# Where is the center?

• B
Summary:
Size and center question
Unless the universe is infinite, and I don’t see how if the Big Bang is true…

There must be a finite point beyond which the universe has yet to expand. It seems if measured from side to side we could determine it’s general diameter, allowing for undulations that result in a possibly non-perfect sphere.

And if that sphere can be determined then shouldn’t there be a theoretical center to that sphere? We may never know where it is but shouldn’t it exist?

Tex

PeroK

Bandersnatch
In Big Bang cosmology there can be no empty space into which matter expands, similar to an explosion. The observed expansion is predicted by general relativity for a distribution of matter that never ends - it has to be roughly the same everywhere, and a central lump with an edge beyond which there is no matter is obviously not such a distribution.
The two possible ways for such distribution to exist is if the universe is either spatially infinite, or is closed, similar to how the surface of a sphere is closed (i.e. you can go forever in any direction on the surface of a sphere, and never fall over the edge). In either case there is no edge, and no centre.

PeterDonis
Mentor
2020 Award
Unless the universe is infinite, and I don’t see how if the Big Bang is true
Our best current model of the universe does indeed say the universe is spatially infinite. This is perfectly compatible with an FRW spacetime geometry with an initial singularity. It should be noted, though, that our best current model of our actual universe does not claim that there was an initial singularity; the term "Big Bang" in that model is properly used to refer to the hot, dense, rapidly expanding state that is the earliest state for which we have good evidence. In inflationary cosmology, that "Big Bang" state occurs at the end of inflation.

There must be a finite point beyond which the universe has yet to expand.
No, there mustn't. There is no requirement in geometry that any manifold must be embedded in a higher dimensional manifold. In particular, there is no requirement that our universe, with its curved spacetime geometry, must be embedded in some higher dimensional space into which it is expanding. The physics of our universe can be formulated entirely in terms of its intrinsic geometry, with no embedding at all.

elcaro
Drakkith
Staff Emeritus
Unless the universe is infinite, and I don’t see how if the Big Bang is true…
This is likely because you are under the impression that the universe erupted from a single spatial point and expanded outwards from there. This is not true. Or at least it's not what the big bang theory and modern cosmology postulates.

The key is to look at what we have now and to extrapolate backwards. If we do so, we find that the average density of space increases as the universe 'collapses'. As we go further and further back in time the density and temperature increases everywhere, and at some point in time we find that the density everywhere goes to infinity. Not the density at a single point, the density everywhere. In other words, the universe doesn't collapse to a single point of infinite density. It collapses until the entire universe has infinite density. This is the big bang singularity.

Note that I mentioned nothing about the size of the universe. It could be either finite or infinite and this could happen. The reason we believe it is infinite is that a finite universe has some potentially problematic 'boundary conditions' and we use a healthy dose of "we can't see an edge, our model doesn't require an edge, edges introduce some unnecessary problems, so let's not assume there is one".

mathman