Yukei Murakami studies the expansion history of the universe using Type Ia supernovae (SNeIa), a type of exploding star that provides a powerful tool for measuring distances in the universe. SNe Ia exhibit a well-defined luminosity–color–duration relationship, allowing them to function as standardizable candles and enabling precise measurements of cosmic distances across billions of years of cosmic history. However, remaining variations in the observed post-standardization luminosities of SNe Ia are the dominant source of systematic uncertainty in modern cosmology, limiting the potential of next-generation surveys such as the Nancy Grace Roman Space Telescope and the Rubin Observatory Legacy Survey of Space and Time (LSST). Murakami’s research focuses on understanding the astrophysical origins of these variations and incorporating them into physically motivated models of supernova luminosity. As a Ph.D. student at Johns Hopkins University, he investigated key sources of supernova variability, including spectroscopic differences between explosions, dust extinction in host galaxies, and the properties of nearby supernova samples used to anchor cosmological measurements. As a Brinson Prize Fellow, Murakami will develop a unified, astrophysics-driven model linking supernova properties to their host galaxies and progenitor systems across cosmic time. By reducing the dominant systematic uncertainties in supernova cosmology, this work will enable next generation surveys to measure key cosmological parameters, such as the properties of dark energy, with unprecedented precision and test whether current cosmological tensions point to new physics.