View Full Version : galaxy collision
2003-Dec-11, 01:57 PM
Check out the APOD for today: http://antwrp.gsfc.nasa.gov/apod/astropix.html
I am wondering about where the dark matter is and what effects it has in galaxy collisions. I thought dark matter is part of the repulsive force that keeps galaxies pushing away from each other. Enlighten me somebody!
2003-Dec-11, 02:45 PM
All I have to say on the subject is that, in my opinion, the evidence for dark matter and the evidence for galaxy collisions are based on the same false assumption; namely that gravity is the only force explaining the motions of galaxies. If electrical forces are included, there is no need for either dark matter or collisions. They should be extremely rare anyway, because an expanding Universe doesn't allow for the enormous amount of collisions that have been assumed already.
2003-Dec-11, 04:12 PM
Dark matter, if any, is supposed to increase the attractive nature of gravitational fields as is dark energy, if any. For aesthetic purposes, it is comfortable to assume they provide a flat or Euclidian geometry for space-time. The sum of all mass and energy is supposed to work in opposition to whatever is causing the expansion. This may be larger than we can get our arms around at the moment. It's hard to know whether we should grow longer arms or shrink the problem---not that we know how to do either. :unsure:
2003-Dec-12, 08:29 AM
Dark matter composes the majority of matter in the universe. As such it contributes to the total gravitational field of the universe. This field counters the expansion caused by the initial Big Bang and the consequent period of inflation.
Dark energy or quintessence is a repulsive force that has been hypothesized to account for the accelerating expansion of the universe indicated by the study of type Ia supernovae. Prior to this data it was assumed that the expansion of the universe was slowing due to gravitation.
Galactic collisions take place on a local scale and do not in any way contradict what is known as the Hubble Flow, the separation of galaxies due to the expansion of the universe as a whole. Galaxies within clusters and superclusters are subject to gravitational attraction and can indeed exhibit blue shifts. This is exactly what is happening in the Local Group with M31 and the Milky Way, which are expected to merge in a few billion years or so. When the universe was a much smaller place such mergers were commonplace.
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