Modified Newtonian dynamics

Modified Newtonian dynamics (MOND) is theory that proposes a modification of Newton's second law to account for observed properties of galaxies. Its primary motivation is to explain galaxy rotation curves without invoking dark matter, and is one of the most well-known theories of this class. However, it has not gained widespread acceptance, with the majority of astrophysicists supporting the Lambda-CDM model as providing the better fit to observations.^[1][2]

MOND was developed in 1982 and presented in 1983 by Israeli physicist Mordehai Milgrom.^[3] Milgrom noted that galaxy rotation curve data, which seemed to show that galaxies contain more matter than is observed, could also be explained if the gravitational force experienced by a star in the outer regions of a galaxy decays more slowly than predicted by Newton's law of gravity. MOND modifies Newton's laws for extremely small accelerations (characteristic of the outer regions of galaxies, or the inter-galaxy forces within galaxy clusters), fitting the galaxy rotation curve data.^{[4][dubious – discuss]}

Since Milgrom's original proposal, MOND has seen scattered successes. It is capable of explaining several observations in galaxy dynamics,^[5][6] some of which can be difficult for Lambda-CDM to explain.^[7] However, MOND struggles to explain a range of other observations, such as the acoustic peaks of the cosmic microwave background and the Bullet cluster; furthermore, because MOND is not a relativistic theory, it struggles to explain relativistic effects such as gravitational lensing and gravitational waves. Finally, a major weakness of MOND is that galaxy clusters show a residual mass discrepancy even when analyzed using MOND.^[5][8][9]

A minority of astrophysicists continue to work on the theory. Jacob Bekenstein developed a relativistic generalization of MOND in 2004, TeVeS, which however had its own set of problems. Another notable attempt was by Constantinos Skordis and Tom Złośnik, who proposed a relativistic model of MOND in 2021 that claimed to explain the cosmic microwave background, but appears to be highly contrived.^[1]