Stochastic tachyon fluctuations, marginal deformations, and shock waves in string theory

Starting with exact solutions to string theory on curved spacetimes we obtain deformations that represent gravitational shock waves. These may exist in the presence or absence of sources. Sources are effectively induced by a tachyon field that randomly fluctuates around a zero condensate value. It is shown that at the level of the underlying conformal field theory (CFT) these deformations are marginal and moreover all α' corrections are taken into account. Explicit results are given when the original undeformed four-dimensional backgrounds correspond to tensor products of combinations of two-dimensional CFT's, for instance, SL(2,openR)/openR direct-product SU(2)/U(1)