Time-Delay Cosmography (H0LiCOW)

The H0LiCOW and TDCOSMO collaborations measure H₀ by exploiting the time delays between multiple lensed images of background quasars. When a foreground galaxy acts as a gravitational lens, photons traveling along different paths arrive at different times; the delay encodes the time-delay distance D_Δt, a combination of angular diameter distances between observer, lens, and source that is inversely proportional to H₀. Independent of the CMB or any local calibrator, this method consistently yields H₀ values in the range of 73–74 km/s/Mpc, in close agreement with the SH0ES distance ladder and in tension at approximately three to four sigma with the Planck CMB inference of 67.4 km/s/Mpc. The robustness of the result across multiple lens systems and independent lens modeling codes makes a systematic origin increasingly difficult to sustain.

Successive Collision Theory explains this tension through the same spatial mechanism that drives the SH0ES discrepancy. We reside within the KBC supervoid, an underdense region roughly 300 Mpc in extent where the local gravitational binding strength λ_local is approximately 20 percent below the cosmic mean. Because the effective cosmological term Λ_eff = C × Λ_parent/λ_local is inversely proportional to λ_local, the locally measured expansion rate is systematically elevated above the global mean. Time-delay cosmography with lensing systems at redshifts z ~ 0.3–0.7 probes the expansion history along lines of sight that pass primarily through our local cosmic environment. The inferred H₀ from these systems therefore reflects the locally enhanced expansion rate within and around the KBC void, naturally yielding values 5–7 km/s/Mpc above the globally averaged Planck result without any systematic observational error in either dataset.

The temporal evolution of Λ_eff adds a further contribution: the tensor mesh has been dissipating since decoupling, so the effective H₀ at z = 0 is genuinely higher than the value inferred from CMB physics at z = 1100. The SCT prediction is therefore that the tension between time-delay H₀ and CMB H₀ is real, not systematic, and that it will persist at the same significance as lens samples grow — a prediction distinguishable from systematic-error models that predict convergence as methodology improves. SCT also predicts a mild anisotropy in time-delay H₀ correlated with the direction of the supervoid axis, testable with the growing all-sky lens sample from Euclid and Rubin Observatory.