Overproduction of cosmic superstrings

TL;DR

This work shows that the end of brane–antibrane inflation generically produces a high initial density of topological defects, with a correlation length set by the string scale $M_s^{-1}$ rather than the Hubble radius $H^{-1}$. By combining analytic tachyon-evolution near defect cores with lattice simulations for both kink and vortex formation in compact spaces, the authors demonstrate robust defect production even when extra dimensions are causally connected. They contrast this runaway tachyon dynamics with conventional scalar field theories, highlighting the absence of symmetry restoration and the resulting potential overclosure in certain brane-inflation scenarios, especially when defects wrap compact dimensions. The cosmological implications include rapid approach to a scaling regime for string-like defects, possible domain-wall and monopole problems depending on the compactification, and a need for detailed anisotropic, higher-dimensional simulations to assess viability of specific models.

Abstract

We show that the naive application of the Kibble mechanism seriously underestimates the initial density of cosmic superstrings that can be formed during the annihilation of D-branes in the early universe, as in models of brane-antibrane inflation. We study the formation of defects in effective field theories of the string theory tachyon both analytically, by solving the equation of motion of the tachyon field near the core of the defect, and numerically, by evolving the tachyon field on a lattice. We find that defects generically form with correlation lengths of order M_s^{-1} rather than H^{-1}. Hence, defects localized in extra dimensions may be formed at the end of inflation. This implies that brane-antibrane inflation models where inflation is driven by branes which wrap the compact manifold may have problems with overclosure by cosmological relics, such as domain walls and monopoles.