Center of Mass of Universe: Is it Nowhere, Everywhere?

[Quinzio]

Center of mass of ...

I just don't know where to post this...feel free to move it.

Assuming our universe is the 3-dimensional surface of a 4-dimensional sphere, my question is: where is the center of mass of the universe ?
Nowhere, everywhere, is it in the center of the sphere ? There's no center of mass ?
I cannot think anything.

(By the way, how much is the radius of the sphere if there's any estimation of that ?)

 

[Bill_K]

Quinzio, our universe is not the 3-dimensional surface of a 4-dimensional sphere. As best we can tell, it is flat.

 

[khemist]

The center is any point you chose.

 

[AstrophysicsX]

There is no center or edge of our universe.

 

[mrspeedybob]

If you model the universe as you say then it makes the most sense to use time as the 4'th dimension. The past is toward the center of the sphere and the future is outside the sphere, hence the universe expands with time. When pictured in this way it becomes obvious that the center of mass of the universe would be at T=0, the center of the sphere, the instant of the big bang.

 

[khemist]

If you model the universe as you say then it makes the most sense to use time as the 4'th dimension. The past is toward the center of the sphere and the future is outside the sphere, hence the universe expands with time. When pictured in this way it becomes obvious that the center of mass of the universe would be at T=0, the center of the sphere, the instant of the big bang.

But that center is the entire universe.

Basically, everything is expanding from everything else. If I stand on earth, it appears that everything is moving away. However, if I stand in a star system 100 light years away, everything will seem to be moving away from that point. Therefore, there is no center, yet everything is the center.

 

[mrspeedybob]

But that center is the entire universe.

Basically, everything is expanding from everything else. If I stand on earth, it appears that everything is moving away. However, if I stand in a star system 100 light years away, everything will seem to be moving away from that point. Therefore, there is no center, yet everything is the center.

You are correct in saying that no point on the surface of the sphere can be identified as the center, but that does not mean a sphere has no center.

 

[JaredJames]

You are correct in saying that no point on the surface of the sphere can be identified as the center, but that does not mean a sphere has no center.

But if every point can be seen as the centre, you don't have an edge and so nothing to give you an actual centre point?

 

[mrspeedybob]

But if every point can be seen as the centre, you don't have an edge and so nothing to give you an actual centre point?

The Earth is a 3 dimensional sphere. You cannot give me the co-ordinates of the center of the Earth in terms of latitude and longitude because those only define points on it's surface. You can tell me the center is approximately 6400 km below any given point on the surface.

I am saying the situation in the universe is similar. It makes no sense to try to define the center with only the 3 dimensions of space, that would be like trying to define the center of the Earth with latitude and longitude. Adding time as the 4'th co-ordinate to the universe is like adding altitude/depth to the 2 dimension of latitude and longitude. The center of the universe is 13.7 billion years ago no matter where in the universe you are, just like the center of the Earth is 6400 km down no matter where on the surface of the Earth you are.

 

[Quinzio]

If you model the universe as you say then it makes the most sense to use time as the 4'th dimension. The past is toward the center of the sphere and the future is outside the sphere, hence the universe expands with time. When pictured in this way it becomes obvious that the center of mass of the universe would be at T=0, the center of the sphere, the instant of the big bang.

This is interesting.
In such a way, the center of mass becomes a point in space-time and not only in space.
But how to deal with that object ?

 

[Nabeshin]

If you model the universe as you say then it makes the most sense to use time as the 4'th dimension. The past is toward the center of the sphere and the future is outside the sphere, hence the universe expands with time. When pictured in this way it becomes obvious that the center of mass of the universe would be at T=0, the center of the sphere, the instant of the big bang.

This is perhaps a curious way of thinking about it but is not quite right. For example, what about when the universe eventually begins to recollapse? How do you account for the fact that time is a monotonic dimension (we move only forward)? At any rate, although I will admit it is an interesting way to look at it, I would not recommend using it much. Better to think of each individual 3-sphere as a slice of a 4-dimensional object, the slicing parameter being time.

As you say, given your coordinates on the surface of a 3-sphere we cannot hope to determine the "coordinates" of the center of that 3-sphere. What we can do is measure the radius of curvature of the 3-sphere however, and this is precisely what is done. As other posters have pointed out, the universe is quite close to being flat, or, if the universe is a 3-sphere, its radius of curvature is very, very large compared with the scales on which we make observations.

 

[mrspeedybob]

This is perhaps a curious way of thinking about it but is not quite right. For example, what about when the universe eventually begins to recollapse? How do you account for the fact that time is a monotonic dimension (we move only forward)?

The best observations that I know of indicate that the universe will expand forever so re-collapse isn't really an issue for me.

We perceive objects to move through time in only 1 direction but that may just be a perception. If the past of an object and the future of an object are viewed as simply 1 more dimension of its existence then objects do not move through time they exist in it. An object has duration in the same sense that it has length.

 

[shashankac655]

i don't think there can be a point called the center of mass of the universe because by saying 'center of mass' you are trying to specify a point in space,but before big bang there was no space or time or space-time,so you cannot point out at point in space and say that it is from this place where all the matter originated and so it is the center of mass of the universe because it only after the big bang that space ,time or space time and matter,antimatter and whatever.. came to existence.

i don't think anybody knows for sure in what fashion matter is distributed in the universe and how many kinds matter there are. There is something called dark matter and nobody knows what it is except that it is not ordinary matter,there is also dark energy,there is also a mystery why we are not able to find the same quantity of anti-matter as there is matter.

So there are lots of things in the universe about which we don't know of the locations or it's existence itself.

 

[Chronos]

We are at the temporal edge of the universe [the most ancient point observable], yet, we see the past equidistant in every direction.

 

[Cosmo Novice]

We are at the temporal edge of the universe [the most ancient point observable], yet, we see the past equidistant in every direction.

This is a very clear way of saying what I have been trying to put into words for the past few days!

Thanks