Probing Cosmic Superstrings with Gravitational Waves

TL;DR

This paper models the gravitational wave background from cosmic superstrings, showing that heavier string types produce distinctive signals detectable by current and future detectors, and deriving constraints on string tension from NANOGRAV data.

Contribution

It introduces a semi-analytical velocity-dependent model for cosmic superstrings, highlighting the importance of heavier strings and realistic loop sizes for gravitational wave predictions and constraints.

Findings

Heavier string types leave detectable signatures in the gravitational wave spectrum.

NANOGRAV data constrains fundamental string tension to Gμ_F<3.2×10^{-9}.

Including heavier strings refines the parameter space probed by gravitational wave observations.

Abstract

We compute the stochastic gravitational wave background generated by cosmic superstrings using a semi-analytical velocity-dependent model to describe their dynamics. We show that heavier string types may leave distinctive signatures on the stochastic gravitational wave background spectrum within the reach of present and upcoming gravitational wave detectors. We examine the physically motivated scenario in which the physical size of loops is determined by the gravitational backreaction scale and use NANOGRAV data to derive a conservative constraint of $G\mu_F<3.2 \times 10^{-9}$ on the tension of fundamental strings. We demonstrate that approximating the gravitational wave spectrum generated by cosmic superstring networks using the spectrum generated by ordinary cosmic strings with reduced intercommuting probability (which is often done in the literature) leads, in general, to weaker observational constraints on $G\mu_F$. We show that the inclusion of heavier string types is required for a more accurate characterization of the region of the $(g_s,G\mu_F)$ parameter space that may be probed using direct gravitational wave detectors. In particular, we consider the observational constraints that result from NANOGRAV data and show that heavier strings generate a secondary exclusion region of parameter space.