Lecture 8 - Rotation of the Milky Way Galaxy
Dark matter, whose presence can only be inferred from its gravitational attraction, is pushing the boundaries of our known physics. This lecture explains how the first hint of the existence of this 'Dark Matter' came from examining the nature of the Rotation of our Galaxy.
All spiral galaxies rotate, and so does our Milky Way Galaxy. Around 1920, it was discovered that our galaxy rotates ‘differentially’, like the planets in the solar system. But in order to determine the precise law of rotation – how the angular velocity depends on the distance from the galactic center – one had to wait for the discovery of the 21 cm radiation from neutral hydron atoms in the interstellar medium. When the ‘rotation curves’ of galaxies were finally measured using the 21 cm radiation, it threw up a shocking result, namely, all galaxies are embedded in a giant halo of “Dark Matter”, whose nature is still unclear. This lecture is devoted to these exciting developments.