Predictions and Outcomes for the Dynamics of Rotating Galaxies (2004.14402v1)

Abstract: A review is given of a priori predictions made for the dynamics of rotating galaxies. One theory - MOND - has had many predictions corroborated by subsequent observations. While it is sometimes possible to offer post hoc explanations for these observations in terms of dark matter, it is seldom possible to use dark matter to predict the same phenomena.

Dynamics of Rotating Galaxies and the MOND Paradigm

The paper by Stacy McGaugh critically examines the prevailing unresolved issues in extragalactic systems, particularly focusing on the anomalies in mass distribution that traditional dark matter theories have struggled to account for. It challenges the lambda cold dark matter (ΛCDM) model, which has failed to provide predictive success for the dynamics of rotating galaxies when compared to the Modified Newtonian Dynamics (MOND) framework.

Predicted Dynamics by MOND

One significant assertion in this paper is that MOND continues to align closely with empirical observations, elucidating aspects of galaxy dynamics that are problematic for dark matter. MOND proposes a modification of classical gravitational dynamics at low accelerations, offering a coherent explanation for galactic rotational curves without invoking dark matter. It asserts an empirical acceleration threshold—approximately $1.2 \times 10^{-10} \, \text{m s}^{-2}$—below which gravitational behavior diverges from Newtonian expectations. This threshold has remained stable across decades of observation.

A key element that MOND addresses is the Tully-Fisher relation, which correlates the luminosity of a galaxy to its rotational velocity. In the MOND context, this is reframed as the Mass-Asymptotic Speed Relation (MASR), predicting a strict relation between baryonic mass and rotational speed, with a slope of four times the velocity rather than the cubic relationship typically inferred by dark matter models. The paper highlights successes of MOND in predicting rotational velocities across a wide spectrum of galaxy masses, including those at lower ranges previously unmeasured.

Challenges to ΛCDM

The paper critically assesses the predictive inadequacies of ΛCDM, especially concerning varying mass distribution and rotational dynamics. While dark matter models require complex simulation environments—often modifying initial parameters to fit empirical data—MOND stands out by offering predictions prior to observations. This distinction underscores the need for reevaluating leading cosmological paradigms in favor of or integrating alternative explanations such as MOND or emerging theories involving novel dark matter candidates or modified gravity models.

MOND's consistent prediction of galaxy rotation curves' shapes, from high to low surface brightness, further challenges the necessity of dark matter dominance. It presents an archetype where rotation curves derived from observable mass distributions in galaxies correlate closely with observed curves without recourse to unroused dark matter influences. This offers a parsimonious alternative vis-à-vis the inherently variable interpretations permitted within the dark matter framework.

Implications and Future Directions

The findings compel the scientific community to reassess its stance on dark matter, fostering interest in the theoretical foundations that MOND offers. A significant implication arises from MOND's capability to address discrepancies in small-scale galactic dynamics that ΛCDM fails to resolve consistently. It opens avenues for revisiting the theoretical assumptions regarding gravity itself, as well as its interplay across cosmic scales. Future efforts may hinge on bridging the gap between MOND and general relativity, developing a cohesive model that encompasses both resulting in a more comprehensive cosmological theory.

The paper serves as an impetus for deepening explorations into alternative models for dark matter and gravity, potentially incorporating elements of superfluidity, emergent gravity, or novel particle theories that better accommodate empirical data. Furthermore, it calls for an intensified focus on laboratory-based searches for dark matter and efforts to reconcile observational data with theoretical expectations.

In conclusion, the paper by Stacy McGaugh is a thought-provoking critique of prevailing cosmological models, emphasizing the predictive successes of MOND and demanding fidelity in future research toward a more accurate understanding of galactic dynamics and cosmic structure formation.