The following responds to the entries that asked questions that I can answer:

Sticks: I heard one idea that Young's Slit experiment proved the existence of parallel universes.
Young's slit experiment had nothing to do with parallel universes.

Mootzy: How is the emission of single photons confirmed in this experiment?
The interference pattern observed is fully accounted for by assuming that a plane wave corresponding to the a single photon fell perpendicularly onto the plane of the slit, thereby generating a pair of synchronized cylindrical waves at the slits, those waves then traveling to the screen and interfering with each other constructively and destructively at the predicted locations.