  The q-model is a random walk model used to describe the flow of stress in a
stationary granular medium. Here we derive the exact horizontal and vertical
correlation functions for the q-model in two dimensions. We show that close to
a critical point identified in earlier work these correlation functions have a
universal scaling form reminiscent of thermodynamic critical phenomena. We
determine the form of the universal scaling function and the associated
critical exponents $\nu$ and $z$.
