Another Voice In the Gravity And Tully-Fischer Conversation (And More)

None of this is unfamiliar to me, but it is nice to see more people having this epiphany. Of course, the next step that this author needs to take after this initial baby step is to imagine what kind of physics would be necessary to produce this kind of structure.

The flattening of spiral-galaxy rotation curves is unnatural in view of the expectations from Kepler's third law and a central mass. It is interesting, however, that the radius-independence velocity is what one expects in one less dimension. In our three-dimensional space, the rotation curve is natural if, outside the galaxy's center, the gravitational potential corresponds to that of a very prolate ellipsoid, filament, string, or otherwise cylindrical structure perpendicular to the galactic plane. While there is observational evidence (and numerical simulations) for filamentary structure at large scales, this has not been discussed at scales commensurable with galactic sizes. If, nevertheless, the hypothesis is tentatively adopted, the scaling exponent of the baryonic Tully--Fisher relation due to accretion of visible matter by the halo comes out to reasonably be 4. At a minimum, this analytical limit would suggest that simulations yielding prolate haloes would provide a better overall fit to small-scale galaxy data.

Felipe J. Llanes-Estrada, "Elongated Gravity Sources as an Analytical Limit for Flat Galaxy Rotation Curves" 7(9) Universe 346 arXiv:2109.08505 (September 16, 2021) DOI: 10.3390/universe7090346

UPDATE September 21, 2021

A couple more articles in the same vein. 

The first is very akin to Deur's effort to infer dark sector phenomena from an analysis of General Relativity that grapples with the removal of simplifying assumptions often used to make it possible to obtain a clean analytic solution, although this approach is inspired by statistical mechanics rather than by quantum chromodynamics.

Inspired by the statistical mechanics of an ensemble of interacting particles (BBGKY hierarchy), we propose to account for small-scale inhomogeneities in self-gravitating astrophysical fluids by deriving a non-ideal Virial theorem and non-ideal NavierStokes equations. These equations involve the pair radial distribution function (similar to the two-point correlation function used to characterize the large-scale structures of the Universe), similarly to the interaction energy and equation of state in liquids. Within this framework, small-scale correlations lead to a non-ideal amplification of the gravitational interaction energy, whose omission leads to a missing mass problem, e.g., in galaxies and galaxy clusters. 

We propose to use a decomposition of the gravitational potential into a near- and far-field component in order to account for the gravitational force and correlations in the thermodynamics properties of the fluid. Based on the non-ideal Virial theorem, we also propose an extension of the Friedmann equations in the non-ideal regime and use numerical simulations to constrain the contribution of these correlations to the expansion and acceleration of the Universe. 

We estimate the non-ideal amplification factor of the gravitational interaction energy of the baryons to lie between 5 and 20, potentially explaining the observed value of the Hubble parameter (since the uncorrelated energy account for ∼ 5%). Within this framework, the acceleration of the expansion emerges naturally because of the increasing number of sub-structures induced by gravitational collapse, which increases their contribution to the total gravitational energy. A simple estimate predicts a non-ideal deceleration parameter qni ' -1; this is potentially the first determination of the observed value based on an intuitively physical argument. We show that another consequence of the small-scale gravitational interactions in bound structures (spiral arms or local clustering) yields a transition to a viscous regime that can lead to flat rotation curves. This transition can also explain the dichotomy between (Keplerian) LSB elliptical galaxy and (non-Keplerian) spiral galaxy rotation profiles. Overall, our results demonstrate that non-ideal effects induced by inhomogeneities must be taken into account, potentially with our formalism, in order to properly determine the gravitational dynamics of galaxies and the larger scale universe. 

P. Tremblin, et al., "Non-ideal self-gravity and cosmology: the importance of correlations in the dynamics of the large-scale structures of the Universe" arXiv:2109.09087 (September 19, 2021) (submitted to A&A, original version submitted in 2019).

The second generalized modified gravity approaches to explaining dark matter in galaxies in the traditional geometric paradigm.

We obtain more straightforwardly some features of dark matter distribution in the halos of galaxies by considering the spherically symmetric space-time, which satisfies the flat rotational curve condition, and the geometric equation of state resulting from the modified gravity theory. In order to measure the equation of state for dark matter in the galactic halo, we provide a general formalism taking into account the modified f(X) gravity theories. Here, f(X) is a general function of X∈{R,,T}, where R, and T are the Ricci scalar, the Gauss-Bonnet scalar and the torsion scalar, respectively. These theories yield that the flat rotation curves appear as a consequence of the additional geometric structure accommodated by those of modified gravity theories. Constructing a geometric equation of state wX≡pX/ρX and inspiring by some values of the equation of state for the ordinary matter, we infer some properties of dark matter in galactic halos of galaxies.

Ugur Camci, "On Dark Matter As A Geometric Effect in the Galactic Halo" arXiv:2109.09466 366 Astrophys. Space. Sci. 91 (September 17, 2021) DOI: 10.1007/s10509-021-03997-5

While not in the same vein, so it doesn't get lost in the shuffle, I also note a new paper which identifies a potential source of systemic error that could help explain the discrepancy in the Hubble constant measurements at high z and low z. 

While papers with new physics explanations for the Hubble constant anomaly abound, given the history of prior Hubble constant anomalies, and prior anomalies in fundamental physics generally, papers identifying potential sources of systemic error deserve outsized attention, because most anomalies in fundamental physics are resolved by discovering them.

The bias in the determination of the Hubble parameter and the Hubble constant in the modern Universe is discussed. It could appear due to statistical processing of data on galaxies redshifts and estimated distances based on some statistical relations with limited accuracy. This causes a number of effects leading to either underestimation or overestimation of the Hubble parameter when using any methods of statistical processing, primarily the least squares method (LSM). The value of the Hubble constant is underestimated when processing a whole sample; when the sample is constrained by distance, especially when constrained from above, it is significantly overestimated due to data selection. The bias significantly exceeds the values of the error the Hubble constant calculated by the LSM formulae.

These effects are demonstrated both analytically and using Monte Carlo simulations, which introduce deviations in both velocities and estimated distances to the original dataset described by the Hubble law. The characteristics of the deviations are similar to real observations. Errors in estimated distances are up to 20%. They lead to the fact that when processing the same mock sample using LSM, it is possible to obtain an estimate of the Hubble constant from 96% of the true value when processing the entire sample to 110% when processing the subsample with distances limited from above.

The impact of these effects can lead to a bias in the Hubble constant obtained from real data and an overestimation of the accuracy of determining this value. This may call into question the accuracy of determining the Hubble constant and significantly reduce the tension between the values obtained from the observations in the early and modern Universe, which were actively discussed during the last year.

S.L.Parnovsky "Bias of the Hubble constant value caused by errors in galactic distance indicators" arXiv:2109.09645 (September 20, 2021) (Accepted for publication at Ukr. J. Phys).