The Hubble Constant and the ΛCDM Cosmology: A Magnified View using Strong Lensing

Recently, a significant tension has been reported between two measurements of the Hubble constant (H_0) from early-Universe (e.g., cosmic microwave background) and late-Universe probes (e.g., cosmic distance ladder). If systematic errors are ruled out in these measurements, then new physics extending the ΛCDM model will be required to resolve the tension. Therefore, different independent probes of H_0 -- such as the strong-lensing time-delay -- are essential to confirm or resolve the tension. The measured time-delays between the lensed images of a background quasar constrain H_0, as they depend on the absolute physical distances in the lens configuration. I led a team from the STRong-lensing Insights into the Dark Energy Survey (STRIDES) collaboration to analyze the lens DES J0408-5354. I modeled the mass distribution of this lens using Hubble Space Telescope imaging, and combined it with analyses from my collaborators to infer H_0 = 74.2-3.0+2.7 km s^-1 Mpc^-1 with the highest precision (3.9 per cent) from a single lens to date. This measurement agrees well with both the previous sample of six lenses from the H0LiCOW collaboration and other late-Universe probes, thus it increases the aforementioned tension. To confirm or resolve this tension at the 5σ level -- the gold standard of detecting new physics -- we need to increase the sample size and improve precision per system while keeping the systematics under control. The large amount of required investigator time (~1 year per lens) is currently the main bottleneck to increase the sample size. I present an automated lens-modeling framework that will enable rapid increment of the sample size in the near future. I also show, through simulation, that incorporating the spatially resolved kinematics of the lensing galaxy improves the precision of H_0 per system. Additionally, I develop the first general method to efficiently compute the lensing properties of any given elliptical mass distribution. By allowing any radial shape of mass profile, this method helps to avoid any systematic that may potentially arise from adopting only a few specific parameterizations. I forecast that a sample of ~40 lenses with spatially resolved kinematics will provide sub-per-cent precision in H_0 within the next decade