Modified Dynamics and Galactic Structure

Astrophysical observations of galaxies reveal persistent discrepancies between the gravitational acceleration inferred from luminous matter and the motions of stars and gas. Traditional remedies invoke non-baryonic dark matter, yet an alternative paradigm modifies the laws of gravity or inertia in the weak-acceleration regime. Such modified dynamics naturally account for the flat rotation curves of spiral galaxies and reproduce empirical scaling relations, most notably the baryonic Tully–Fisher relation and the radial acceleration relation. Extensions of the basic framework embed these modifications within relativistic theories, enabling the study of phenomena such as gravitational lensing, cosmological evolution and cluster dynamics. Tests at different scales—from wide binary stars in the Solar neighbourhood to weak-lensing maps of galaxy haloes—probe the universality and limits of the modified law. In parallel, high-resolution simulations, improved mass models and kinematic surveys refine our understanding of how modified dynamics shapes galactic structure, bar formation and disc stability. This field thus bridges fundamental gravitational physics and the observed diversity of galaxy morphologies, offering concrete predictions that can be confronted by upcoming observational facilities.

Research from Nature Portfolio

No recent Nature Portfolio content available.

Modified Dynamics and Galactic Structure publication trend

The graph below shows the total number of articles in modified dynamics and galactic structure across all publications each year (not limited to Nature Index journals).

Technical terms

Modified Newtonian Dynamics (MOND): A theoretical framework positing that Newton’s second law or the law of gravitation alters below a characteristic acceleration a₀, thereby explaining galaxy rotation curves without invoking dark matter.

Baryonic Tully–Fisher relation: An empirical scaling law linking the total baryonic mass of a spiral galaxy to the fourth power of its asymptotic circular velocity, reflecting a universal mass–velocity coupling.

Radial Acceleration Relation (RAR): A tight empirical correlation between the observed centripetal acceleration in galactic rotation curves and that predicted solely by the distribution of visible matter.

External Field Effect (EFE): A prediction of certain modified gravity theories that a system’s internal dynamics can depend on an external gravitational field, in tension with the strong equivalence principle.

Virial radius: The radius within which the system’s self-gravity, whether in dark matter or modified gravity contexts, maintains dynamical equilibrium and binds the outer parts of galaxies or clusters.

References

1. Indefinitely Flat Circular Velocities and the Baryonic Tully–Fisher Relation from Weak Lensing. The Astrophysical Journal Letters (2024).
2. Breakdown of the Newton–Einstein Standard Gravity at Low Acceleration in Internal Dynamics of Wide Binary Stars. The Astrophysical Journal (2023).
3. Radial acceleration relation of galaxies with joint kinematic and weak-lensing data. Journal of Cosmology and Astroparticle Physics (2024).
4. Modified Newtonian Dynamics (MOND): Observational Phenomenology and Relativistic Extensions. Living Reviews in Relativity (2012).
5. From Galactic Bars to the Hubble Tension: Weighing Up the Astrophysical Evidence for Milgromian Gravity. Symmetry (2022).