MOND vs. dark matter in light of historical parallels (1910.04368v3)

Abstract: MOND is a paradigm that contends to account for the mass discrepancies in the Universe without invoking dark' components, such asdark matter' and `dark energy'. It does so by supplanting Newtonian dynamics and General Relativity, departing from them at very low accelerations. Having in mind readers who are historians and philosophers of science, as well as physicists and astronomers, I describe in this review the main aspects of MOND -- its statement, its basic tenets, its main predictions, and the tests of these predictions -- contrasting it with the dark-matter paradigm. I then discuss possible wider ramifications of MOND, for example the potential significance of the MOND constant, $a_0$, with possible implications for the roots of MOND in cosmology. Along the way I point to parallels with several historical instances of nascent paradigms. In particular, with the emergence of the Copernican world picture, that of quantum physics, and that of relativity, as regards their initial advent, their development, their schematic structure, and their ramifications. For example, the interplay between theories and their corollary laws, and the centrality of a new constant with converging values as deduced from seemingly unrelated manifestations of these laws. I demonstrate how MOND has already unearthed a number of unsuspected laws of galactic dynamics (to which, indeed, $a_0$ is central) predicting them a priori, and leading to their subsequent verification. I parallel the struggle of the new with the old paradigms, and the appearance of hybrid paradigms at such times of struggle. I also try to identify in the history of those established paradigms a stage that can be likened to that of MOND today.

Comparative Analysis of MOND and Dark Matter Paradigms

The paper by Mordehai Milgrom presents a detailed analysis of Modified Newtonian Dynamics (MOND) and its competition with the Dark Matter (DM) paradigm. This work is particularly insightful for researchers engaged in astrophysics and cosmology. Both paradigms attempt to address the mass discrepancies observed in the dynamics of galaxies and the universe at large, albeit through fundamentally different approaches.

Core Concepts and Comparison

MOND challenges conventional paradigms, proposing that discrepancies in observed stellar motion can be explained without resorting to dark components like DM and dark energy. It offers a modification to Newtonian dynamics and General Relativity at very low accelerations, introducing a constant $a_0$, which sets the boundary between Newtonian behavior and MOND dynamics.

Dark Matter, on the other hand, postulates the existence of non-baryonic matter that accounts for gravitational effects not explained by visible matter alone. In this view, galaxies and the universe contain substantial amounts of DM to reconcile the observed gravitational behavior with Newton's laws and Einstein's relativity.

MOND's Predictions and Observational Testing

The paper discusses various predictive laws within the MOND framework, many of which hinge on the scale invariance of the deep-MOND limit. Several notable predictions include:

• Asymptotic flatness of rotation curves: MOND predicts that rotational velocities become independent of radius for large-scale orbits.
• The mass-asymptotic-speed relation (MASR): Suggests a proportional relationship between the asymptotic speed and the mass of celestial bodies.
• Virial relations in self-gravitating systems: Predicts the relation between intrinsic speed and mass without dependence on system size.

Importantly, the paper illustrates that many of these predictions have been corroborated by astronomical observations, especially in analyzing galaxy rotation curves and the dynamics of dwarf galaxies. For instance, the MASR has shown consistent results with MOND when comparing observed versus predicted rotational speeds.

Critique of the Dark Matter Paradigm

Despite being the mainstream approach, DM is critiqued for several reasons:

1. Lack of detection: There is no direct evidence or successful detection of DM particles.
2. Assumptions and complexity: The DM paradigm requires complex assumptions about particle interactions which have not been empirically validated.
3. Correlative rather than predictive: DM explanations often retrospectively fit data rather than offer genuine predictions.

MOND advocates argue that the observed tight correlations between baryonic matter properties and dynamics align poorly with DM models that suggest baryonic and dark matter should evolve differently.

Historical Parallels in Scientific Paradigms

Milgrom draws historical analogies between the ascendancy of MOND and earlier scientific paradigm shifts such as the Copernican revolution or the development of quantum theory. Each faced significant resistance from entrenched theories but gradually emerged as superior tools to explain anomalous observations.

Implications and Future Directions

The work speculates on the deeper cosmological connections of MOND, noting its relation to cosmological constants like the Hubble constant and the cosmic acceleration linked to dark energy. This paper challenges researchers to rethink foundational paradigms in light of the evidential shortcomings of DM and the predictive successes of MOND.

Advancing the field will require bridging these paradigms or developing hybrid models that incorporate both dark matter's cosmological strengths and MOND's galactic-level predictive accuracy. Further observational and theoretical research is crucial for defining a more comprehensive understanding of cosmic dynamics and the fundamental forces at play.