Galaxy rotation problem

The discrepancy between galaxies’ observed rotation speeds and those predicted from visible matter under standard gravitational theory. It is commonly explained by dark matter or, alternatively, by modified gravity.

Galaxy rotation curve: A graph of orbital speed versus distance from a galaxy’s center. Flat outer portions of these curves show the discrepancy most directly.
Vera Rubin: An American astronomer whose measurements established that spiral galaxies have unexpectedly flat rotation curves. Her observations made the mismatch compelling across many galaxies.
Lambda-CDM model: The standard cosmological model combining cold dark matter with a cosmological constant. It places galaxy rotation curves within a broader model of cosmic structure and dark matter.
Dark matter detection: The experimental and observational effort to identify dark matter through its effects or interactions. Galaxy dynamics provide gravitational evidence, though not a direct particle detection.
Newtonian gravity: A theory describing gravitational attraction between masses through Newton’s laws and law of universal gravitation. Visible matter under Newtonian gravity predicts declining speeds beyond most of a galaxy’s mass.
Kent Ford: An American astronomer who developed sensitive spectrographic methods used to measure galaxy rotation. His instruments enabled Rubin’s detailed velocity measurements.
MOND: An empirical framework associated with modified Newtonian dynamics that relates low-acceleration gravity to galaxy rotation. It is the best-known alternative framework for explaining galactic rotation without particle dark matter.
Gravitational lensing: The deflection and distortion of light by gravity from intervening mass. Lensing estimates galaxy mass independently of stellar and gas rotation.
Virial theorem: A relation connecting time-averaged kinetic and potential energies in a bound system. It helps estimate masses of gravitationally bound systems from their motions.
Horace Babcock: An American astronomer who reported a rising rotation curve in the outer regions of the Andromeda Galaxy in 1939. His early measurements hinted that outer galactic motions were unusual.

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