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

Friedmann Cosmology at T < 1 second

  1. Oct 31, 2007 #1

    Jorrie

    User Avatar
    Science Advisor
    Gold Member

    MTW section 27.7 states:
    It then proceeds to derive the relationships: [itex]\rho_m a^3 = [/itex] constant and [itex]\rho_v a^4 = [/itex] constant that pervect has shown before.

    My question: does this mean one cannot push the basic Friedmann equations that were discussed so exhaustively in the locked thread 'Firedmann Fun' to a time before t ~ 1 second without gross errors?

    If so, can someone please point to a fairly recent accessible paper showing how to deal with the time since inflation ended, up to 1 second? (Or better still, tell us how!:smile:)
     
  2. jcsd
  3. Oct 31, 2007 #2

    Wallace

    User Avatar
    Science Advisor

    We don't precisely know the physics of the very very early universe and therefore do not know the equations of state of the energy components and any couplings between them. This info is needed to solve the Friedmann equations.

    What would solving the Friedmann equations for T<1 second tell you anyway?
     
  4. Oct 31, 2007 #3

    Jorrie

    User Avatar
    Science Advisor
    Gold Member

    Thanks! Just thought it would be interesting to know how the densities evolved in that time bracket.
     
  5. Oct 31, 2007 #4

    jal

    User Avatar

    Density determines the Schwarzschild radius.
    It would be interesting to know how to get out of the black hole.
    jal
     
  6. Oct 31, 2007 #5

    Jorrie

    User Avatar
    Science Advisor
    Gold Member

    I do not think the homogeneous universe as a whole ever had or will have a Schwarzschild radius, because it's a local inhomogeneity phenomenon.
     
  7. Oct 31, 2007 #6

    Chris Hillman

    User Avatar
    Science Advisor

    Two corrections

    Actually, mass, not density. Specifically, in the Schwarzschild vacuum solution, the event horizon is located at [itex]r= 2 m[/itex] in geometric units, where m is a parameter whose interpretation involves some discussion but which, to shorten a longer story, can be identified with "mass" of the (nonrotating) black hole modeled by this solution (or even better, by closely related solutions such as the Oppenheimer-Snyder model of a collapsing dust ball).

    For various reasons (extensively discussed in well-known textbooks such as MTW), "density" can be tricky unless this refers to a scalar quantity (as in, a component of the stress-energy tensor, evaluted in a frame comoving with matter). Unfortunately you clearly can only be referring to a dubious notion of "density in the large", since the Schwarzschild vacuum is a vacuum solution.

    http://www.math.ucr.edu/home/baez/physics/Relativity/BlackHoles/universe.html

    In future, please try to perform some elementary fact checking before making unsupported claims.
     
    Last edited: Oct 31, 2007
  8. Oct 31, 2007 #7

    jal

    User Avatar

    Thank you Chris Hillman.
    That's what I'm trying to do... "fact checking"
    jal
    ---------
    I read your link
    I'll quote "Perhaps the truth is even stranger. In other words, who knows?"
    --------
    I'm now reading .... trying to get more insight into everyones opinions.
    http://arxiv.org/abs/0710.5721
    The radiation equation of state and loop quantum gravity corrections
    Authors: Martin Bojowald, Rupam Das
    (Submitted on 30 Oct 2007)
     
    Last edited: Oct 31, 2007
  9. Oct 31, 2007 #8

    Chris Hillman

    User Avatar
    Science Advisor

    Hi, jal, fair enough (and I appreciate the "thanks"!), but in future I suggest that you phrase such posts as "is the following correct?..." and "I know Wikipedia is unreliable but I happened to notice that article A in version V says in part... and wonder if this is correct".
     
    Last edited: Oct 31, 2007
  10. Oct 31, 2007 #9

    jal

    User Avatar

    I wish I had had the occasion to become a "math kid" like you. I would have loved to spend time over a beer with you.

    Sorry, out of time and got to run. Give me some insight on the bounce and black holes.
    jal
     
  11. Oct 31, 2007 #10

    pervect

    User Avatar
    Staff Emeritus
    Science Advisor


    My take on this is that while MTW has made this assumption that there is no interconversion of energy to matter, all that really *has* to be done is that both the dynamic and static Friedmann equations (which I called F1 and F2) must be satisfied. If both Friedmann equations are satisfied, the Einstein field equations will be satisfied.

    I think that one first must pick a spatial curvature factor K, which specifies the spatial part of the geometry, and optionally chose some value for the cosmological constant [itex]\Lambda[/itex]. Given this choice of K, one can subsequently specify any (well, actually any twice differentiable) function a(t) , and then use the Friedmann equations to compute rho(t) and P(t) from a(t) and K and [itex]\Lambda[/itex].

    However, some of these solutions, for instance solutions with rho(t) < 0, are probably not physically interesting.

    The trick is to find out which a(t) corresponds to a P(t) and rho(t) that has the desired relationship between P and rho.

    The bigger trick is to figure out some theoretical grounds for some "equation of state" that one expects P and rho to satisfy.
     
    Last edited: Nov 2, 2007
  12. Nov 1, 2007 #11

    Chris Hillman

    User Avatar
    Science Advisor

    If I can find time, I might try to post something like "Intro to FRW", or "Survey of Cosmological Models", or "Signals in Spacetimes" in an appropriate forum. The application of superstring theory to cosmological speculations doesn't interest me as much as the application to enumerative geometry, however.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?



Similar Discussions: Friedmann Cosmology at T < 1 second
  1. Friedmann equation (Replies: 3)

  2. Friedmann recantation (Replies: 7)

  3. Friedmann Equation (Replies: 9)

Loading...