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Dark matter subhalos and higher order catastrophes in gravitational wave lensing

Luka Vujeva1,*, Jose María Ezquiaga1, Daniel Gilman2, Srashti Goyal3, and Miguel Zumalacárregui3

  • *Contact author: luka.vujeva@nbi.ku.dk
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Phys. Rev. D 114, 083024 – Published 9 October, 2026

DOI: https://doi.org/10.1103/kp86-q1h2

Abstract

Gravitational lensing is an invaluable probe of the nature of dark matter, and the structures it forms. Lensed gravitational waves in particular allow for unparalleled sensitivity to small-scale structures within the lenses, due to the precise time resolution in combination with the continuous monitoring of the entire sky. In this work, we show two distinct ways of using strongly lensed gravitational waves to identify the presence of dark matter subhalos; (i) through higher order caustics generating high relative magnification (μr>2), short time-delay image pairs that break the caustic universality relations of single dark matter halos, which occur for ∼1–10 percent of strongly lensed events in our cold dark matter models, and (ii) through the presence of more than three highly magnified images, which occur for ∼0.01–1 percent of the same simulated events. We find that these results are highly sensitive to the concentrations of subhalos in our simulations, and more mildly to their number densities. The presence of low-mass subhalos increases the probability of observing wave-optics lensing in lensed gravitational waves, which is studied by solving the diffraction integral with the stationary phase approximation, as well as numerically. We also report distinct quantitative and qualitative differences in the distributions of relative magnifications and time delays for subhalo populations with increased number densities or concentrations. With the upcoming detection of strongly lensed events by ground- and space-based detectors, comparisons against these simulated distributions will provide insight into the nature of dark matter.

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Phys. Rev. D 114, 083024– Published 9 October, 2026
  • Received 17 December 2025
  • Accepted 13 August 2026
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