| Watsisname | Date: Saturday, 02.03.2013, 01:42 | Message # 16 |
 Galaxy Architect
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| Perhaps an easy way to make sense of what is going on is to look at the data. 
This is Edwin Hubble's original figure for radial velocity versus distance of nearby galaxies, published in 1929. The first thing you notice is there is a strong linear trend, showing that the more distant a galaxy is, the faster it is receding.
But there's a bit more to it than that. You'll also notice there is some deviation from the trend-line, with some points above it and others below. This is because there are two factors that determine the radial velocity of a galaxy:
1: Redshift due to the expansion of the universe (which makes the trend-line)
2: Doppler shift due to the galaxy's physical motion through space (this is known as 'peculiar velocity') and causes vertical deviation from the trend-line.
For galaxies that are close to one another, the effect of universal expansion can be smaller than the peculiar velocity, and if the peculiar velocity is negative then the galaxies are approaching one another. Thus why there are a few points on the diagram below the horizontal axis.
Another way to think of it is that for objects sufficiently close together, other forces may overwhelm the effect of universal expansion, so they remain together. Galaxies (and even galaxy clusters) aren't expanding because they are bound by their mutual gravitation. The Earth isn't expanding for the same reason. Our bodies aren't expanding because we are bound by electrical forces. The expansion of the universe is only relevant over very large distances.
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