Abstract
Cosmological tensions may be hinting at flaws in the standard cosmological model, ΛCDM, within which the nature of dark matter and dark energy are poorly understood. My research contributes to making gravitational lensing as a probe of these non–luminous, elusive constituents of our Universe possible.The arc shapes and magnifications of bright, lensed objects reveal critical information about dark matter substructure on the lens plane, constraining dark matter in an as–yet–unconstrained mass regime. And, by analysing the sizes of Einstein rings relative to the redshifts of background sources, we learn about changes to the expansion rate of the Universe from different cosmic epochs, which is driven by the nature of dark energy.
In this thesis, we précis the state of the art of cosmography with gravitational lensing, and explore the current standards for lens modelling. We emphasise the advantages of systems with three or more galaxies in alignment on the optical axis, and go on to extract significant information about dark matter and dark energy from a cosmography–grade example of such a curiosity, the triple source plane lens SDSSJ0946+1006 (J0946).
We present the first models of J0946 that include all three sources and reconstruct observations from multiple data bands simultaneously. This tests for the presence and properties of the most confidently detected sub-galactic scale dark matter halo in our Universe so far. Using a Bayesian framework, we show a 5.9𝜎 confidence in a dark matter substructure with mass log10(𝑀200/𝑀⊙) = 10.3+1.2 and concentration log10 𝑐 = 2.4+0.5, marking it as a 2.6–3.3𝜎 outlier against mass–concentration relations in cold dark matter simulations. We highlight a degeneracy between the accuracy of background source morphology reconstruction and the detection confidence of small–scale lens features, necessitating careful future consideration into sensible priors on what an unlensed galaxy looks like.
We also introduce the first application of GPU-accelerated, fully auto-differentiable and probabilistically programmed lens modeling code to a multiple source plane lens. We use it to simultaneously obtain background source–marginalised posteriors on mass model parameters, lens light parameters and cos- mological parameters. Our posterior on the equation of state parameter for dark energy, 𝑤 , and energy density of matter, Ω𝑚, is orthogonal to the Planck satellite’s cosmic microwave background constraints, which jointly yield 𝑤 = −1.49+0.32 and Ω𝑚 = 0.21+0.07. We show that a more advanced treatment of the −0.27−0.04 mass profile of a lens is needed when it is probed at such a large range of radii by the multiple sources, but forecast a potential ∼ 13% constraint on 𝑤 once the full focusing of all three source planes by the lens in J0946 is made possible.
We end by summarising our findings and looking ahead to the next–generation, statistical era of strong gravitational lensing. In it, a vast influx of gravitational lenses will be discovered, inevitably including exotica like multiple source plane systems, and more precision cosmography will be made viable by the fast, robust lens modelling methodologies developed in this thesis.
| Date of Award | 18 Oct 2024 |
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| Original language | English |
| Awarding Institution |
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| Supervisor | Thomas Collett (Supervisor), Adam Amara (Supervisor) & David Bacon (Supervisor) |