What happens inside of black holes? What happened at the Big Bang? Why do we live in a universe with three dimensions of space and one of time? Answering these questions requires one to push our best theory of gravitation, Einstein's General Relativity, into the realm of the very small, dense, the realm of quantum mechanics. The first guess at what such a theory looks like is to treat gravity the same way we treat electromagnetism, and assume that gravity comes in point-like indivisible quanta, known as gravitons. Nature does not seem to work this way, since any such theory completely fails to make any sensible predictions. Instead, the best candidate is a theory based on the idea that quanta are not point-like, but in fact are one dimensional; this idea goes by the name of string theory. The price one pays for a consistent theory of quantum gravity is the prediction that there exist extra spatial dimensions beyond the three that we experience every day. This would seem to be a disaster, but these extra dimensions can be hidden from our view by making them small (``compactifying" them), or by confining the strings we are made out of to a subsection (a ``brane") of a higher dimensional space, as if we were cartoon characters confined to a sheet of paper. In fact, in string theory the configuration of the extra dimensions, or our embedding in them, dictates the masses and interactions of particles in the standard model of particle physics. Research on string theory in the York HEP group focuses on the phenomenology of string theory, in particular the implications for the properties and evolution of the very early universe. Energies much higher than those obtained in particle physics experiments, such as the LHC, can be accessed in the early universe making cosmological observables the ideal, and perhaps only, feasible experimental probe of string theory.