The paper re-examines distances to nearby megamaser galaxies and finds a lower value of the Hubble constant H0 when local galaxy motions are modeled more carefully. Megamasers are rare, bright radio sources in galaxy centers. Their distances can be measured directly, without using the usual “distance ladder,” so they offer an independent check on how fast the Universe is expanding today.

The authors reanalyzed the same six megamaser galaxies used in a previous study, but they applied a new reconstruction of the local velocity field (called M25). This reconstruction combines a redshift survey (2M++) with constrained cosmological simulations. That lets it predict both large-scale coherent flows and smaller, non‑Gaussian galaxy motions that older methods did not fully capture. The older analyses either treated peculiar velocities as simple Gaussian noise or used a coarser reconstruction (C15). The new M25 corrections change the redshifts used to infer H0.

Peculiar velocities are motions of galaxies that are not due to cosmic expansion. At the low redshifts of the megamaser sample (z < 0.04), these motions matter. In a sample of only six objects, a single large peculiar velocity can skew the result. The authors point out that one galaxy (NGC6323) has a peculiar velocity that would be almost three times the spread expected in a Gaussian model with σ = 250 km/s. They also show that the three most distant megamasers lie in the same coherent outflow, so correcting for that flow systematically reduces their redshifts and the inferred H0.

Using the M25 velocity corrections and a simple distance–redshift fit in a flat ΛCDM model (matter density Ωm fixed at 0.3), the authors obtain a value of H0 that agrees with the Cosmic Microwave Background result (Planck H0 = 67.4 ± 0.5 km/s/Mpc) and is in tension with the higher local distance‑ladder value (H0 = 73.5 ± 0.8 km/s/Mpc) at greater than the 2σ level. This contrasts with the earlier megamaser analysis, which was closer to the distance‑ladder number and in tension with the CMB. The paper therefore argues that the Hubble tension may come from an unknown systematic in the distance ladder rather than new physics.