Strong-lens time-delay cosmography measures a “time-delay distance” from the arrival-time differences between multiple images of a variable background source, giving an essentially one-step, absolute distance scale that is highly sensitive to H0 and largely independent of the traditional distance ladder (Refsdal 1964; Treu & Marshall 2016). H0LiCOW and related programs typically infer H0˜72–76 km s?¹ Mpc?¹, in significant tension with the lower value inferred from Planck CMB data in flat ΛCDM, and this discrepancy persists even after careful treatment of key lensing systematics such as the mass-sheet degeneracy and line-of-sight structure, forcing ΛCDM either to accept substantial, finely tuned systematics or to invoke new physics beyond its minimal model (Suyu et al. 2017; Wong et al. 2020).