Charged-particle creation in cosmology

Three modifications of Maxwell's equations are discussed which allow for the possibility of nonconservation of electric charge. The first, which arises from Wheeler-Feynman theory, is shown to lead to the excitation of negative-energy modes in the electromagnetic field, but does not satisfy the steady-state condition for a Lyttleton-Bondi universe. The second, constructed by Lyttleton and Bondi especially for their cosmology, does not excite the negative-energy modes. The third, due to Watson, is shown to be physically uninteresting.