Gravity waves and density perturbations

[Discman]

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.

 

[Chalnoth]

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.

 

[Discman]

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?

 

[Chalnoth]

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?

They don't do any oscillation before re-entering the horizon. They do evolve somewhat, but not enough to oscillate.

 

[Discman]

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?

 

[Chalnoth]

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?

The waves still expand (increase in wavelength) during the period while they're outside the horizon. They just don't oscillate.

 

[Discman]

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?

 

[Chalnoth]

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?

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.

 

[Discman]

Thank you. From your answer I now understand that perturbations were re-entering all the time, also before the beginning of the matter era.

 

[Chalnoth]

Yes, depending upon their wavelengths.