PHYSICS 20B Lecture Notes - Lecture 23: Modified Newtonian Dynamics, Alternatives To General Relativity
4 Jun 2018
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PHYSICS 20B - Lecture 23 - Dark Matter, Modified Gravity, and Tidal Dwarf Galaxies
*Images from Pawlowski’s lecture
Volume of a cylinder
●r hV = π 2
Two Possibilities
●Visible matter + Newtonian Gravity: Galaxies would not be bound
1. There is more mass, we just can’t see it: Dark Matter
2. The gravity works differently on the scale of galaxies: Modified Gravity
One leading alternative: Modified Newtonian Dynamics (MOND)
●Suggests that at very low accelerations, the laws of gravity deduced on Earth and the
solar system are not correct anymore
○Using Newtonian Gravity for the dynamics of Galaxies is an extreme
extrapolation
●MOND is actively investigated by a small community of astronomer
Did we ever modify gravity? Two analogies from the Solar System
●Anomalies in the Orbit of Uranus lead to the prediction of a, as of then unseen, planet
(in a sense this was like postulating dark matter)
Document Summary
Physics 20b - lecture 23 - dark matter, modified gravity, and tidal dwarf galaxies. Visible matter + newtonian gravity: galaxies would not be bound: there is more mass, we just can"t see it: dark matter, the gravity works differently on the scale of galaxies: modified gravity. Suggests that at very low accelerations, the laws of gravity deduced on earth and the solar system are not correct anymore. Using newtonian gravity for the dynamics of galaxies is an extreme extrapolation. Mond is actively investigated by a small community of astronomer. Anomalies in the orbit of uranus lead to the prediction of a, as of then unseen, planet (in a sense this was like postulating dark matter) The planet (neptune) was discovered in 1846, where it was predicted to be. Anomalies in the precession of the planet mercury lead to the prediction of a planet dubbed vulcan .
