From galactic bars to the Hubble tension: weighing up the astrophysical evidence for Milgromian gravity (2110.06936v9)

Abstract: Astronomical observations reveal a major deficiency in our understanding of physics $-$ the detectable mass is insufficient to explain the observed motions in a huge variety of systems given our current understanding of gravity, Einstein's General theory of Relativity (GR). This missing gravity problem may indicate a breakdown of GR at low accelerations, as postulated by Milgromian dynamics (MOND). We review the MOND theory and its consequences, including in a cosmological context where we advocate a hybrid approach involving light sterile neutrinos to address MOND's cluster-scale issues. We then test the novel predictions of MOND using evidence from galaxies, galaxy groups, galaxy clusters, and the large-scale structure of the Universe. We also consider whether the standard cosmological paradigm ($\Lambda$CDM) can explain the observations and review several previously published highly significant falsifications of it. Our overall assessment considers both the extent to which the data agree with each theory and how much flexibility each has when accommodating the data, with the gold standard being a clear $a~priori$ prediction not informed by the data in question. Our conclusion is that MOND is favoured by a wealth of data across a huge range of astrophysical scales, ranging from the kpc scales of galactic bars to the Gpc scale of the local supervoid and the Hubble tension, which is alleviated in MOND through enhanced cosmic variance. We also consider several future tests, mostly on scales much smaller than galaxies.

• The paper demonstrates that MOND naturally accounts for galaxy rotation curves and the baryonic Tully-Fisher relation, reducing the need for dark matter fine-tuning.
• It employs observational tests like the radial acceleration relation and external field effect to substantiate MOND’s predictive accuracy.
• The analysis spans galaxies to clusters, suggesting a hybrid MOND model could address challenges such as the Hubble tension and ΛCDM discrepancies.

Astrophysical Evidence for Milgromian Gravity

The paper by Banik and Zhao systematically assesses the astrophysical evidence for Milgromian dynamics (MOND), critically evaluating MOND against the standard $\Lambda$ Cold Dark Matter ($\Lambda$CDM) paradigm across various scales and astrophysical phenomena. This review provides a detailed and comparative analysis of these two paradigms in light of recent and comprehensive observational data, considering phenomena ranging from galaxy dynamics to large-scale cosmic structures.

Rotation Curves of Disc Galaxies

The rotation curves (RCs) of disc galaxies remain pivotal in testing gravitational theories. MOND demonstrates significant predictive success across many galaxies, directly linking baryonic mass to observed dynamics without invoking non-baryonic dark matter. The paper highlights the consistent fitting of MOND to RCs and its capability to account for observed correlations such as the baryonic Tully-Fisher relation (BTFR). Conversely, $\Lambda$CDM requires extensive fine-tuning of halo parameters to accommodate these phenomena, lacking the same level of predictability.

Radial Acceleration Relation

A cornerstone in this analysis is the radial acceleration relation (RAR), which reveals a tight correlation between the observed centripetal acceleration in galaxies and the predicted Newtonian acceleration due to baryonic matter. MOND naturally predicts this relation, and the observed consistency in the RAR across diverse galaxies bolsters its standing. In contrast, $\Lambda$CDM's statistical modeling struggles with the tightness and universality of the RAR, primarily due to the stochastic nature of baryonic feedback processes affecting dark matter halo profiles.

Dynamics of Elliptical and Dwarf Galaxies

The paper extends its analysis to elliptical and dwarf spheroidal galaxies, which serve as critical tests due to their dynamic environments and lower accelerations. Here, MOND continues to perform well, aligning with the observed velocity dispersion profiles and the BTFR even in lower surface brightness galaxies. Challenges remain in reconciling $\Lambda$CDM predictions with the observed dynamics without invoking substantial amounts of unseen baryonic or non-baryonic matter.

External Field Effect

Crucial to MOND is the External Field Effect (EFE), a non-conventional prediction that the internal dynamics of a system are influenced by external gravitational fields. The authors showcase substantial empirical support for the EFE by linking discrepancies in galaxy RCs within dense environments to external gravitational influences, a prediction unique to MOND.

Implications for Galaxy Clusters and Large-Scale Structures

Galaxy clusters present a complex challenge where MOND alone does not fully account for the observed gravitational binding, requiring additional mass such as light sterile neutrinos. This hybrid approach is posited to reconcile MOND with the $\Lambda$CDM paradigm's successes on larger scales, like the cosmic microwave background (CMB) anisotropies.

Moreover, large-scale structures exhibit more pronounced features than anticipated in $\Lambda$CDM, with the paper highlighting the existence of the KBC void and the associated local Hubble tension as significant challenges for standard cosmology. The MOND framework may provide alternative insights by considering a more rapid structure formation, potentially alleviating these tensions.

Future and Conclusion

This extensive comparison culminates in Banik and Zhao advocating for continued exploration of MOND, particularly through future tests encompassing wide binaries and cosmic expansion history clarifications. They also emphasize the potential need for MOND to integrate further components beyond modified gravity to fully describe cosmological phenomena across scales, an approach that may ultimately enrich the current gravitational framework.

In summary, this paper underscores the efficacy of MOND in providing a consistent framework for galaxy-scale phenomena, urging further investigation and the potential integration of hybrid approaches to reconcile MOND with observed large-scale structures, a field where $\Lambda$CDM remains compelling.