Modified Newtonian Dynamics

In physics, Modified Newtonian dynamics (MOND) is a hypothesis that proposes a modification of Newton's law of gravity to explain the galaxy rotation problem. When the uniform velocity of rotation of galaxies was first observed, it was unexpected because Newtonian theory of gravity predicts that objects that are farther out will have lower velocities. For example, planets in the Solar System orbit with velocities that decrease as their distance from the Sun increases.

MOND was proposed by Mordehai Milgrom in 1983 as a way to model this observed uniform velocity data. Milgrom noted that Newton's law for gravitational force has been verified only where gravitational acceleration is large, and suggested that for extremely small accelerations the theory may not hold. MOND theory posits that acceleration is not linearly proportional to gravitational force at small values.

MOND stands in contrast to the more widely accepted theory of dark matter. Dark matter theory suggests that each galaxy contains a halo of an as yet unidentified type of matter that provides an overall mass distribution different from the observed distribution of normal matter. This dark matter accounts for the uniform rotation velocity data without modifying Newton's law of gravity.