Personal website of Ryan Brady
Measuring the Hubble Constant
Despite the successes of ΛCDM in explaining a wide range of cosmological observations, a persistent and statistically significant discrepancy has appeared in recent years between measurements of the Hubble constant (H0) derived from observations of the early Universe and those from the local Universe. Measurements of the cosmic microwave background (CMB) anisotropies by the Planck satellite, when interpreted within the ΛCDM framework, yield a value of H0 = 67.4 ± 0.5 km s−1 Mpc−1 [Planck 2018]. In contrast, direct distance ladder measurements, such as those conducted by the SH0ES collaboration using Cepheid-calibrated Type Ia supernovae, report a significantly higher value of H0 = 73.04 ± 1.04 km s−1 Mpc−1 [Riess 2022]. This discrepancy, often referred to as the “Hubble tension,” now exceeds the 5σ threshold and suggests either unknown systematic errors in one or more of the measurement techniques or, more provocatively, the need for new physics beyond ΛCDM [Di Valentino 2021].
Composite red-green-blue (RGB) images of the eight doubly imaged quasar systems in the sample. Each figure presents an image constructed from the HST observations, with F160W data mapped to the red channel, F475X data to the blue channel, and F814W data to the green channel. To enhance visualization, the intensity scaling of each band is adjusted individually.
Time-delay cosmography (TDC) of strongly lensed quasars has emerged as an independent and complementary method for determining H0 that circumvents many of the systematics inherent in other techniques. First proposed by [Refsdal 1964], this method relies on the fact that multiple images of a background quasar, lensed by a foreground galaxy, arrive at the observer at different times due to differences in both the geometric path length and the gravitational potential traversed by the light rays. By accurately measuring the time delays between the lensed images and modeling the mass distribution of the lens, one can infer the so-called time-delay distance, which is inversely proportional to H0 [Suyu 2010], [Treu 2016]. The H0LiCOW collaboration has applied this technique to a sample of well-characterized quadruply imaged quasar systems and reported values of H0 consistent with the local distance ladder measurements, finding H0 = 73.3+1.7−1.8 km s−1 Mpc−1 [Wong 2020]. These results reinforce the observed tension and emphasize the potential for time-delay cosmography to arbitrate between the competing measurements of H0 and to probe possible extensions to the standard cosmological model. My work therefore aims to obtain a percent-precision measurement of H0 using doubly imaged quasars, which are ~4x more common than quads in the Universe.
Gravitational Lens Modeling of Doubly Imaged Quasars: A Pathway to Determining the Hubble Constant
My undergraduate honors thesis, detailing the Lenstronomy pipeline that I developed to robustly model our six doubly imaged quasar systems and showcasing the relevent extracted cosmological information.