Green and Schwarz prove that Veneziano’s Superstring Theory is the only self-consistent theory that includes quantum gravity. But the main obstacle is that the mathematics needed to solve String Theory is not fully understood, and furthermore, there are countless solutions (millions and millions) for the possible geometries of the extra dimensions. If the mathematics does not uniquely identify some geometries, then perhaps all are solutions of the theory. This would imply (but was not clear in 1984) that there exists a plethora of universes with different geometries (the String Theory Landscape), and that we are in this because it could not be otherwise (the Weak Anthropic Principle). Our Big Bang would therefore not have been unique, but only one of countless in the Multiverse.
