The Hubble constant—the rate at which the universe expands—is being measured two different ways, and the two numbers do not agree. This project will use exploding stars to settle whether that disagreement signals a flaw in the standard model of cosmology or simply a measurement error. The tension between the local Hubble constant (measured from nearby supernovae) and the value inferred from the early universe has grown too large to ignore. If it reflects real new physics—such as massive neutrinos or modified gravity—it would upend decades of cosmological theory. If it is a systematic error, the field needs cleaner measurements to find out. This project will build three independent ways to measure the Hubble constant. It will use strongly lensed supernovae—rare events where a galaxy’s gravity bends light from a more distant explosion—to achieve 1.3% accuracy. It will also construct a cleaner distance ladder from supernovae observed by a single survey, the Zwicky Transient Facility, avoiding calibration mismatches. Finally, it will test whether the Hubble constant varies across the sky. The research is fundamental science. It will not produce a new battery or a faster computer. But resolving whether the standard model of the universe is incomplete would reshape particle physics and our understanding of cosmic history—much as the discovery of dark energy did two decades ago.
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Cosmology is at the threshold of a new era. The standard, Lambda-CDM, model, till recently, has been an excellent explanation of observations in the early and late universe. The recent tension between the local Hubble constant (H0) and the inference from early universe, is a strong challenge to Lambda-CDM, with profound consequences for particle physics and cosmology. While this tension can be a sign of novel physics, e.g., a contribution from massive neutrinos or a modified theory of gravity, it can also be a realization of unknown systematic effects. It is, therefore, urgent to explore such measurements from more than one method. This project will tackle the open question "is there new physics beyond Lambda-CDM?" with cutting edge transient surveys in three steps: 1) Developing strongly lensed supernovae as an exciting new probe of H0, independent of either the early universe or the local distance scale. 2) sidestepping known systematics in local H0, by constructing a unique distance ladder from Type Ia supernovae (SNe Ia) observed by a single survey, the Zwicky Transient Facility (ZTF) 3) pioneering an empirical test of the directional dependence of H0 using the SN Ia magnitude-redshift relation. The Vera Rubin Observatory (VRO), online imminently, is expected to discover hundreds of lensed SNe. Utilizing state-of-the-art spectrographs, e.g., NOT and 4MOST, we will build a sample of lensed SNe from the VRO and measure H0 with 1.3% accuracy. Implementing modern methods for distances to the ZTF SN Ia host galaxies from tip of the red giant branch (TRGB) stars, we will overcome the current largest distance scale error sources, i.e. host galaxy bias and calibration. Forthcoming spectroscopy surveys will allow precise inference of ZTF SN Ia host galaxy redshifts, shedding light on potential anisotropies in the Hubble constant. These interconnected channels for measuring the Hubble constant and its directional dependence will enable a precise and accurate test of Lambda-CDM.
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