Gravitational lensing of gravitational waves: from detection to cosmology and population inference

Seo, Eungwang (2026) Gravitational lensing of gravitational waves: from detection to cosmology and population inference. PhD thesis, University of Glasgow.

Abstract

The direct detection of gravitational waves has transformed our view of the Universe, elevating them from a confirmation of General Relativity to a tool for precision astrophysics and cosmology. Unlike electromagnetic radiation, which is subject to absorption, scattering, and extinction by baryonic matter, gravitational waves propagate through the Universe largely unimpeded, carrying relatively clean information about both their sources and the intervening gravitational structures.

Gravitational lensing, another prediction of General Relativity, naturally arises along such propagation paths. Acting as a cosmic telescope, it magnifies distant signals while encoding the lens properties into the observed waveform. This phenomenon has long been exploited in electromagnetic observations to probe dark matter, the expansion history of the Universe, and the population properties of compact objects. When applied to gravitational waves, lensing provides a complementary probe of these same questions, with sensitivity to regimes that remain inaccessible to light-based observations.

In this thesis, I investigate both the detection of gravitationally lensed gravitational waves and their scientific applications. Current searches focus on two regimes: strong lensing, in which massive structures such as galaxies or clusters produce multiple images with time delays ranging from minutes to years, and microlensing, induced by smaller-scale structures such as stars, compact remnants, or dark matter subhalos, which imprint frequency-dependent interference patterns due to the wave nature of gravitational waves. Up to the third observing run of the LIGO–Virgo–KAGRA Collaboration, no compelling evidence for gravitational-wave lensing has been reported. I present results from a search for isolated point-mass microlensing in the first part of the fourth observing run, highlighting an interesting candidate event, GW231123_135430.

I then transition to a more astrophysically realistic lensing scenario. While an isolated point-mass lens provides a useful approximation for the effect of an individual microlens, microlensing is expected to occur within more complex lens environments, where compact objects can be embedded in a larger-scale gravitational potential. I therefore extend the search to scenarios involving combined strong lensing and microlensing. The complex and highly degenerate nature of such signals poses a challenge for existing model-dependent pipelines. To address this, a model-independent search framework based on residual tests is developed, enabling rapid identification of candidate events, several orders of magnitude faster than fully model-dependent analyses, while remaining sensitive to a broad range of lensing-induced deviations from unlensed waveforms.

Once detected, gravitationally lensed gravitational waves offer powerful applications in cosmology. One of the central challenges in modern cosmology is the Hubble tension, the discrepancy between measurements of the Hubble constant inferred from the early Universe and those obtained from local distance indicators. Gravitational waves provide an independent way to measure cosmic distances through their amplitude, without relying on the traditional distance ladder. In the presence of strong lensing, this capability is further enhanced: combining multiple lensed images with an accurate reconstruction of the lens mass distribution significantly improves the precision of luminosity distance estimates, offering a promising avenue to alleviate this tension.

In addition to cosmological applications, gravitational-wave lensing provides a direct means to probe the population of compact objects and the small-scale structure of matter. Because gravitational waves are insensitive to baryonic effects such as dust extinction, lensing signatures reflect the underlying gravitational field alone, enabling a mapping of the otherwise invisible components of the Universe. Such measurements constrain the abundance and mass function of intermediate-mass black holes, primordial black holes, and dark matter subhalos, thereby providing insights into both compact-object formation channels and the small-scale structure of dark matter, which are difficult to access through electromagnetic observations only.

As we look toward the next decade of gravitational-wave astronomy—with the continued operation of the LVK network and the anticipation of next-generation detectors and space-based detectors—the detection of a definitively lensed GW event is no longer a matter of if but when. This work establishes the essential methodologies required to both identify such events and fully exploit their scientific potential, paving the way toward precision cosmology and a deeper understanding of the dark Universe.

• Item Type: Thesis (PhD)
• Qualification Level: Doctoral
• Subjects: Q Science > QB Astronomy
• Colleges/Schools: College of Science and Engineering > School of Physics and Astronomy
• Supervisor's Name: Hendry, Professor Martin
• Date of Award: 2026
• Depositing User: Theses Team
• Unique ID: glathesis:2026-86238
• Copyright: Copyright of this thesis is held by the author.
• Date Deposited: 22 Sep 2026 15:10
• Last Modified: 22 Sep 2026 15:10
• Thesis DOI: 10.5525/gla.thesis.86238
• URI: https://theses.gla.ac.uk/id/eprint/86238

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