is no such thing a "small bang before the big bang". Where are you getting this nonsense from ?
You are just making stuff up as you go along and as it suits you.

25. Originally Posted by Markus Hanke
Originally Posted by Bud
There was a small bang before the "big" bang, the first of wich forming the first dozen or however many super blue giants. The BIG bang was the 10 stars, exploding in a chain reaction due to proximity, a supernatrually large explosion.
This is getting more and more ridiculous - there is no such thing a "small bang before the big bang". Where are you getting this nonsense from ?
You are just making stuff up as you go along and as it suits you.
pretty much

26. Originally Posted by Bud
Originally Posted by Markus Hanke

This is getting more and more ridiculous - there is no such thing a "small bang before the big bang". Where are you getting this nonsense from ?
You are just making stuff up as you go along and as it suits you.
pretty much
Well, thanks for the admission. This is technically called trolling.

27. what about the points made in the video?If this universe has a finite amount of mass, it has to have a center of mass.

28. Originally Posted by Naggy Doggy
what about the points made in the video?If this universe has a finite amount of mass, it has to have a center of mass.
No, this is only true for a system with boundary. If a boundary does not exist then there is no centre of mass. Besides, the universe is 4-dimensional.

29. Originally Posted by Naggy Doggy
what about the points made in the video?If this universe has a finite amount of mass, it has to have a center of mass.
You would benefit from learning a bit about the topic. I recommend Ned Wright's cosmology faq. Here's a link to the specific entry about the center question:

Cosmology FAQ: Where is center of the Big Bang?

30. Originally Posted by Markus Hanke
Originally Posted by Naggy Doggy
what about the points made in the video?If this universe has a finite amount of mass, it has to have a center of mass.
No, this is only true for a system with boundary. If a boundary does not exist then there is no centre of mass. Besides, the universe is 4-dimensional.
What do you mean by a boundary?

In the video, there is no boundary surrounding the billiard ball.

Yet, we can clearly calculate the position of the centre of mass. As the long as the universe has a finite amount of mass, just like the billiard ball.

31. Originally Posted by tk421
Originally Posted by Naggy Doggy
what about the points made in the video?If this universe has a finite amount of mass, it has to have a center of mass.
You would benefit from learning a bit about the topic. I recommend Ned Wright's cosmology faq. Here's a link to the specific entry about the center question:

Cosmology FAQ: Where is center of the Big Bang?

but i'm not referring to the Big Bang, I'm referring to the center of mass.

32. Originally Posted by tk421
Originally Posted by Naggy Doggy
what about the points made in the video?If this universe has a finite amount of mass, it has to have a center of mass.
You would benefit from learning a bit about the topic. I recommend Ned Wright's cosmology faq. Here's a link to the specific entry about the center question: Cosmology FAQ: Where is center of the Big Bang?
Your link does not answer the question. If the universe is finite, then the barycenter can be determined regardless of whether I am located at point A or point B. However, the nature of the universe (such as that described by the theory of relativity) may prevent us from determining the barycenter. According to Wikipedia, the question of whether the universe is infinite or finite is undetermined, because of ... you guessed it ... the nature of the universe.
According to general relativity, space can expand faster than the speed of light, although we can view only a small portion of the universe due to the limitation imposed by light speed. Since we cannot observe space beyond the limitations of light (or any electromagnetic radiation), it is uncertain whether the size of the universe is finite or infinite.
Universe - Wikipedia, the free encyclopedia

33. Originally Posted by Naggy Doggy
Originally Posted by tk421
Originally Posted by Naggy Doggy
what about the points made in the video?If this universe has a finite amount of mass, it has to have a center of mass.
You would benefit from learning a bit about the topic. I recommend Ned Wright's cosmology faq. Here's a link to the specific entry about the center question:

Cosmology FAQ: Where is center of the Big Bang?

but i'm not referring to the Big Bang, I'm referring to the center of mass.
I guess you don't see the connection. Let's work up to it in steps: If the universe has no center, then please provide your formula for the center of mass.

Possibly helpful hint: Consider the popular balloon analogy in visualizing your answer.

ETA: Note that Kalster made this suggestion at the top of this thread. Guess you're good at ignoring suggestions.

34. Originally Posted by tk421
Originally Posted by Naggy Doggy
Originally Posted by tk421
Originally Posted by Naggy Doggy
what about the points made in the video?If this universe has a finite amount of mass, it has to have a center of mass.
You would benefit from learning a bit about the topic. I recommend Ned Wright's cosmology faq. Here's a link to the specific entry about the center question:

Cosmology FAQ: Where is center of the Big Bang?

but i'm not referring to the Big Bang, I'm referring to the center of mass.
I guess you don't see the connection. Let's work up to it in steps: If the universe has no center, then please provide your formula for the center of mass.

Possibly helpful hint: Consider the popular balloon analogy in visualizing your answer.

Let's consider this: Does the universe have a finite amount of mass?

So I assume, YES it does.

Now I just measure the mass of every object in the universe, measure the distances between the objects, and use the well-known formula to determine the position of the center of mass.

I could even do the same if the universe had a non-isotropic distribution of an infinite amount of mass.

35. Unless you want to bring in GR.

36. Originally Posted by Osby
Your link does not answer the question. If the universe is finite, then the barycenter can be determined regardless of whether I am located at point A or point B.
No, that is an unfortunately common misconception. Much depends on the topology assumed. You have apparently restricted yourself to a particular choice, such a flat 2D universe. But we don't live in one (GR and all that, you know). If you understand the balloon analogy (and I stress that it's an analogy), you can have a finite universe with no defined barycenter. That's sufficient to falsify the OP's original assertion.

To go further, I asked the OP to offer his/her proposal for a mathematical formula yielding the location of the barycenter. I know of none that is sensible without discarding GR. In the absence of evidence to the contrary, I am assuming GR to be correct.

37. Originally Posted by Naggy Doggy
Let's consider this: Does the universe have a finite amount of mass?

So I assume, YES it does.

Now I just measure the mass of every object in the universe, measure the distances between the objects, and use the well-known formula to determine the position of the center of mass.
Yup, that's what I figured. You've made the mistake that one would expect of someone whose math education does not extend much beyond high school geometry. To see your error (which is the same one that Osby made), apply your approach to the balloon analogy (I guess you didn't take Kalster's and my hint; too bad). If the entire universe is the surface of the balloon (thus one does not consider anything "above" or "inside" the universe) and consists of a finite amount (as you have assumed) of homogeneously distributed matter (we can revisit this assumption later), calculate where the center is.

I could even do the same if the universe had a non-isotropic distribution of an infinite amount of mass.
I will have to ask you for a specific equation, not a verbal description, so that I can show you specifically how and where your math fails.

38. Originally Posted by Naggy Doggy
Now I just measure the mass of every object in the universe, measure the distances between the objects, and use the well-known formula to determine the position of the center of mass.
And what formula, exactly, would you use for this, considering that the universe as a whole has a non-Euclidean geometry, and an unknown topological structure ? I am not aware of any such formula, and if it existed it would be very complicated indeed. But maybe you can write it down here for us.

39. Originally Posted by Markus Hanke
Originally Posted by Naggy Doggy
Now I just measure the mass of every object in the universe, measure the distances between the objects, and use the well-known formula to determine the position of the center of mass.
And what formula, exactly, would you use for this, considering that the universe as a whole has a non-Euclidean geometry, and an unknown topological structure ? I am not aware of any such formula, and if it existed it would be very complicated indeed. But maybe you can write it down here for us.
I agree, because I'm so used to visualising Euclidean space, and I can't visualize higher order geodesics.

We can assume the universe is non-euclidean because of GR, right?

Do you think there is a finite amount of mass in the universe? If there is, and the universe is euclidean, this formula http://hyperphysics.phy-astr.gsu.edu/hbase/cm.html should suffice?

40. Originally Posted by tk421
Originally Posted by Osby
Your link does not answer the question. If the universe is finite, then the barycenter can be determined regardless of whether I am located at point A or point B.
No, that is an unfortunately common misconception. Much depends on the topology assumed. You have apparently restricted yourself to a particular choice, such a flat 2D universe. But we don't live in one (GR and all that, you know). If you understand the balloon analogy (and I stress that it's an analogy), you can have a finite universe with no defined barycenter. That's sufficient to falsify the OP's original assertion.

To go further, I asked the OP to offer his/her proposal for a mathematical formula yielding the location of the barycenter. I know of none that is sensible without discarding GR. In the absence of evidence to the contrary, I am assuming GR to be correct.
Where can I find resources on GR for laymen, but with a fair bit of math?

41. Originally Posted by Naggy Doggy
Where can I find resources on GR for laymen, but with a fair bit of math?
Our very own Markus Hanke has an excellent thread on the Einstein Field Equations. I strongly recommend reading it.

42. Originally Posted by Naggy Doggy
We can assume the universe is non-euclidean because of GR, right?
Yes, and because of the fact that we can actually observe the metric expansion of space through the red-shift of far away objects. It is definitely non-Euclidean.

Do you think there is a finite amount of mass in the universe?
This question is really much more complicated than it initially appears. Do you mean just mass as in the classical sense of the word, or do you mean all sources of gravity, which means mass plus all other forms of energy. Remember that all forms of energy are sources of gravity, including EM fields, stress, momentum, vacuum energy etc etc. I don't know the answer to your question - my personal opinion (!) would be that there is a finite amount of gravitational energy in a finite universe, however, in an infinite universe it could go either way, depending on how such a universe started and what the quantum structure of space-time really is.

Where can I find resources on GR for laymen, but with a fair bit of math?
Just google it, there are loads of free PDFs which can teach the basics. You need to be fairly proficient in tensor calculus and differential geometry though, so you may need to start with that first.

Our very own Markus Hanke has an excellent thread on the Einstein Field Equations. I strongly recommend reading it.
The solution presented there is only a local solution though, the Schwarzschild Metric. Perhaps one rainy afternoon when I have nothing better to do I will add a cosmological solution, the Friedmann-Lemaitre-Robertson-Walker metric, to that thread. The FLRW metric is rather more complicated, so typing the LaTeX will be pretty time consuming.

43. Originally Posted by Naggy Doggy
Where can I find resources on GR for laymen, but with a fair bit of math?
Try this for starters, it is a very straightforward and basic introduction to the mathematical principles underlying GR :

http://preposterousuniverse.com/grnotes/grtinypdf.pdf

44. Originally Posted by Naggy Doggy
Originally Posted by Markus Hanke
Originally Posted by Naggy Doggy
what about the points made in the video?If this universe has a finite amount of mass, it has to have a center of mass.
No, this is only true for a system with boundary. If a boundary does not exist then there is no centre of mass. Besides, the universe is 4-dimensional.
What do you mean by a boundary?

In the video, there is no boundary surrounding the billiard ball.

The boundary is the surface of the ball. It separates "is the Ball" from what "isn't the Ball".

Yet, we can clearly calculate the position of the centre of mass. As the long as the universe has a finite amount of mass, just like the billiard ball.
Wrong, because, unlike the ball, there is no boundary that separates the universe from "not the universe". Even if the universe is finite. The Ball is finite and bounded, the universe can be finite and unbounded.

Not to be redundant, but again, consider the surface of the balloon in the balloon analogy. The surface is finite in size, but has no edge or boundary. Now before you say "But why isn't the surface itself the boundary just like the surface of the Ball is?" You have to understand something. In this analogy the surface of the balloon is the entire universe, and as pointed out by tk421, there is no inside or outside of the balloon.

You see, even the balloon itself is a poor analogy, unfortunately its the best we can do. It is a concession to the fact that we can't directly visualize Non-euclidean geometries. Instead, we use a Euclidean geometry analogy to try and get the idea across. The problem is that are brains just aren't wired to deal with them intuitively. That's because on the scales that we deal with on an every day basis, things behave almost exactly like they would under Euclidean geometry.

To explain what I mean, we have to go back the balloon analogy. Imagine that we draw a circle on the surface of the balloon. We now draw a line cutting it in half to represent the diameter. If we measure the length of this line and compare it to the circumference of the circle, we find the because of the "bump" inside the circle caused by the fact that we are drawing this on a surface of a sphere, the ratio will be smaller than pi. The larger the circle we draw, the more pronounced the difference. On the other hand, if we draw smaller and smaller surfaces, the height of the Bump get less and less compared to the circumference of the circle, and the ratio gets closer and closer to pi. Eventually we reach a point where the difference is so small that for all practical purposes it does not exist. And that is the range where we live out our lives and evolved. This became ingrained into the way we think, and the rules of Euclidean Geometry seem "natural" to us. So much so, that even when we try to discuss Non-Euclidean geometry, we are reduced to using Euclidean Geometry examples.

For example, sometimes you'll here the Hyper-sphere example, where the universe is the 3-D "surface" of the 4-D hypersphere. (Its like the balloon Analogy but with the Third spatial Dimension added back to the Universe. To do this, there has to be a fourth spatial dimension added. The thing is, Non-Euclidean geometry does not require a 4th spatial direction, only that things somewhat behave like there is. For example, in the hyper-sphere universe if you head in any direction in a straight line you end up back where you started. With Non-Euclidean geometries you can do this without "curving" through a 4th spatial dimension, its just how the rules of the geometry work.

45. Originally Posted by tk421
Originally Posted by Osby
Your link does not answer the question. If the universe is finite, then the barycenter can be determined regardless of whether I am located at point A or point B.
No, that is an unfortunately common misconception. Much depends on the topology assumed. You have apparently restricted yourself to a particular choice, such a flat 2D universe. But we don't live in one (GR and all that, you know). If you understand the balloon analogy (and I stress that it's an analogy), you can have a finite universe with no defined barycenter. That's sufficient to falsify the OP's original assertion. To go further, I asked the OP to offer his/her proposal for a mathematical formula yielding the location of the barycenter. I know of none that is sensible without discarding GR. In the absence of evidence to the contrary, I am assuming GR to be correct.
No, I am not restricting myself to a 2D universe. It was your link that I was referring to. You quoted only one sentence of my post.

46. No, I am not restricting myself to a 2D universe. It was your link that I was referring to. You quoted only one sentence of my post.
Then you need to make sure that you understand the analogy correctly - the universe is the surface of the ball/balloon, not its inside. The volume has a boundary, the surface does not, even though it is finite. Likewise, the sphere has a centre point, but not the manifold which constitutes its surface. It works the same way in 4D, only then you can no longer visualize it.

47. Originally Posted by Markus Hanke
No, I am not restricting myself to a 2D universe. It was your link that I was referring to. You quoted only one sentence of my post.
Then you need to make sure that you understand the analogy correctly - the universe is the surface of the ball/balloon, not its inside. The volume has a boundary, the surface does not, even though it is finite. Likewise, the sphere has a centre point, but not the manifold which constitutes its surface. It works the same way in 4D, only then you can no longer visualize it.
So the balloon analogy attempts to describe a 4D universe with the 2D surface of the balloon? So does that mean if we could travel to the end of the universe we would find out that we were already there to begin with?

48. Originally Posted by Osby
So the balloon analogy attempts to describe a 4D universe with the 2D surface of the balloon? So does that mean if we could travel to the end of the universe we would find out that we were already there to begin with?
Because the balloon analogy is an analogy (which is why I used italics in several instances), and not a rigorous model of the universe, one must be careful not to ask questions that may lie outside the analogy's range of validity. The question of the geometry of our universe is as yet not settled. But if it turns out that our universe is like the balloon, then yes, one implication would be that you could travel in "one" direction and eventually end up where you started.

49. Originally Posted by Bud
Originally Posted by Markus Hanke
Originally Posted by Bud
There a trillions and trillions of origination points. But there was a unique one indeed, the first of which, and had about 10 stars formed from this big bang. Not so big when you think about it now eh? After these 10 stars were formed, they die and explode, now there are 10 different origination points for hundreds of others, so on and so forth.
What nonsense. There never was any unique point of origin, and there were no stars until about 150 million years after the Big Bang ( the "Reionization Period" ).
That does not disprove me.
The battle cry of the crank "PROVE ME WRONG"

50. Originally Posted by Markus Hanke
Originally Posted by Naggy Doggy
Where can I find resources on GR for laymen, but with a fair bit of math?
Try this for starters, it is a very straightforward and basic introduction to the mathematical principles underlying GR :

http://preposterousuniverse.com/grnotes/grtinypdf.pdf

Cool site.

I wonder if there are any videos which quantitatively demonstrate GR using CG.

I see a lot of videos for laymen though.

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