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

B Gravity waves and density perturbations

  1. Dec 15, 2016 #1
    I am totally confused. What is concerning the anisotropy the difference between re-entering gravity waves and re-entering density perturbations and more important for me what is then happening in the photon-baryon plasma between end of inflation and these re-enterings.
     
  2. jcsd
  3. Dec 15, 2016 #2

    Chalnoth

    User Avatar
    Science Advisor

    Density perturbations are perturbations in the density of matter and radiation. Gravity waves are perturbations in the curvature of space-time. The two are related, but not completely: because plasmas do not support twisting forces, gravity waves that lead to twisting forces don't interact with matter very much.

    By large, before the perturbations re-enter the horizon, they are frozen* by the rapid expansion and speed of light limitation. This means that there is no oscillation and the relative density between different regions stays about the same as the universe expands. Once the expansion slows down to the point that the waves can oscillate, they do: normal matter starts falling into overdense regions and bounces back out.

    * The perturbations aren't completely frozen, but they don't evolve very much.
     
  4. Dec 15, 2016 #3
    Thank you so far for your explanation. But I still don't understand the correlation between the acoustic waves which are laid down by the inflation which are oscillating during the radiation era and the re-entering ± 50.000 years later. Must I see them as a continuum?
     
  5. Dec 15, 2016 #4

    Chalnoth

    User Avatar
    Science Advisor

    They don't do any oscillation before re-entering the horizon. They do evolve somewhat, but not enough to oscillate.
     
  6. Dec 15, 2016 #5
    Then still there is the problem that the length of the basic tone is calculated right from the end of inflation till the sound horizon and not calculated from the re-entering till the sound horizon. Is the fluctuation in the radiation era a part of the total wavelength?
     
  7. Dec 15, 2016 #6

    Chalnoth

    User Avatar
    Science Advisor

    The waves still expand (increase in wavelength) during the period while they're outside the horizon. They just don't oscillate.
     
  8. Dec 15, 2016 #7
    Sorry, I'm not very clear. I am talking now about the subhorizon fluctuations and my question is what happens to them when perturbations re-enter the horizon. Does a great photon concentration from the radiation era now fall together with the baryons in a potential well of the re-entering perturbation?
     
  9. Dec 15, 2016 #8

    Chalnoth

    User Avatar
    Science Advisor

    If the density perturbation had a wavelength large enough that it didn't re-enter the horizon until after the radiation-dominated era, then yes, it will fall into the potential well only after the density perturbation re-enters the horizon.

    As I understand it, before the emission of the CMB, when the universe was a plasma, the photons and baryons moved together as a single fluid as they interacted strongly.
     
  10. Dec 15, 2016 #9
    Thank you. From your answer I now understand that perturbations were re-entering all the time, also before the beginning of the matter era.
     
  11. Dec 15, 2016 #10

    Chalnoth

    User Avatar
    Science Advisor

    Yes, depending upon their wavelengths.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted



Similar Discussions: Gravity waves and density perturbations
  1. Gravity waves? (Replies: 38)

  2. Gravity waves (Replies: 10)

  3. Density Perturbations (Replies: 1)

Loading...