Time-dependent backgrounds of 2D string theory: Non-perturbative effects

We study the non-perturbative corrections (NPC) to the partition function of a compactified 2D string theory in a time-dependent background generated by a tachyon source. The sine-Liouville deformation of the theory is a particular case of such a background. We calculate the leading as well as the subleading NPC using the dual description of the string theory as matrix quantum mechanics. As in the minimal string theories, the NPC are classified by the double points of a complex curve. We calculate them by two different methods: by solving Toda equation and by evaluating the quasiclassical fermion wave functions. We show that the result can be expressed in terms of correlation functions of the bosonic field associated with the tachyon source and identify the leading and the subleading corrections as the contributions from the one-point (disk) and two-point (annulus) correlation functions.Comment: 37 pages, 2 figure

Exponentially-improved asymptotics of single and multidimensional integrals

Two different approaches for finding the exponentially improved asymptotic behaviour of integrals with saddlepoints are presented. Both rely on the deformation of the contours of integration and can be applied to single and multidimensional integrals alike. The class of integrals studied is of the form J J g(z1, ,zp)e-f{zu 'Zp,X)dz1 dz

Logarithmic catastrophes and Stokes's phenomenon in waves at horizons

Waves propagating near an event horizon display interesting features including logarithmic phase singularities and caustics. We consider an acoustic horizon in a flowing Bose-Einstein condensate where the elementary excitations obey the Bogoliubov dispersion relation. In the hamiltonian ray theory the solutions undergo a broken pitchfork bifurcation near the horizon and one might therefore expect the associated wave structure to be given by a Pearcey function, this being the universal wave function that dresses catastrophes with two control parameters. However, the wave function is in fact an Airy-type function supplemented by a logarithmic phase term, a novel type of wave catastrophe. Similar wave functions arise in aeroacoustic flows from jet engines and also gravitational horizons if dispersion which violates Lorentz symmetry in the UV is included. The approach we take differs from previous authors in that we analyze the behaviour of the integral representation of the wave function using exponential coordinates. This allows for a different treatment of the branches that gives rise to an analysis based purely on saddlepoint expansions, which resolve the multiple real and complex waves that interact at the horizon and its companion caustic. We find that the horizon is a physical manifestation of a Stokes surface, marking the place where a wave is born, and that the horizon and the caustic do not in general coincide: the finite spatial region between them delineates a broadened horizon.Comment: 34 pages, 12 figure