Does the Universe Have a Center?
- The universe is infinite.
- The universe is finite, but wraps around so that if you go far enough in one direction you will return to the starting point.
- The universe is finite and has boundaries, but the geographical center is outside the universe. For example, if the universe has the shape of a 3-d torus (doughnut).
Now if none of these 3 conditions apply, then yes the universe has a center. But that center would likely be far outside the observable universe and there probably wouldn’t be anything special about it. We wouldn’t know where it is. And it wouldn’t necessarily be an ‘origin point’ of the universe. So we treat the universe as if it has no center, since for most practical purposes it does not matter.
Q. Doesn’t the Cosmological Principle say there are no boundaries (and therefore center) because space is the same everywhere?
The Cosmological Principle is a philosophical idea that works fine as a default assumption for the observable universe. But it makes no sense to assume that because we do not see a boundary in the observable universe, there must not exist any boundaries anywhere. Because any such boundaries would likely be found in only a very small percent of the total volume of the universe.
Q. Doesn’t the FLRW and Lamda-CDM models of a “flat universe” prove the universe has no center and boundaries?
The reason an apparently “flat” universe does not have boundaries is because none of the 3 curvature possibilities in the FLRW model have boundaries.
https://en.wikipedia.org/wiki/Shape_of_the_universe
For example, in the Friedmann–Lemaître–Robertson–Walker (FLRW) model, the universe is considered to be without boundaries…
And the reason why the FLRW model does not include modelling boundaries is simply because it is very hard to model boundaries, not because they are unlikely.
One of the unanswered questions about the universe is whether it is infinite or finite in extent…Assuming a finite universe, the universe can either have an edge or no edge. Many finite mathematical spaces, e.g., a disc, have an edge or boundary. Spaces that have an edge are difficult to treat, both conceptually and mathematically. Namely, it is very difficult to state what would happen at the edge of such a universe. For this reason, spaces that have an edge are typically excluded from consideration.
https://iopscience.iop.org/article/10.1088/1475-7516/2023/02/049/pdfWe also assume for concordance cosmology that this large-scale spatial 3-manifold has no boundary hypersurfaces. This is not an essential feature of spacetime