Time-Delay Cosmography (H0LiCOW)

When a massive galaxy or cluster sits between Earth and a distant quasar, gravity bends the quasar's light into multiple images — a phenomenon called strong gravitational lensing. Because each image traces a path of different length and through different depths of the gravitational potential, the light from each image arrives at slightly different times. These time delays, measured precisely over years of monitoring, encode the Hubble constant H₀ in a way that is entirely independent of the local distance ladder and of the CMB. The H0LiCOW collaboration analyzed six such lens systems and derived H₀ = 73.3 ± 1.7 km/s/Mpc — in striking agreement with SH0ES Cepheid measurements and in strong tension (roughly 3.1 sigma) with Planck's CMB-derived value of ~67.4 km/s/Mpc.

ΛCDM has no mechanism to reconcile these two anchors without modifying either early-universe physics or late-universe expansion. Proposed fixes — such as a mass-sheet degeneracy in the lens models, or unknown systematics in the quasar host environments — have been carefully investigated and found insufficient to close the gap. The TDCOSMO collaboration subsequently reanalyzed the lenses with more flexible mass models, widening the uncertainty but still finding H₀ values preferring the high side. The persistence of a high H₀ from this wholly independent geometric method, using no standard candles and no CMB priors, makes the tension genuinely multi-front and difficult to dismiss as a calibration artifact.