Speaker
Description
The growing tension between early- and late-Universe measurements of the Hubble constant ($H_0$) has elevated time-delay cosmography to a primary probe of the expansion rate. However, the precision of this "one-step" method is fundamentally limited by the mass-sheet degeneracy (MSD) and model mis-specification. In this work, we present a systematic exploration of the $H_0$ likelihood surface using a brute-force grid-search of $10^6$ lens models applied to high-fidelity mock systems from the IllustrisTNG hydrodynamical simulations. We introduce a sequential filtering pipeline—moving through astrometric, chronometric, photometric, and kinematic gates—to quantify the hierarchy of observational constraints. We find that image positions and time delays alone result in a catastrophic systematic floor, with $H_0$ values permitted across a global range of $0$ to $125$ km s$^{-1}$ Mpc$^{-1}$. The inclusion of absolute magnification—representative of a lensed Type Ia supernova—successfully breaks the primary MSD, narrowing the uncertainty to a $\sim 30\%$ interval. Finally, we demonstrate that the addition of stellar velocity dispersion serves as the critical anchor, resolving the residual profile-slope degeneracy and recovering the true $H_0$ with $\sim 3\%$ precision. Our results provide a forensic decomposition of the information gain in lensing systems and underscore the necessity of auxiliary photometric and kinematic data for resolving the Hubble tension in the era of Euclid and the Rubin Observatory.
| Other topic / keywords: | Gravitational Lensing, Cosmology, the Hubble Tension |
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