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

Redshift and Hubble constant

  1. Jul 31, 2009 #1
    Which statement is true?

    A The apparent speed of recession of a galaxy is given by the product of the Hubble constant and the distance to the galaxy.

    B The further away a galaxy is, the lower its apparent speed of motion away from us.

    C A galaxy with a redshift of 5.37 is situated at a look-back time of 4.1 billion years.

    D Space is expanding uniformly so that the distance between galaxies remains constant.
  2. jcsd
  3. Jul 31, 2009 #2


    User Avatar
    Science Advisor

    Which statment(s) do you think is/are true and why?
  4. Jul 31, 2009 #3
    I think statement A is true but I'm not 100% sure whether D is false and there is only one true statement.
  5. Jul 31, 2009 #4
    Well, to rule out C you can have a look at Ned Wrights Javascript cosmology calculator to determine the lookback time. Just plug in 5.37 for the redhshift. The light travel time is the "lookback time".

  6. Jul 31, 2009 #5


    User Avatar
    Science Advisor

    Well, isn't it generally true that the farther a galaxy is from us, the faster it appears to be receding?
  7. Jul 31, 2009 #6
    Yeah, I didn't think B or C were the answers.
    I'm still stuck between whether A or D is the right answer and I really need help.
  8. Jul 31, 2009 #7


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Does it make any sense for the distance between things to remain constant in an *expanding* universe???

    When we say that a galaxy is moving away from us, what does that mean? Does it not mean that it is getting farther away with time?
  9. Sep 22, 2009 #8
    Statement A is definitely correct.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook