Galactic Motion

About Expansion of the Universe: If the universe is expanding like a rubber balloon being inflated where all the spots are moving away from each other then how is it that some galaxies are on a collision course? I've read somewhere that this is because of the influence of the dark matter. Is this correct? If yes, could you pl. explain how?
- Ajit (age 65)
Pune, India

The expansion gives only the average large-scale picture. On a smaller scale, ordinary random motions are more important. These can be calculated rather well from simple Newtonian physics using the initial positions and velocities of galaxies and assuming they interact via just gravity. The dark matter is important because it provides most of the mass of the galaxies, so it's responsible for most of the gravity. There are other conceivable models, including some modified models of gravity that can fit the galactic dynamics data without requiring dark matter. The consensus, however, is that the best explanation for the combination of galactic dynamics and the details of the Cosmic Microwave Background ripple patterns is ordinary gravity with some dark matter.

Mike W.

(published on 12/24/2013)