A new paper notes a subtle, but quite general, flaw of conventional dark matter models, by comparing a little studied feature of actual hydrogen gas distributions that are observed with the distributions predicted by one of the leading dark matter simulations.
Atomic hydrogen (H I) gas, mostly residing in dark matter halos after cosmic reionization, is the fuel for star formation. Its relation with properties of host halo is the key to understand the cosmic H I distribution. In this work, we propose a flexible, empirical model of H I-halo relation. In this model, while the H I mass depends primarily on the mass of host halo, there is also secondary dependence on other halo properties. We apply our model to the observation data of the Arecibo Fast Legacy ALFA Survey (ALFALFA), and find it can successfully fit to the cosmic H I abundance (ΩHI), average H I-halo mass relation ⟨MHI|Mh⟩, and the H I clustering. The best fit of the ALFALFA data rejects with high confidence level the model with no secondary halo dependence of H I mass and the model with secondary dependence on halo spin parameter (λ), and shows strong dependence on halo formation time (a1/2) and halo concentration (cvir).
In attempt to explain these findings from the perspective of hydrodynamical simulations, the IllustrisTNG simulation confirms the dependence of H I mass on secondary halo parameters. However, the IllustrisTNG results show strong dependence on λ and weak dependence on cvir and a1/2, and also predict a much larger value of H I clustering on large scales than observations. This discrepancy between the simulation and observation calls for improvements in understanding the H I-halo relation from both theoretical and observational sides.
Zhixing Li, Hong Guo, Yi Mao, "Theoretical Models of the Atomic Hydrogen Content in Dark Matter Halos" arXiv:2207.10414 (July 21, 2022).
An analysis finds that modified gravity theories, generally are a better fit to the character of the data than dark matter particle or modified inertia theories. It is one of the rare papers to compare modified inertia theories to modified gravity theories.
Mass discrepancy in galaxies invokes dark matter, or alternatively modification of gravity or inertia. These theoretical possibilities may be distinguished by the statistical relation between the centripetal acceleration of particles in orbital motion and the expected Newtonian acceleration for the observed distribution of baryons in galaxies. Here predictions of cold dark matter halos, modified gravity, and modified inertia are compared and tested by a statistical sample of rotation curves of galaxies.
Modified gravity under an estimated mean external field correctly predicts the observed statistical relation of accelerations. Cold dark matter halos predict systematically deviating relations and modified inertia is inconsistent with the apparently seen difference between the inner and outer parts. All aspects of rotation curves are most naturally explained by modified gravity.
Kyu-Hyun Chae, "Distinguishing Dark Matter, Modified Gravity, and Modified Inertia by the Inner and Outer Parts of Galactic Rotation Curves" arXiv:2207.11069 (July 22, 2022).
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