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  • Open Access

Gravitational waves from low-scale cosmic strings without scaling

Kai Schmitz1,2,* and Tobias Schröder1,†

  • 1Institute for Theoretical Physics, University of Münster, 48149 Münster, Germany
  • 2Kavli IPMU (WPI), UTIAS, The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8583, Japan

  • *Contact author: kai.schmitz@uni-muenster.de
  • †Contact author: schroeder.tobias@uni-muenster.de
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Phys. Rev. D 112, 083517 – Published 8 October, 2025

DOI: https://doi.org/10.1103/bv4k-xkmt

Abstract

Cosmic strings are predicted in many extensions of the Standard Model and constitute a plausible source of gravitational waves (GWs) from the early Universe. In a previous article [K. Schmitz and T. Schröder, Phys. Rev. D 110, 063549 (2024)], we pointed out that the GW spectrum from a population of string loops in the scaling regime can exhibit a sharp cutoff frequency associated with the fundamental oscillation mode of string loops. In this paper, we study the effect of particle decay due to kink-kink collisions and cusps on the GW spectrum in the nonscaling scenario introduced in P. Auclair et al., [Phys. Rev. D 101, 083511 (2020)]. We find analytical conditions for the existence of a cutoff frequency in the fundamental spectrum and provide expressions for this frequency. In large regions of parameter space, our results in the nonscaling model turn out to be identical to those in the scaling model. Finally, we demonstrate how the spectrum changes when transitioning from the regime with a cutoff frequency to the regime without a cutoff frequency. Our analytical estimates are validated at qualitatively different benchmark points by comparing them with numerical spectra.

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Phys. Rev. D 112, 083517– Published 8 October, 2025
  • Received 12 May 2025
  • Accepted 17 September 2025
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