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Galaxy clusters

  1. Jul 10, 2009 #1


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    Is there a rigorous definition for what constitutes a galaxy cluster or a super cluster, or are they just identified in an ad-hoc manner?
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  3. Jul 10, 2009 #2


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    Galactic clusters are gravitationally bound groups of galaxies. Gravitationally bound means their mutual gravitational attraction surpasses the repulsive effects of cosmological expansion [at least for the forseeable future]. Superclusters are gravitationally bound groups of galactic clusters. Again, their gravitational attraction appears to at least slow the effects of cosmological expansion. This is, however, an interesting question. See, for example:

    "The Influence of the Cosmological Expansion on Local Systems"
  4. Jul 10, 2009 #3


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    Cosmic expansion is not a replusive effect. You can call "dark energy" a repulsive effect; but it is far too small to be of much significance on the scale of a cluster.

    The proper definition of "gravitationally bound" is that the velocities of galaxies in the cluster is insufficient to escape gravitational attraction. It is precisely the same definition as used to tell whether a comet is gravitationally bound to the Sun.

    Cheers -- sylas

    PS. The paper you cite does not back up your proposed definition of gravitationally bound. What it does is say that the effects of expansion are not significant on the scales of a cluster, or less.
  5. Jul 14, 2009 #4
    Hi Sylus, what's wrong with saying cosmological expansion has "repulsive effects"? Expansion (with Dark Energy or without) is important and inhibits the growth of structure like galaxy clusters. As long as one doesn't associate "repulsion" with "force" it seems as good a word as any to describe what's going on.

  6. Jul 15, 2009 #5


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    Replusive effect suggests something driving the expansion, whereas this is not a necessary part of expansion at all.

    Dark energy confuses the issue, because, it is a "repulsive effect" that accelerates expansion (makes things expand more). But even before dark energy was a part of the standard model, we still have expansion. The difference is that a few decades ago, the questions were centered on the idea that the universe was slowing down in the expansion... and that means the "effect" is not repulsive at all, in any way. It is attractive. The effect considered was gravity and mutual attraction of massive objects... which SLOWS DOWN expansion as it pulls things together.

    It's true that expansion can inhibit the growth of clusters... but NOT because of a repulsive effect. It is rather because of expansion itself. Things that are moving apart from each other don't cluster together. Even with dark energy, that remains the case.

    Something started the universe expanding, to be sure... but that was way back when the universe was a hot "soup" of dense matter and energy. There was, we strongly suspect, a period of "inflation" in the very early universe that kicked off a rapid expansion. But that inflation stopped, and after that the expansion of the universe was not something driven by repulsive effects, but simply the nature of motion everything had been given back then. The major actual force involved is gravity (dark energy only became more significant than gravity after the universe was more than half its present age) and that is an attractive effect to pull clusters together. The main thing gravity has to overcome is NOT a repulsion effect at all, but only the motion itself of material in a universe where everything is moving apart from everything else.

    Cheers -- sylas
  7. Jul 15, 2009 #6
    I think were on the same page as far as the physics goes; I think it's the semantics that we were disagreeing on.

    Strictly speaking, I think you're correct, "repulsion" is typically associated with some external force (e.g. electric repulsion). However, in cosmology terms are thrown around that aren't strictly correct (e.g. hubble friction) but help one intuitively get a feel for a physical process.

    Yet, when people talk about repulsion in cosmology, they are usually referring to accelerating expansion (caused by, or at least parametrized by DE). So I think you're right, not because repulsion is technically incorrect (hubble friction is technically incorrect), but because people mostly use "repulsion" to describe accelerating expansion.
  8. Jul 15, 2009 #7
    In other words, it's ad-hoc.
  9. Jul 15, 2009 #8


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    Uh... no. This definition is not ad hoc at all.
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