Quick MOND Hits

Enticing, but with issues. Lots of self-citation, an arXiv review delay, very short, the author is primarily a mathematician and not primarily an astronomer, although his does have an institutional affiliation to a legitimate cosmology research center.

Gas-rich ultra-diffuse galaxies (UDGs) are an unusually sharp test for gravity models tied to the baryonic Tully--Fisher relation because several systems appear to rotate too slowly for their baryonic masses. This study revisits the six isolated gas-rich UDGs analysed by Mancera Piña et al. with the current outer-radius prescription of hyperconical modified gravity (HMG), using the published baryonic masses and circular velocities at the outer radii. The scan over the neighbourhood-scale parameter drives the model towards the asymptotic branch of HMG. For that limit, the HMG velocities are still systematically high for four of the six galaxies. Relative to the observed values, the fixed asymptotic branch gives χ2≃18.1 for six objects, whereas Newtonian baryons alone give χ2≃9.7, but MOND interpolation is much worse (χ2≃615.7). Using combined uncertainties, the per-galaxy HMG tension ranges from 0.2σ to 2.1σ, very similar to the 0.1σ to 1.7σ found for Newtonian baryons, and much smaller than the 3.7σ to 5.9σ obtained for MOND. We conclude that the present outer-radius HMG implementation alleviates the difficulties of MOND, but is still not sufficient to account for the published central values of the UDG sample. Gas-rich UDGs therefore provide a useful discriminant between MOND and HMG.

Robert Monjo, "Gas-rich ultra-diffuse galaxies: alleviating the MOND tension with HMG" arXiv:2604.09652 (March 30, 2026) (4 pages, 1 figure).

More credible. An established MOND astrophysicist. A big blow to MOND critics.

It is a common miss-conception that 1E 0657-56, the "Bullet Cluster", is somehow inconsistent with MOND expectations. The argument centres on the fact that the baryonic matter distribution of this system is dominated by the X-ray emitting gas, while the total projected surface density required under General Relativity to explain the observed lensing signal, centres on the observed galaxies. This is sometimes interpreted as being in conflict with MOND, as under such an interpretation, it is naively assumed that all dark matter being absent, the gravitational potential should necessarily be dominated by the largest mass distribution, that of the gas. 

However, just as under General Relativity, under MOND, the total gravitational potential of a system depends sensitively upon the volume density and not just on the total mass. It is shown in this letter that the surface density which QUMOND predicts will be inferred under a standard gravity framework from the total gravitational potential of the Bullet Cluster, closely matches what General Relativity inferences of lensing observations return. The close-to-point-like galaxies imply under QUMOND a relatively much larger surface density signal than what is expected from the Mpc scale gas distribution.

X. Hernandez, "A consistent MOND modelling of the Bullet Cluster" arXiv:2604.10811 (April 12, 2026).

The Hubble Tension Is Real

The Hubble constant is a measurement of the expansion of the universe, sometimes attributed to a cosmological constant in General Relativity (and the source of more than two-thirds of the mass-energy of the universe in conventional cosmology). Except, it appears that the Hubble constant isn't quite constant. So the explanation must be more complicated than a simple cosmological constant.

The Hubble tension isn't huge in relative terms, 10% over measurements more than ten billion years removed from each other. 

But it is highly statistically significant at the five sigma plus level, and isn't a simple methodological artifact of late time Hubble constant measurements (although it could be a methodological artifact of model dependent cosmic microwave background radiation measurements).

Context. The direct empirical determination of the local value of the Hubble constant (H(0)) has markedly advanced thanks to improved instrumentation, measurement techniques, and distance estimators. However, combining determinations from different estimators is nontrivial due to their correlated calibrations and different analysis methodologies.

Aims. Using covariance weighting and leveraging community expertise, we have constructed a rigorous and transparent “Distance Network” to find a consensus value and uncertainty for the locally measured Hubble constant.

Methods. Experts across all relevant distance measurement domains were invited to critically review the available datasets spanning parallaxes, detached eclipsing binaries, masers, Cepheids, the tip of the red giant branch, Miras, carbon-rich asymptotic giant branch stars, Type Ia (SNe Ia) and Type II supernovae, surface brightness fluctuations, the fundamental plane, and Tully–Fisher relations. Before any calculations, the group voted for first-rank indicators to define a “baseline” Distance Network. Other indicators were included to assess the robustness and sensitivity of the results. We provide open-source software and data products to support full transparency and future extensions of this effort.

Results. Our key findings are as follows: (1) The local H(0) is robustly determined, with first-rank indicators internally consistent within their uncertainties. (2) A covariance-weighted combination yields a relative uncertainty of 1.1% (baseline) or 0.9% (all estimators). (3) The contribution from SNe Ia is consistent across compilations of optical or NIR magnitudes. (4) Removing either Cepheids or the tip of the red giant branch has a minimal effect on the central value of H0. (5) Replacing SNe Ia with galaxy-based indicators changes H(0) by less than 0.1 km s^−1 Mpc^−1 while doubling its uncertainty. (6) The baseline result is H(0) = 73.50 ± 0.81 km s^−1 Mpc^−1, 7.1σ from the early Universe plus ΛCDM result 67.24 ± 0.35 km s^−1 Mpc^−1 and 5.0σ from BBN+BAO within a flat ΛCDM DESI DR2 (68.51 ± 0.58 km s^−1 Mpc^−1).

Conclusions. A networked approach, such as the one presented here, is invaluable for enabling further progress in Hubble constant measurements, as it provides the much needed advances in accuracy and precision without overreliance on any single method, sample, or group.

H0DN Collaboration, "The Local Distance Network: A community consensus report on the measurement of the Hubble constant at ∼1% precision" 708 A&A, A166 (April 10, 2026).

Worth noting that in Deur's approach, there is no cosmological constant, and that the apparent cosmological constant varies over time, and is expected to increase as galaxy and cluster structure increase somewhat over time. And, in Deur's approach, galaxy formation comes earlier than in ΛCDM.