Modified Newtonian dynamics

Modified Newtonian dynamics (MOND) modifies gravity or inertia below a characteristic acceleration to explain galaxy rotation without dark matter. It reproduces several regularities in galactic dynamics but faces challenges in clusters and cosmology.

MOND acceleration scale: The characteristic acceleration below which MOND predicts departures from Newtonian dynamics. It sets the threshold at which MOND’s modified behavior becomes significant.
Galaxy rotation curve: A plot of orbital speed against distance from the center of a galaxy. MOND aims to explain why outer galactic speeds remain unexpectedly high.
Dark matter: Matter inferred from gravitational effects but not observed through emitted or absorbed light. It explains galaxy rotation by adding unseen mass rather than modifying dynamics.
Mordehai Milgrom: An Israeli physicist who proposed MOND in 1983 to account for galaxy rotation without dark matter. He introduced the characteristic acceleration scale and core phenomenology.
Bullet Cluster: A merging galaxy cluster whose separated hot gas and gravitational lensing mass provide evidence for unseen gravitating matter. Its lensing pattern challenges simple MOND explanations without additional unseen mass.
Interpolation function: A function connecting Newtonian and modified dynamics across the transition between high and low accelerations. Its form determines how MOND predictions approach Newtonian gravity.
Baryonic Tully–Fisher relation: An empirical relation linking a spiral galaxy’s total baryonic mass to the fourth power of its rotation speed. MOND predicts this tight mass–speed relation from its low-acceleration dynamics.
Lambda-CDM model: The standard cosmological model combining cold dark matter with a cosmological constant. It fits cosmological observations that basic MOND does not address by itself.
Galaxy rotation problem: The mismatch between observed galactic rotation speeds and predictions based on visible matter under Newtonian gravity. This discrepancy motivated MOND’s original proposal.
Gravitational lensing: The bending of light by gravity, observable as distortions or multiple images of distant sources. Relativistic MOND must reproduce lensing signals as well as galaxy motions.

Linked from 21 pages