Moving Five-Branes and Membrane Instantons in Low Energy Heterotic M-Theory

We study cosmological solutions in the context of 4-dimensional low energy Heterotic M-theory with moving bulk branes. In particular we present non-trivial, analytic axion solutions generated by new symmetries of the full potential-free action which are similar to 'triple axion' solutions found in Pre-Big-Bang (PBB) cosmologies. We also consider the presence of a non-perturbative superpotential, for which we find cosmological solutions with and without a background perfect fluid. In the absence of a fluid the dilaton and the T-modulus go to the potential-free solutions at late time, while the moving brane tries to avoid colliding with the boundary and stabilize within the bulk. When the fluid is included, we find that the real parts of the fields track its behaviour and that the moving brane gets stabilized at the middle point between the boundaries. In this latter case we can make analytic approximations for the evolution of the fields whether or not axions are included and we consider the possibility of this set up being a realization of the quintessential scenario.