Lorentz Symmetry Violation in String-Inspired Effective Modified Gravity Theories

We discuss situations under which Lorentz symmetry is violated in effective gravitational field theories that arise in the low-energy limit of strings. In particular, we discuss spontaneous violation of the symmetry by the ground state of the system. In the flat space-time limit, the effective theory of the broken Lorentz Symmetry acquires a form that belongs to the general framework of the so-called Standard Model Extension (SME) formalism. A brief review of this formalism is given before we proceed to describe a concrete example, where we discuss a Lorentz-symmetry-Violating (LV) string-inspired cosmological model. The model is a gravitational field theory coupled to matter, which contains torsion, arising from the fundamental degrees of freedom of the underlying string theory. The latter, under certain conditions which we shall specify, can acquire a LV condensate, and lead, via the appropriate equations of motion, to solutions that violate Lorentz and CPT (Charge-Parity-Time-Reversal) symmetry. The model is described by a specific form of an SME effective theory, with specific LV and CPT symmetry Violating coefficients, which depend on the microscopic parameters of the underlying string theory, and thus can be bounded by current-era phenomenology.