Bubble Experiment Finds Universal Laws

My guess is it’s not really constrained at either end, there actually is no such thing as big or small, the universe is scale-free and everything is relative.

Roger Penrose has suggested as the universe spreads out and you get a very large, boring, empty universe it will eventually become equivalent mathematically to a very small, dense universe and another “Big Bang” will occur, not because the universe got small but because the big started looking the same as the small.

Or something.

A problem in the study of what happens when black holes form is that at such insanely small scales the general relativity model of gravity breaks down because of quantum effects.

A similar problem occurred in the formation of drops and bubbles where at those tiny scales right before it pinches off molecular effects come in to play.

However, through a mathematical method, scientists were able to ignore those molecular effects.

Black strings are entirely hypothetical. Scientists are hoping they can be pinched off in the same way as bubbles and that, if they can, they can use the same method above to ignore quantum effects (in the way molecular effects are ignored in bubbles). If so, it could (theoretically) be applied to the study of black holes, allowing scientists to study black holes without a quantum theory of gravity.

The fifth dimension is theoretical, yes. It turns out that you can get classical electricity and magnetism out of the Einstein equations by adding a fifth, rotationally symmetric spatial dimension (https://en.wikipedia.org/wiki/Kaluza%E2%80%93Klein_theory). Some people had hopes that quantum mechanics could fall out of the einstein equations in a similar fashion and this hope sort of lead to string theory, which needs to add further dimensions beyond the rotationally symmetric U(1) group of Kaluza-Klein theory to support the SO(3) (I believe) group of the electro-weak force (so they add 4 more dimensions) then further dimensions for the strong force symmetries. This latter part isn't my specialty so I am hazy on the details, but it falls under gauge theory.

So, while the fifth dimension is at the moment largely theoretical, it holds some interest in the field of gravitational physics due to some elegant constructions that fall out of it. It may be that the scalar field it predicts could describe dark matter: https://arxiv.org/abs/hep-ph/0207125.