In the open string the gauge charges are carried by the Chan-Paton degrees of freedom at the endpoints. In the closed string the charges are carried … - Joseph Polchinski

On notable classmate was Dan Friedan. Friedan stunned me, and I think everyone else, at his Ph.D. seminar, when he showed that Einstein's equation, the basic equation of general relativity, could be interpreted in terms of one of the basic objects in QFT, the β function that governs the energy scale. I did not see what this could possibly mean, but a few years later it showed up as one of the key ideas in string theory.

D-branes provide a simple description of various nonperturbative objects required by string duality, and give new insights into the quantum mechanics of black holes and the nature of spacetime at the shortest distances. ... D-branes are extended objects, topological defects in a sense, defined by the property that strings can end on them.

In all there are four arguments here for the multiverse: the failure of conventional methods for understanding why the cosmological constant is not large, the success of the environmental theories for doing so, the successful prediction of the nonzero cosmological constant, and the string landscape.