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

Definition of Energy in Friedmann equations?

  1. Nov 13, 2015 #1
    The first Friedmann equation for a flat Universe is given by:

    $$\bigg(\frac{\dot{a}(t)}{a(t)}\bigg)^2 = \frac{8 \pi G}{3} \rho(t)$$

    The energy density ##\rho(t)## is given by:

    $$\rho(t) \propto \frac{E(t)}{a(t)^3}$$

    where ##E(t)## is the energy of the cosmological fluid in a co-moving volume.

    Is the energy ##E## the energy measured by a local observer at time ##t## or is it the energy measured with respect to a (global) reference observer at the present time ##t_0## where ##a(t_0)=1##?
     
  2. jcsd
  3. Nov 13, 2015 #2

    PeterDonis

    User Avatar
    2016 Award

    Staff: Mentor

    No, you have it backwards. The energy density ##\rho(t)## is a direct observable (it is the energy density measured by a comoving observer); it isn't derived from anything. If you want to try to define a "total energy" ##E(t)##, then you could do it along the lines of ##E(t) \propto \rho(t) a(t)^3##. But the physical meaning, if any, of such an ##E(t)## would be an interesting question.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Definition of Energy in Friedmann equations?
  1. Friedmann equation (Replies: 3)

  2. Friedmann Equation (Replies: 9)

Loading...