The Hubble Constant

[Ephram]

It is my understanding that the Hubble constant interprets as, we are moving away from everything else in the universe proportional to a given objects distance from us. For this to be true for all points in the universe, all objects need to be moving away from each other, no?

If that is true, how do galaxies collide? None would seem to be moving towards each other (hubble constant) so none can come in contact with each other.

[neufer]

It is my understanding that the Hubble constant interprets as, we are moving away from everything else in the universe proportional to a given objects distance from us. For this to be true for all points in the universe, all objects need to be moving away from each other, no? If that is true, how do galaxies collide? None would seem to be moving towards each other (hubble constant) so none can come in contact with each other.

There is a lot of scatter of velocities around the simple underlying Hubble expansion terms...especially within gravitationally bound clusters like the Virgo Cluster.

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Fit of redshift velocities to Hubble's law; patterned after William C. Keel (2007). The Road to Galaxy Formation. Berlin: Springer published in association with Praxis Pub., Chichester, UK. ISBN 3540725342.Various estimates for the Hubble constant exist. The HST Key H₀ Group fitted type Ia supernovae for redshifts between 0.01 and 0.1 to find that H₀ = 71 ± 2(statistical) ± 6 (systematic) km s−1Mpc−1, while Sandage et al. find H₀ = 62.3 ± 1.3 (statistical) ± 5 (systematic) km s−1Mpc−1.

[Ann]

Another way of putting this is that in galaxy clusters, the mutual gravity between the galaxies is stronger than the Hubble constant. That is particularly true toward the center of the galaxy clusters. Therefore it is not at all hard for galaxies to collide if they are close to the center of a galaxy cluster.

But the Milky Way and the Andromeda Galaxy are also close enough that their mutual gravity is stronger than the Hubble constant. That is why our galaxy may collide with the Andromeda galaxy a couple of billion years from now.

Ann