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

B Demystifying the Hubble Constant

  1. Aug 12, 2018 #1
    This year alone we have conflicting speeds for the Hubble Constant with 67.66 (+ or - 0.42) from the Planck Mission and 73.45 (+ or - 1.66) from the Hubble Space Telescope.

    The answer is simply found, and is between those figures.

    The furthest thing we can see (in theory) is 13.8 billion light years away, the Universe being that old.

    Divide 13.8 billion light years into one million megaparsec sections, as that is how the Hubble Constant is measured and you get 13.8 billion divided by 3.26 million equals just over 4,233.

    The speed of light in kms is 298,051 kms/sec which is the apparent speed of expansion at 13.8 billion light years from us. Divide that by 4,233 and you get a Hubble Constant of 70.409 kms/sec for expansion.
  2. jcsd
  3. Aug 12, 2018 #2


    Staff: Mentor

    No, this is not correct. The furthest thing we can see is about 47 billion light-years away. It has moved away from us all the time that the light it emitted 13.8 billion years ago was traveling to us, due to the universe expanding. This means that the simple relationship you are assuming between light travel time and distance does not hold (it only holds in flat spacetime, and the spacetime describing the universe as a whole is not flat).

    Huh? This is not the correct number; the correct number is 299,792.458 km/s.

    The rest of your post appears to be personal speculation based on the above mistake. Please review the PF rules on personal speculations.

    Thread closed.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook