Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Hubble constant

  1. Oct 18, 2011 #1
    i think Hubble constant is not a constant because of relativistic speeds of galaxies. is it?
  2. jcsd
  3. Oct 18, 2011 #2


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    You're right that the thing we call the Hubble parameter, H, which relates recessional velocity to proper distance, is not constant. It changes with time, and so it can be expressed as a function H(t). However, your reason is wrong: it has nothing to do with "relativistic speed" of galaxies. It changes with time in a manner determined by the Friedmann equations, which describe the large-scale dynamics of the universe in terms of its matter and energy content.

    The thing we we call the Hubble constant, H0, is the value of the Hubble parameter NOW. This is, in fact, a constant.

    Note that although H changes with time, it is constant throughout space (i.e. it does not vary with location). Therefore, H is the same everywhere at any given instant.
  4. Oct 19, 2011 #3
    this is not the correct answer!. what is NOW ? this indicates lack of understanding of theory of reletivity
  5. Oct 19, 2011 #4


    User Avatar
    Science Advisor
    Gold Member

    Asking a question you already 'know' the answer to is commonly referred to as trolling. Of course, that is not an issue in this case. I suggest a refresher course on the concept of 'NOW'.
  6. Oct 19, 2011 #5
    The space time coordinate of the current location.

    Be very careful on this forum with asserting that people don't understand things.
  7. Oct 19, 2011 #6
    sorry. what i mean is that the distant galaxy is not moving at calculated speed NOW but is was moving like that long back!
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook