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Clustering Galaxies

  1. Aug 18, 2004 #1
    Question: When Galaxies are being drawn into clusters, does expansion of the universe enter into the equation? In other words, does expansion effect clustering? :shy:
  2. jcsd
  3. Aug 18, 2004 #2


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    Cosmological expansion does not affect gravitationally bound systems, like galaxy clusters.

    - Warren
  4. Aug 18, 2004 #3
    This is the hardest thing for me to understand in cosmology. From what I understand the expanding Universe moves galaxies apart. But mapping shows them moving towards each other. i sure want to find out where my thinking is wrong.
  5. Aug 19, 2004 #4


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    It might help to think about just the nearby ~100 Mpc of the universe ("Mpc" = megaparsec, a unit of distance which is approx 320 million light years). Near to us is the Virgo cluster of galaxies, a relatively modest cluster. The galaxies in this cluster attract each other, as they each have a mass of several million to several billion (and more) Msol (this is a unit of mass, equal to the mass of the Sun). This mutual attraction, together with whatever relative motions each proto-galaxy had when it was formed (either in the first ~few hundred million years after the Big Bang, or from the collision in which it was created), makes for a very complicated picture - galaxies whizzing this way and that, passing close by each other and changing their 'orbits', etc. However, the total mass of the cluster is large enough that no (or very few) galaxy can 'escape' the combined gravitational grip of the whole cluster.

    What about expansion of the universe? The Virgo galaxies, if there were no gravity, would expand away from each other, as they are part of the universe. However, there is gravity, and for the Virgo galaxies it is far 'stronger' than the universal expansion, so the net result is the Virgo cluster isn't 'pulled apart'.

    Note that I've presented a very simplified word picture - there are many other things going on which make it all quite complicated. However, if you set up a model - using equations from Relativity etc - it all becomes a lot clearer. In fact, there are simulations which show what happens to 'point masses' as the universe expands (sorry that I don't have a link to one for you) - you see that most quite quickly clump together (as clusters and super-clusters) and the clumps move apart as the universe continues to expand.
  6. Aug 20, 2004 #5
    Dark matter would increase the gravitational attraction. As I understand it, the strongest evidence for dark matter comes from cosmologists studying the behaviour of galaxies. although do they take into account the possible existence of black holes at the centre of each galaxy?
  7. Aug 20, 2004 #6


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    Indeed expansion does not affect clustering in a way that it is visible today, lets say, galaxies inside clusters moving apart from each other.

    But the dynamics of expansion affects the dynamics of clustering. As far as I know the cosmological models reveal that clusters tend to form faster or slower depending on the strengh of expansion.

    Specifically, clusters form easier in models with no acceleration of the expansion (no dark energy). One should be able to find more or less of clusters at high redshifts (number count) depending on whether expansion decellerated or accelerated during the formation of galaxy clusters.

    Deeper elaborations are behind my level, but as fas as I remenber there is a formula called Press-Schechter which may be used to describe this phenomenon.

  8. Aug 20, 2004 #7


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    The Local Groupe of gallaxies, in which we reside, is also held together by gravity.

    But the Virgo Cluster is moving away from the Local Groupe. Cosmic expansion is sufficient to move the Local Groupe away from Virgo, but will probably be overcome some day, and the Milky Way and all the rest of us will begin moving towards Virgo.

    So on the "small" scale, like superclusters and gallactic groupes, gravity prevails, pulling things closer together. And on the very large scale, like the distance between supurclusters, expansion dominates gravity, and moves things farther apart.

    And the distance between the Local Groupe and the Virgo Supercluster is kinda on the boarderline between those two.

    Hmmmm... Virgo Supercluster; does that have nougat and caramell?
  9. Aug 20, 2004 #8


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    Well, it's probably more astronomers studying galaxies :wink: (cosmologists only run computer models and make theories :tongue2:)

    On the BH in galactic nuclei: these are, in fact, the easiest to 'see' - the inferred motions of stars, gas clouds, etc in the inner parts of a galaxy are fairly easy to measure (lots of light), and the physics is very straight-forward (pre-dates Newton, in a way; Kepler's Laws). The dark matter which is 'observed' is via gravitational lensing (or other gravitational distortions in images of more distant objects), via galaxy rotation curves (the outer parts indicate far more mass than is inferred from the light), via cluster X-ray (assumed to come from inter-galactic gas in equilibrium), and via galaxy motions in a cluster (mass estimates via the Virial Theorem). The good news for cosmologists is that the amounts of dark matter so 'observed' is approx the same as they think they need to have in order for their models to match other observations. :smile:
  10. Aug 22, 2004 #9
    Yes expansion effects clustering
  11. Aug 23, 2004 #10
    Thanks for the replies, I sure would like to find the link to the simulations Neried mentioned. I'm away from home right now and look forward to having time at home to converse on this subject more.
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