Questions about Baryonic Density Perturbations

[cp05]

I am completely (or mostly) lost on these two questions.

For the first one, I'm supposed to describe how and why adiabatic perturbations of baryons ALONE cannot be responsible for structure formation in the expanding universe. I need two pieces of observational evidence and two facts from theory.

I have one for observations: the observed temperature fluctuations in the CMB are too low compared to those predicted by baryon perturbations.

And possibly one for theory: Inflationary theory predicts universe has flat geometry, so assuming a critical density of 1 then most of the mass in the universe is non-baryonic (dark matter).

Can you help me think of two more?


The second question asks me to describe what happens to baryonic density perturbations on physical scales corresponding to masses <<10^16 solar masses between the time they enter the particle horizon and to times well after the epoch of recombination.
I have no idea what this is even asking. How does something even enter the particle horizon?? And what does that have to do with the epoch of recombination??

Thanks so much for your help!

 

[cepheid]

I am completely (or mostly) lost on these two questions.

For the first one, I'm supposed to describe how and why adiabatic perturbations of baryons ALONE cannot be responsible for structure formation in the expanding universe. I need two pieces of observational evidence and two facts from theory.

I have one for observations: the observed temperature fluctuations in the CMB are too low compared to those predicted by baryon perturbations.

You need a theoretical element here to explain why this is problem for structure formation. That theoretical element is that the amplitude of the perturbation, Δ = δρ/ρ, grows linearly with scale factor in the matter dominated era (Δ ~ a). We also know that the perturbations are at a level of 10^-5 at the time of last-scattering (recombination). Therefore, linear perturbation theory says that today the amplitude of the perturbations should only be at a level of 10^-2, since the scale factor (a) grows by a factor of 1000 between then and now. This level is still too small to explain the structure we see today. Theory predicts that the universe should still be too smooth to allow for the presence of galaxies, stars, planets and people!

And possibly one for theory: Inflationary theory predicts universe has flat geometry, so assuming a critical density of 1 then most of the mass in the universe is non-baryonic (dark matter).

Okay, but in order to make this argument work, you have to state the observational evidence that that the entire critical density cannot be accounted for by baryons, i.e. that Ω_b < 1. One such piece of evidence is the constraint on the baryon density from primordial nucleosynthesis models (which are in pretty good agreement with all attempts to measure primordial abundances of the light elements so far).

[The second question asks me to describe what happens to baryonic density perturbations on physical scales corresponding to masses <<10^16 solar masses between the time they enter the particle horizon and to times well after the epoch of recombination.
I have no idea what this is even asking. How does something even enter the particle horizon?? And what does that have to do with the epoch of recombination??

Thanks so much for your help!

The particle horizon scale is just the largest distance over which light (and hence, information) can have traveled since the beginning of the universe. Earlier on, this size scale is small, so it is conceivable that the perturbation in question is larger than it. In other words, the perturbation doesn't entirely "fit" inside the particle horizon. Of course, as time goes on, the particle horizon scale grows, and eventually it will become big enough that it encompasses the entire perturbation. In other words, there is point at which the entire perturbation just fits inside the horizon. This is what we mean when we say that the perturbation "enters" the particle horizon.