1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Variation of Hubble constant in model universe

  1. Jan 12, 2014 #1
    1. The problem statement, all variables and given/known data

    For a problem I'm doing, I am considering a universe in which k=0, and I'm told that I can consider most of the expansion to have happened during a phase when only one of the density parameters was dominant (I know which one, as well), but I don't know the scale factor or the critical density.
    The density is dominated for most of the expansion by one component only, so I know how the scale factor varies with time (to a good approximation). I've completed what is required of me in the question but I can only get the answer in terms of H_0.

    Now, my question is: do you think it is appropriate to use the consensus value for H_0 in our universe? It worries me because our universe is not k=0 (but only approximately spatially flat) and also the approximation of radiation leading to most of the expansion is also an approximation. Or am I worrying about nothing as the answer isn't required to a great level of precision?
    Thanks in advance

    2. Relevant equations

    3. The attempt at a solution
  2. jcsd
  3. Jan 13, 2014 #2
    It's very appropriate to use the "direct" measurement value, for example http://arxiv.org/abs/1103.2976. This is based on distance laddering up to supernovae, and is fairly independent of any cosmological assumptions.

    I wouldn't use the Planck value though, as that has much more modelling baggage hidden behind it.
  4. Jan 13, 2014 #3
    Thanks, that might be the way to go. What I'm asked for in the question is a ratio of scale factors, between now and a given time in the past t_1 (for which I don't know the redshift). I can use the given value of t_1 but then I need to make an estimate of t_0, or I can get the answer in terms of H_0, as I mentioned. If I compare the answers using these two approaches, they are wildly different. I think perhaps using the value of t_0 in our universe is inappropriate, but if only I could work out how to get a numerical answer not involving H_0 or t_0...
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted