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

What is the current value of the scale factor?

  1. Jul 11, 2008 #1
    Do cosmologists know what is the current value of the scale factor? I know we have a current value for the hubble constant H, and I know H=a'/a. But what about the value of a?

  2. jcsd
  3. Jul 11, 2008 #2


    User Avatar
    Staff Emeritus
    Science Advisor

  4. Dec 20, 2011 #3
    What is the current value of the scale factor(R₀)?

    a= R(t)/R₀

    Give me the number.

  5. Dec 20, 2011 #4


    User Avatar
    Gold Member

    R0= 1, which is a dimensionless number. Why? Because I define it as 1. I can define it as any present number I want to, and it will not change the physics. I can arbitrarily define R at one time only; the value at all other times will scale with my choice of R at the single time I selected.
    Last edited: Dec 20, 2011
  6. Dec 20, 2011 #5
    Yeah, the scale factor is traditionally set to unity at the current time. In other words, a(t=2011 A.D.) = 1. Then you can apply the cosmology of your choice (e.g. flat universe with cosmological constant) and answer questions such as how old was the universe when things were 1000 times closer together: a(time in past) / a (now) = 1 / 1000 and solve for the time in the past.
  7. Dec 21, 2011 #6
    Thanks, but here appears another question. So why do we define a=R/R0 at all, when R0=1? It means a(t)=R(t), and it is just changing names, isn't it?!
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook