It is relatively straightforward, in the context of general relativity, to construct spacetimes where the number of “large” (i.e., macroscopic) dimensions changes with time. Not only could you have had a large extra dimension in the past, you can have one in the future, or you could even construct a spacetime where that number oscillates, or gets larger-and-smaller-and-larger again over time.

So that first part is awesome: we can have a Universe where a fourth — or extra — spatial dimension begins to appear.

Now, that’s cool, but what would it be like? We don’t normally think about it, but the four fundamental force — gravity, electromagnetism, and the two nuclear forces — only have the properties and strengths that they do because they exist in the number of dimensions our Universe has. If we reduced or increased the number of dimensions, we’d change the way that, for example, field lines of force spread out.

This would be catastrophic if electromagnetism or the nuclear forces were affected immediately.