Cosmic strings from preheating

We investigate nonthermal phase transitions that may occur after post-inflationary preheating in a simple model of a two-component scalar field with the effective potential $\lambda ({\varphi }_i^2-{\mathrm{v}}^2{)}^2\mathrm{/}4$, where ${\varphi }_1$ is identified with the inflaton field. We use three-dimensional lattice simulations to investigate the full nonlinear dynamics of the model. Fluctuations of the fields generated during and after preheating temporarily make the effective potential convex in the ${\varphi }_1$ direction. The subsequent nonthermal phase transition with symmetry breaking leads to formation of cosmic strings even for $\mathrm{v}\gg {10}^{16}$ GeV. This mechanism of string formation, in a modulated (by the oscillating field ${\varphi }_1$) phase transition, is different from the usual Kibble mechanism.