Growth of Cosmic Strings with Field-Dependent Tension

TL;DR

This work develops a dynamical-systems framework to study cosmic-string growth when the string tension depends on a rolling modulus in Type IIB string compactifications. It shows that a Growth Region (GR) exists in the phase space defined by $X$ and $Y$, enabling comoving growth not only during kination but also at stable scaling fixed points, for both fundamental F-strings and effective strings from wrapped branes in fibred Calabi–Yau geometries. For F-strings, growth is driven by the volume modulus with the kinating fixed point inside the GR, while for effective strings in fibred geometries, D3- and NS5-strings can reside in GRs, allowing extended growth away from the minimum. These results imply potentially observable gravitational-wave signatures from percolating string networks and motivate further exploration of currents, particle spectra, and multi-messenger probes in this string cosmology context.

Abstract

A novel mechanism to produce a cosmic network of fundamental superstrings based on a time-varying string tension has been recently proposed. It has been found that fundamental superstrings can grow in a kinating background driven by the rolling of the volume modulus of Type IIB string compactifications towards the minimum of its potential. In this talk, I will generalise this analysis using dynamical systems techniques. First, I will analyse the cosmological growth of strings with a field-dependent tension in a Universe filled with a perfect fluid, finding a growth region in the phase space of this system. This machinery is then applied to both fundamental superstrings and effective strings obtained from wrapping $p$-branes on $(p-1)$- dimensional cycles. I will show how cosmological growth can be achieved in both cases not only in kinating backgrounds, but also in scaling fixed points. This talk is based on arXiv:2502.14953.