A simple cosmological model has nothing to predict about the fate of collision of particles at high energies, or any other question about short-distance physics, for that matter. To describe the Universe not just in a broad macroscopic sketch, but also in the microscopic details, one needs a quantum theory, and string theory is the only known consistent theory that reconciles particle physics (such as gauge theories with chiral fermions) and general relativity (the latter including cosmological models).