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Hubble's law in a void

  1. Jul 23, 2009 #1
    Would the greatest voids in the universe support Hubble's law to a substantially lesser radius than the neighborhoods of galaxies?
  2. jcsd
  3. Jul 24, 2009 #2


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    Well, yes, I would tend to think that Hubble's law would apply at smaller distances in the absence of strong gravitational wells. My reasoning is thus:

    If you take an expanding universe that includes lots of stuff moving at various peculiar velocities, and ignore gravitational interaction, then over time those peculiar velocities tend to get damped by the expansion: as the universe expands, it more and more closely approaches Hubble's law.

    Why is this? Well, it's easiest to think of it in terms of one-dimensional expansion. If we take this case, and consider an object receding at a bit faster than the expansion rate for its distance, then over time it will catch up with objects further away that are moving faster, until it finds itself at just the right location, just by coasting, as stuff that's moving as fast as it is. If it's moving a bit slower, then stuff closer-in will catch up with it, and it will eventually find itself again at the right distance to match the Hubble flow.

    Gravitational interactions counteract this, of course, as when things fall into a gravitational well, they accelerate, and their orbits around gravitational wells are obviously non-Hubble motions. So in a void where there are fewer deep gravitational wells, I would tend to expect that the damping induced by the Hubble flow would indeed result in a Hubble law that holds more strongly. I can't say precisely what the magnitude of the effect would be without some calculations, of course, but it seems likely to be sound.
  4. Jul 25, 2009 #3

    What would the Hubble effect have on damping matter which undergoes "cosmological constant" acceleration? Without gravitational interaction, would large z bodies still tend towards damping's "self-fulfilling prophesy" of eradicating peculiar velocities (or perhaps peculiar accelerations)?
  5. Jul 25, 2009 #4


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    It doesn't really matter how the universe expands, as long as it expands, and the peculiar velocities tend to damp themselves out. So yes, it works in the presence of a cosmological constant. Or without one.

    Now, the existence of a cosmological constant affects the details, of course, and therefore affects how quickly it happens. But it doesn't affect the fact that it does happen.
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