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

Random thought

  1. Apr 5, 2009 #1

    nicksauce

    User Avatar
    Science Advisor
    Homework Helper

    We can write the Minkowski metric as
    [tex]ds^2 = -c^2dt^2 + d\mathbf{x}^2[/tex]
    or if we wanted different units for the metric
    [tex]ds^2 = -dt^2 + \frac{d\mathbf{x}^2}{c^2}[/tex]
    If we make c a function of time we have
    [tex]ds^2 = -dt^2 + \frac{d\mathbf{x}^2}{c(t)^2}[/tex]

    Which looks exactly like the FRW metric where c(t) = 1/a(t). So two questions: is my logic here correct? and if so, is it possible to tell the difference, in a purely gravitational way, between an FRW universe and a Minkowski universe with variable speed of light?
     
  2. jcsd
  3. Apr 5, 2009 #2

    Chronos

    User Avatar
    Science Advisor
    Gold Member

    looks about right to me, nick. just missing einstein's constant. Integrating these functions requires a constant to be mathematically complete.
     
  4. Apr 6, 2009 #3

    Chalnoth

    User Avatar
    Science Advisor

    Sure, because the speed of light is intimately coupled with the strength of the electromagnetic force, so all we have to do is observe to ensure that the properties of electromagnetism were the same in the early universe as they are today. The answer we find is that they agree to within about a percent, though there are some tentative hints that there are some very small deviations at very high redshift (e.g. a fraction of a percent at z=6 or so). This deviation is too small for a varying speed of light to explain the expansion.
     
  5. Apr 7, 2009 #4

    nicksauce

    User Avatar
    Science Advisor
    Homework Helper

    Is there an intuitive way to explain why a variable speed of light would lead to an expanding universe?
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Random thought
  1. Just a thought (Replies: 3)

  2. A Thought Experiment (Replies: 2)

  3. An impossible thought! (Replies: 3)

  4. Thought experiment (Replies: 9)

Loading...