Random Matrices, Boundaries and Branes
  This thesis is devoted to the application of random matrix theory to the study of random surfaces, both discrete and continuous; special emphasis is placed on surface boundaries and the associated boundary conditions in this formalism. In particular, using a multi-matrix integral with permutation symmetry, we are able to calculate the partition function of the Potts model on a random planar lattice with various boundary conditions imposed. We proceed to investigate the correspondence between the critical points in the phase diagram of this model and two-dimensional Liouville theory coupled to conformal field theories with global $\mathcal{W}$-symmetry. In this context, each boundary condition can be interpreted as the description of a brane in a family of bosonic string backgrounds. This investigation suggests that a spectrum of initially distinct boundary conditions of a given system may become degenerate when the latter is placed on a random surface of bounded genus, effectively leaving a smaller set of independent boundary conditions. This curious and much-debated feature is then further scrutinised by considering the double scaling limit of a two-matrix integral. For this model, we can show explicitly how this apparent degeneracy is in fact resolved by accounting for contributions invisible in string perturbation theory. Altogether, these developments provide novel descriptions of hitherto unexplored boundary conditions as well as new insights into the non-perturbative physics of boundaries and branes. 
Benjamin Niedner
math-ph hep-th math.MP