Dark matter -- Modified dynamics: Reaction vs. Prediction (1912.00716v1)

Abstract: The dark energy-cold dark matter paradigm ($\Lambda$CDM) has gained widespread acceptance because it explains the pattern of anisotropies observed in the cosmic microwave background radiation, the observed distribution of large scale inhomogeneities in detectable matter, and the perceived overall expansion history of the Universe. It is further {\it assumed} that the cosmic dark matter component clusters on the scale of bound astronomical systems and thereby accounts for the observed difference between the directly detectable (baryonic) mass and the total Newtonian dynamical mass. In this respect the paradigm fails; it is falsified by the existence of a simple algorithm, modified Newtonian dynamics (MOND), which explains, not only general scaling relations for astronomical systems, but quite precisely predicts the effective gravitational acceleration in such objects from the observed distribution of detectable baryonic matter -- all of this with one additional universal parameter having units of acceleration. On this sub-Hubble scale, the dark matter hypothesis is essentially reactive, while MOND is successfully predictive.

• The paper contrasts the standard Dark Matter paradigm with Modified Newtonian Dynamics (MOND) on sub-Hubble scales, arguing MOND provides a simpler and more predictive explanation for galaxy dynamics.
• Key findings highlight MOND's success in predicting the Radial Acceleration Relation (RAR) and galactic rotation curves solely based on baryonic matter, often surpassing Dark Matter models' reliance on post hoc fitting.
• Empirical evidence from diverse systems like high/low surface brightness galaxies and gas-dominated dwarf galaxies supports MOND's framework, reinforcing challenges to the necessity of dark matter on galactic scales.

Overview of "Dark Matter -- Modified Dynamics: Reaction vs. Prediction"

The paper by Robert H. Sanders explores the current paradigms in astrophysics related to dark matter and modified Newtonian dynamics (MOND), emphasizing the contrast between these models on sub-Hubble scales. The widely accepted dark energy-cold dark matter model ($\Lambda$CDM) accounts for key cosmological observations such as the anisotropies in the cosmic microwave background, the large-scale structure of the universe, and its expansion history. This model posits that a significant portion of the universe's mass-energy content is composed of dark matter, which clusters on astronomical scales to explain discrepancies between observable baryonic mass and the mass inferred from Newtonian dynamics.

In contrast, Sanders argues that $\Lambda$CDM falls short on the scale of galaxies and proposes MOND as a simpler and predictive alternative. MOND introduces a modification to Newtonian dynamics that adjusts gravitational acceleration based on local conditions, governed by a single new universal parameter with units of acceleration, $a_0$. This parameter is crucial in explaining the radial acceleration relation and the observed rotation curves of galaxies without invoking dark matter.

Key Findings

1. Predictive Power of MOND: MOND provides precise predictions for effective gravitational acceleration in astronomical systems based on baryonic matter distribution, often with greater predictive success than dark matter hypotheses which rely on post hoc fitting.
2. Radial Acceleration Relation (RAR): The empirical relationship between observed gravitational acceleration and that predicted by Newtonian dynamics from baryonic matter, known as the RAR, supports MOND's predictions. This relation is used to infer the interpolating function $\mu(g/a_0)$, providing theoretical consistency across a sample of 100 spiral galaxies.
3. Surface Density and Brightness: MOND predicts a critical surface density beyond which galaxy dynamics behave as expected in a Newtonian regime without dark matter. Empirical observations of galaxy kinematics, including the absence of dark matter in high surface brightness systems and its presence in low surface brightness systems, corroborate this.
4. Near-Isothermal Systems: The universality of the Faber-Jackson relation across a wide range of self-gravitating systems from molecular clouds to galaxy clusters aligns well with MOND's framework. Observations show that these systems have internal accelerations near the critical value $a_0$.
5. Gas-Dominated Dwarf Galaxies: In galaxies where baryonic mass is predominantly gas, and stellar contributions are negligible, MOND accurately predicts rotation curves purely from gas distribution, strengthening its credibility as it does so independently of unobserved components.

Implications and Future Prospects

Sanders' analysis suggests significant implications for our understanding of galactic dynamics and the fundamental laws governing them. The success of MOND in predicting detailed rotation curves and galaxy scaling relations challenges the dark matter paradigm, particularly at smaller scales. The presence of a consistent parameter $a_0$, linking local dynamics to cosmological scales, hints at potential theoretical bridges between modified dynamics and cosmological observations.

Future research could explore possible connections between MOND and dark matter theories, potentially reconciling their discrepancies. Further empirical validation across diverse astronomical objects may strengthen MOND’s standing or revise its formulations for broader applications. Advancements in observational technologies and techniques might also provide new data that could refine or challenge existing models. As such, the paper encourages continued scrutiny and kindles discussion on our understanding of gravity and mass distribution in the universe.