More On Wide Binaries, MOND and Deur

A new study concludes very emphatically that wide binary stars behave in a manner more like Newtonian dynamics and less like MOND. This seems pretty definitive, but we've seen contradictory conclusions on the wide binary data before, so I don't consider this to be the final word.

More importantly, it doesn't rule out all gravity based explanations of dark matter phenomena. In particular, Deur's gravitational self-interaction driven approach explains dark matter phenomena with a gravitational solution (with no dark matter or dark energy) over a very wide range of applicability and does not predict significantly non-Newtonian behavior in wide binaries. 

Deur's approach may or may not be consistent with consideration of non-perturbative standard general relativistic effects as claimed. But whether Deur is applying general relativity in an unconventional way that rigorously considers a feature of general relativity that most other researchers neglect, or is actually a subtle gravity modification theory that isn't quite identical to standard general relativity, it works. Deur's approach also roots MOND-like galaxy behavior, and really all dark matter and dark energy theories in a theory that has a fairly simple deep theoretical motivation rather than just being a phenomenological fit to some key data points. And, unlike LambdaCDM and other standard dark energy theories, Deur's approach does so without globally violating conservation of mass-energy.

Deur's approach explains dark matter phenomena not just where MOND works, but also in many circumstances where MOND does not work. 

For example, Deur's approach works in galaxy clusters, in the Bullet cluster, with respect to wide binary star systems, as an explanation for different degrees of dark matter phenomena in differently shaped elliptical galaxies, as an explanation for the two-dimensional arrangement of satellite galaxies around spiral galaxies, and as an explanation for dark energy phenomena as well. It even provides a potential explanation for the Hubble constant tension and can reproduce the cosmic microwave background observed by the Planck collaboration and the early galaxy formation observed by the Webb telescope.

If the conclusion of this paper holds up, it somewhat decisively tips the balance between Deur's approach and other gravitational explanations of dark matter phenomena.

We test Milgromian dynamics (MOND) using wide binary stars (WBs) with separations of 2−30 kAU. Locally, the WB orbital velocity in MOND should exceed the Newtonian prediction by ≈20% at asymptotically large separations given the Galactic external field effect (EFE). 

We investigate this with a detailed statistical analysis of Gaia DR3 data on 8611 WBs within 250 pc of the Sun. Orbits are integrated in a rigorously calculated gravitational field that directly includes the EFE. We also allow line of sight contamination and undetected close binary companions to the stars in each WB. We interpolate between the Newtonian and Milgromian predictions using the parameter αgrav, with 0 indicating Newtonian gravity and 1 indicating MOND. 

Directly comparing the best Newtonian and Milgromian models reveals that Newtonian dynamics is preferred at 19σ confidence. Using a complementary Markov Chain Monte Carlo analysis, we find that αgrav=−0.021+0.065−0.045, which is fully consistent with Newtonian gravity but excludes MOND at 16σ confidence. This is in line with the similar result of Pittordis and Sutherland using a somewhat different sample selection and less thoroughly explored population model. 

We show that although our best-fitting model does not fully reproduce the observations, an overwhelmingly strong preference for Newtonian gravity remains in a considerable range of variations to our analysis. 

Adapting the MOND interpolating function to explain this result would cause tension with rotation curve constraints. We discuss the broader implications of our results in light of other works, concluding that MOND must be substantially modified on small scales to account for local WBs.

Indranil Banik, "Strong constraints on the gravitational law from Gaia DR3 wide binaries" arXiv:2311.03436 (November 6, 2023).

In other dark matter news, fuzzy dark matter theories (a very light boson with wave-like behavior dark matter theory) are compared to cold dark matter theories (with which all sorts of empirical evidence is inconsistent).

As I've noted often, but articulated less often, models with very light dark matter particles (especially bosonic ones) look a lot like quantum gravity theories with a graviton that has zero mass but non-zero mass-energy and self-interaction. Many axion-like dark matter theories, like fuzzy dark matter theories, lean towards this description. So, we are gradually starting to see dark matter theories converge on a mechanism that bears great similarities to a gravity based explanation of dark matter phenomena.

Finally, this paper is interesting:

The immense diversity of the galaxy population in the universe is believed to stem from their disparate merging histories, stochastic star formations, and multi-scale influences of filamentary environments. Any single initial condition of the early universe was never expected to explain alone how the galaxies formed and evolved to end up possessing such various traits as they have at the present epoch. However, several observational studies have revealed that the key physical properties of the observed galaxies in the local universe appeared to be regulated by one single factor, the identity of which has been shrouded in mystery up to date. 

Here, we report on our success of identifying the single regulating factor as the degree of misalignments between the initial tidal field and protogalaxy inertia momentum tensors. The spin parameters, formation epochs, stellar-to-total mass ratios, stellar ages, sizes, colors, metallicities and specific heat energies of the galaxies from the IllustrisTNG suite of hydrodynamic simulations are all found to be almost linearly and strongly dependent on this initial condition, when the differences in galaxy total mass, environmental density and shear are controlled to vanish. The cosmological predispositions, if properly identified, turns out to be much more impactful on galaxy evolution than conventionally thought.

Jun-Sung Moon, Jounghun Lee, "Why Galaxies are Indeed Simpler than Expected" arXiv:2311.03632 (November 7, 2023).