Growth of Cosmic Strings beyond Kination

Abstract: A novel mechanism for the production of a cosmic network of fundamental superstrings based on a time-varying string tension has been recently proposed in the context of a kinating background driven by the volume modulus of string compactifications. In this paper, we generalise the analysis of this growth mechanism by using dynamical system techniques. We first study the cosmological growth of strings in a spatially-flat Universe filled with a perfect fluid and a field-dependent tension, finding the fixed points of the phase space of this system. We then apply this analysis to fundamental strings and EFT strings obtained from wrapping $p$-branes on $(p-1)$-cycles. We find a cosmological growth for fundamental strings even without kination, as in scaling fixed points, and for EFT strings arising from D3- and NS5-branes wrapped around fibration cycles.