Shawn Knabel

From the calendar

Understanding the expansion history of our Universe is an exciting challenge facing contemporary cosmological models. Amid growing evidence of an evolving dark energy equation of state, we struggle to definitively constrain the current-day expansion, the Hubble constant, H0. The so-called “Hubble tension” is the disagreement between early-Universe and late-Universe probes of the Hubble constant. If this tension is real, the proposed physical solutions are as varied as they are exotic. Time delay cosmography with strong gravitational lensing measures cosmological distances in a single step, bypassing any unaccounted for systematics that may be present in local distance ladder measurements which could be the source of the Hubble tension. I will present the TDCOSMO collaboration’s recent milestone results using gravitational time delays from strongly lensed quasars to measure the Hubble constant independently of other distance measures. I will highlight the critical role played by observations of the kinematics of the deflector galaxies and the breakthrough methodological improvements for overcoming sources of systematic uncertainties and unknown bias that have limited the precision of past cosmological constraints from strong lensing time delays. The constraints on H0 reach 5% precision and are robust across different cosmological models, representing a major step toward resolving the “Hubble tension”. The collaboration's next major work, to be released in 2026, will build upon this framework to reach 2.5-3% precision.