Gravitationally Lensed Supernovae as Probes of Cosmology and Astrophysics

Abstract

Strongly gravitationally lensed supernovae (glSNe) are powerful probes of cosmol-ogy and astrophysics. The time delays between multiple images provide a direct measure of the expansion rate of the Universe, while the standardisable nature of Type Ia supernovae (SNe Ia) helps break lens-modelling degeneracies. This enables a precise measurement of the Hubble constant (H0), potentially resolving the tension between early- and late-time measurements. Furthermore, the "cosmic telescope" effect of strong lensing allows for the study of SNe populations at high redshifts, which is vital for identifying any cosmic evolution that might require revised stan-dardisation techniques. Despite their utility, glSNe are exceptionally rare, and estab-lished discovery methods are currently lacking. This thesis evaluates the prospects of lensed supernova cosmology, demonstrates an astrophysical use case, and tests specific search methodologies.
Following an overview of the fundamental pillars of cosmology and the H0 ten-sion, supernova astrophysics and cosmology, and strong lensing, I present popula-tion forecasts for detectable systems across various surveys. Our results indicate that glSN properties in shallow surveys, such as the Zwicky Transient Facility (ZTF), are driven by the need for high-magnification. This biases discoveries toward four-image systems with short time delays and small image separations—a pattern consistent with iPTF16geu and SN Zwicky, but unrepresentative of deeper survey populations. I find that the Legacy Survey of Space and Time (LSST) should discover approxi-mately 180 systems over ten years. Seventy of these will be suited for cosmography, enabling a 1.2% precision measurement of H0.
I further demonstrate that monitoring known Euclid lenses provides a viable alternative to searching the entire LSST alert stream for glSNe. We predict that
∼ 70% of discoverable lensed hosts of glSNe will be previously identifiable by Eu-
clid as galaxy-galaxy lenses; notably, the majority of these systems will belong to the "golden sample" for time-delay cosmography. Regarding their astrophysical utility, a comparison of iPTF16geu data with SN explosion models suggests a preference for delayed detonation models. However, early-time observations are shown to be crit-ical for model discrimination, highlighting the importance of accurate lens models with time-delay predictions for follow-up. Finally, I present results from a live glSN search in ZTF, categorising the primary contaminants encountered.
I conclude by summarising our results and discussing the future of time-delay cosmography with next-generation surveys. In this era, event rates will no longer be the primary bottleneck; instead, the challenge will lie in efficient identification and the coordination of timely follow-up observations to maximise the scientific yield of glSNe.

Date of Award	22 Apr 2026
Original language	English
Awarding Institution	University of Portsmouth
Supervisor	Thomas Collett (Supervisor), Mark Magee (Supervisor) & Or Graur (Supervisor)