B Can we identify the centre of the Universe?

[Alexandra Fabiello]

I think a better question would be "If the universe stopped expanding and started shrinking instead, where in space would end up being in the middle of the final clump of matter before it went singularity on us?" Though that would probably still be 'everywhere', more or less, I suppose.

 

[jbriggs444]

I think a better question would be "If the universe stopped expanding and started shrinking instead, where in space would end up being in the middle of the final clump of matter before it went singularity on us?" Though that would probably still be 'everywhere', more or less, I suppose.

Yes. If the universe is finite then having it become smaller does not help localize the center. It's still everywhere. If the universe is infinite then scaling it down does not help localize the center. It's still infinite. In any case, there is no "final time prior to the singularity" any more than there is a smallest real number greater than zero.

 

[Flo Tur]

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.

The solar sistem has a center of mass. Similar, a galaxy, a cluster of galaxies, etc, have a center of mass. What is the limit from where we cannot speak anymore about a center of a mass, in a finite Universe?

 

[russ_watters]

The solar sistem has a center of mass. Similar, a galaxy, a cluster of galaxies, etc, have a center of mass. What is the limit from where we cannot speak anymore about a center of a mass, in a finite Universe?

Even in a finite universe there is no size limit applicable (it sounds like you're saying size large enough it looks infinite). It's a geometry thing: the surface of the Earth has no center. The universe is like a 3D surface, curved in a higher dimension. It's hard to visualize, but just like flying around the world, you may be able to fly across the universe and end up where you started.

 

[Flo Tur]

So, can we say that, at a large enought scale, the vectors involved in calculating the center of mass are no longer liniar and cannot be added as usual anymore?

 

[mfb]

Either that, or the universe is infinite and taking the limit (to cover an infinite volume) is not meaningful.

 

[Flo Tur]

I thought is a scientic fact that the universe is finite. Do you have a mathematical representation of a universe finit in time but infinite in volume?

 

[mfb]

It could be finite or infinite, we don't know.

Do you have a mathematical representation of a universe finit in time but infinite in volume?

The standard flat ΛCDM model is the easiest example.