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B Hubble's Law Inquiry

  1. Jun 17, 2016 #1
    I am aware of the philosophical annotations with this question as some argue that science should only serve to explain how physical systems occur and not why; however if this question is not confined to this then I hope that there are answers. Is there an explanation to why Hubble's law exist? Or in other words, why is it the case that the further a body is from us, the faster it receeds from us?
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  3. Jun 17, 2016 #2


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    It's because the universe is homogeneous and isotropic. A homogeneous, isotropic universe follows Hubble's law.

    As to why it's homogeneous and isotropic, that's a harder question to answer. It seems likely that there was some mechanism that allowed the observable universe to come into equilibrium early-on, so that it had nearly the same density and temperature everywhere.
  4. Jun 17, 2016 #3
    Please excuse my ignorance but it seems to me that a homogeneous and isotropic universe could be maintained even if the recessional velocity of any body was constant and did not increase as the body gets further from us. Thus, I think that this doesn't explain the existence of Hubble's Law. Please correct me however.
  5. Jun 17, 2016 #4


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    If the recession velocity isn't proportional to distance, then it isn't homogeneous.

    Remember, to be homogeneous, it has to maintain the property that if you move to another location, the universe looks the same from that location.
  6. Jun 17, 2016 #5


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    Adding to what Chalnoth wrote, in a homogeneous universe, a galaxy that is twice as distant from us must recede at twice the rate of the nearer one, and so on. This is Hubble's law. If the reason is not clear to you, please Google the 'balloon analogy'.
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