In classical mechanics a particle with energy lower than the height of a potential barrier is forbidden to overpass it. In quantum mechanics, it turns out that under this circumstance the particle can tunnel through the barrier, instead.

In 1921 Einstein presented to the Prussian Academy a paper by Theodor Kaluza … in which the gravitational and electro-magnetic field are geometrically unified in five dimensions.

It is time [to] finally do away with gravity as a fundamental force.

A radical explanation of nonlocality postulates the existence of a preferred slicing, or foliation (as mathematicians would say), of space-time into spacelike 3-surfaces. This foliation might naturally arise from the space-time geometry; for example, it might consist of the 3-surfaces of constant timelike distance from the big bang.”

Two of the biggest mysteries of modern science are then how these 35 or so parameters are determined.

General relativity and supergravity, in any Spacetime dimension greater than or equal to 2 + 1, can be written as gauge theories, such that the configuration space is the space of a connection field, Aa, on a spatial manifold Σ. The metric information is contained in the conjugate momenta. The gauge symmetry includes the diffeomorphisms of a Spacetime manifold, usually taken to be Σ x R. The dynamics takes a simple form that can be understood as a constrained topological field theory. This means that the action contains one term, which is a certain topological field theory called BF theory, plus another term which generates a quadratic constraint.

Quantum cosmology is a controversial subject.
There are approaches to quantum cosmology that take the mathematical structure of the theory to be the same as that of ordinary quantum mechanics. … All other approaches propose that the formal structure of quantum theory must be modified for the theory to be sensibly applied to cosmology.

….one considers spacetime as a foliation of spacelike hypersurfaces, or “slices”.

[T]he formal content of special relativity is entirely coded in the Lorentz transformations, which were written by Lorentz, not by Einstein, and several years before 1905. What was Einstein’s contribution? It was to understand the physical meaning of the Lorentz transformation.