Questions about bayronic acoustic oscillations

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Buzz Bloom
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Summary:

I have been trying to understand the abstract and introduction to the paper
https://arxiv.org/pdf/astro-ph/0501171.pdf
DETECTION OF THE BARYON ACOUSTIC PEAK IN THE LARGE-SCALE CORRELATION FUNCTION OF SDSS LUMINOUS RED GALAXIES.
I made an effort to find in the paper the answers to several questions, but I did not succeed. It may be that the answers could be in papers cited at the end of the article, but I do not have access to the journals cited.

Main Question or Discussion Point

Summary: I have been trying to understand the abstract and introduction to the paper
https://arxiv.org/pdf/astro-ph/0501171.pdf
DETECTION OF THE BARYON ACOUSTIC PEAK IN THE LARGE-SCALE CORRELATION FUNCTION OF SDSS LUMINOUS RED GALAXIES.
I made an effort to find in the paper the answers to several questions, but I did not succeed. It may be that the answers could be in papers cited at the end of the article, but I do not have access to the journals cited.

I would much appreciate any help readers of this thread might be able to provide. Here is a quote (citations omitted) from the introduction that raised the questions in my mind.

Because the universe has a significant fraction of baryons, cosmological theory predicts that the acoustic oscillationsin the plasma will also be imprinted onto the late-time power spectrum of the non-relativistic matter. A simple way to understand this is to consider that from an initial point perturbation common to the dark matter and the baryons, the dark matter perturbation grows in place while the baryonic perturbation is carried outward in an expanding spherical wave. At recombination, this shell is roughly 150 Mpc in radius.​
Because the central perturbation in the dark matter is dominant compared to the baryonic shell, the acoustic feature is manifested as a small single spike in the correlation function at 150 Mpc separation.​

Below are the questions.
1. What is the standard deviation for this 150 Mpc estimate of the end radius of a propagation spherical shell?​
2. What caused the start of each spherical shell of acoustic propagation? (With my limited imagination, I am guessing it was something like an explosion of some sort. How would this relate to a perturbation in density?)​
3. At what universe age did these acoustic waves begin?​
4. Am I correct that the propagation of these waves ended at the time of recombination (which is the beginning of the CMB)?​
5. What were the speeds of propagation at the start and end of propagation?​
6. Did the speed of propagation change with changes in temperature and density as related to the scale factor?​
 

Answers and Replies

  • #2
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Not sure if this will answer all your questions but PBS Spacetime do a decent video about them here:

 
  • #3
kimbyd
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Below are the questions.
1. What is the standard deviation for this 150 Mpc estimate of the end radius of a propagation spherical shell?
I don't know the precise answer to this, but the error bars are pretty small for sure. I don't think you should take 150Mpc as an accurate value itself, because they are only attempting to give a broad picture of the processes involved in that context, not provide accurate measurement values.


2. What caused the start of each spherical shell of acoustic propagation? (With my limited imagination, I am guessing it was something like an explosion of some sort. How would this relate to a perturbation in density?)
Not any sort of explosion, no. The spherical model is just a heuristic way to understand what's going on, and I don't think it's used in real calculations. The physical picture here is one where there is a spherical overdensity (basically, a collection of dark matter). The baryons fall into the overdensity, then bounce back out.

3. At what universe age did these acoustic waves begin?
The acoustic waves themselves would have started immediately after reheating. However, only waves which are smaller than the horizon scale can oscillate. In the very early universe, the horizon scale was quite short, so most of the pressure differences were "stuck" in place. As the expansion slowed, the horizon scale increased, and the pressure differences started to become traveling waves.

Those initial pressure differences were once quantum vacuum fluctuations during inflation, fluctuations which were expanded to cosmic scales by the rapid accelerated expansion during inflation.

4. Am I correct that the propagation of these waves ended at the time of recombination (which is the beginning of the CMB)?
Yes, essentially.

5. What were the speeds of propagation at the start and end of propagation?
Roughly half the speed of light:
http://www.astro.ucla.edu/~wright/BAO-cosmology.html

6. Did the speed of propagation change with changes in temperature and density as related to the scale factor?
I think only loosely, as photons make up a large fraction of the fluid density/pressure.
 
  • #4
Buzz Bloom
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  • #5
Buzz Bloom
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Not sure if this will answer all your questions but PBS Spacetime do a decent video about them here
Hi Mikee:

My overall impression of the video is that it is poor. I think several of the things said are the kind of descriptions that generally will confuse and mislead most of the people who will see it. Here is a quote of one such faux pas.
. . . from when the universe was subatomic in size.​
It may be simply a careless misspoken phrase, and he meant to say "observable universe".

In spite of my overall impression, I did learn a few things related to my questions. However, the few somewhat useful answers I feel were much more clearly explained in @kimbyd's post #4.

Regards,
Buzz
 
  • #6
Buzz Bloom
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Hi @kimbyd:

Congratulations for your award for the Astro/Cosmo award. I think it was well earned.

I am hoping you can explain about the use of the 150 Mpc (500 Mly) phenomenon in calculating universe curvature. The video in @MikeeMiracle's post #2 described the use as an expected average value related to the closest distance between pairs of galaxies. I can make a guess about how that relates to curvature, but to me this seems to be based on a careful choosing of the galaxy pairs. For example, Andromeda is only 2.5 Mly away from the Milky-way, so Milky-way-Andromeda would not be a good choice.

This reminds me of a life experience story my wife tells about her freshman physics lab at MIT measuring the speed of light. She does not remember details, but she remembers a repetition of getting the wrong answer, and making adjustments before trying again. This process was continued until she got an answer close to the actual value she knew was correct. Then she stopped experimenting. From this experience she decided to switch her major from physics to math.

Regards,
Buzz
 
  • #7
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Hi Buzz

Yeah I think the videos are made so that they can be followed by those without all the technical background knowledge so it's easier to simplify some explanations.....like for myself. :)

I do believe is answered your questions 2, 3 & 5 though which is why I posted it as I thought you might find it useful. Also a brief simplified overview for us less knowledgable users of PF.
 
  • #8
Buzz Bloom
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Hi @MikeeMiracle:

I do appreciate your post, and your effort to help me understand the BAO phenomenon. I am now becoming aware of just how complicated and difficult to understand this topic is. I am now focussing on wanting to understand the diagram discussed at 4 miniutes into the video. I found the source of the diagram as Figure 2 on page 5 of
I have tried to find in the article an explanation which I can understand of what Figure 2 is communicating (not just conclusions from it), but I failed. I plan to soon start a new thread with several specific questions about this diagram.

Regards,
Buzz
 
  • #9
timmdeeg
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I am hoping you can explain about the use of the 150 Mpc (500 Mly) phenomenon in calculating universe curvature.
Regarding the first peak in the CMB angular power spectrum you will find some more background here:

https://lambda.gsfc.nasa.gov/product/map/dr1/pub_papers/firstyear/peaks/wmap_powspec_peaks.pdfPage 4
"In a flat geometry with known b and m, the quantity that is particularly well determined is the acoustic horizon size, rs . From equation 2 we find rs = 143+- 4 Mpc."
 

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