False vacua, topological defects and cosmology

Two studies related to the role of solitons in theories with spontaneous symmetry breaking are presented. Quantised fermions coupled to solitons are known to induce fractional fermion number. I present an example where an unstable topological solution binding a zero mode of a majorana fermion gets stabilised due to the induced fractional number. Thus neither is the cosmic string stable nor is the fermion number conserved, yet the bound state then becomes stable due to Quantum Mechanics. The other phenomenon concerns a metastable vacuum getting destabilised due to the presence of a cosmic string. This happens because the scalar field signalling spontaneous symmetry breaking acquires a vacuum expectation value approaching its true vacuum value in the core of the string. Thus the string acts like a seed nucleating the true vacuum. The instanton mediating between the false vacuum and the exit point into the true vacuum is shown to exist and graphically displayed through a numerical calculation.