Talk:Baryon acoustic oscillations

Figure 2
Where is Figure 2?? Merryjman (talk) 16:05, 26 March 2010 (UTC)merryjman
 * If you look in the page history, you'll see it was automatically removed when the file itself was deleted. Modest Genius talk 21:26, 26 March 2010 (UTC)

Length Scale?
I get that there's a sound wave formed by the competition between gravity and pressure, but why does this yield a well-defined length? To take the "ripple" analogy in the article at face value: In a pond, there is not a constant distance between the ripple and the original center. Instead, the radius increases with time. For the BAO distribution, is the distance set by age-of-universe (or a fixed time-since-inhomogeneities formed) at decoupling? Is this a resonance condition between speed-of-sound and circumference-of-universe at decoupling? Is / was the BAO "ripple" a static distance set like the pressure vs. gravity balance of a star? SMesser (talk) 15:53, 20 January 2014 (UTC)


 * This is a very good point! The radius of the acoustic waves did increase with time, like the ripples on a pond. The distance scale is set by the maximum distance the waves had time to travel before recombination (when the universe became neutral). When the plasma becomes neutral, the pressure from the photons in the plasma, which was pushing the waves, "decouples" from the baryonic matter, and stops pushing it. (This is because the interaction cross-section between the photons and neutral atoms is so small that the photons can "free stream", they can travel essentially the length of the observable universe without hitting anything.) The cosmic sound section explains this, but I have added a short explanation to the opening paragraph as well. Bensaliw (talk) 21:46, 25 October 2018 (UTC)

Confusing sentence in SDSS section
In the last sentence of the first paragraph of the section on the Sloan Digital Sky Survey, the article says:

The SDSS catalog provides a picture of the distribution of matter such that one can search for a BAO signal by noting a larger number of galaxies separated at the sound horizon.

This sentence confuses me - what would I notice at the sound horizon? A large number of galaxies (ie, more than in surrounding space), or a gap in the galaxies (ie, some on either side of the sound horizon)? Maybe a combination of the two? If I knew the answer, I'd just edit the sentence, but I'm not clear about it yet... — Preceding unsigned comment added by 72.234.136.85 (talk) 18:17, 18 November 2015 (UTC)


 * Good point, that's not a very clear sentence. How about this:

"The BAO signal appears as an increase in the number of galaxies separated by a distance equal to the sound horizon, compared to the number expected if there had been no baryon oscillation in the primordial plasma."
 * I think that's a better lead-in to the next paragraph, where the more detailed explanation appears. What do you think? We really should have a plot of the correlation function with the "BAO bump" in the article. I'll see if I can dig up a good one. - Parejkoj (talk) 09:08, 19 November 2015 (UTC)
 * Ok, Figure 14 of Anderson et al. 2014 is a great example. Unfortunately, the published version is still paywalled, but I'll ask for permission to use it. - Parejkoj (talk) 09:26, 19 November 2015 (UTC)

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Inaccuracies in opening section
The opening section had several significant scientific inaccuracies, which I have corrected. The BAO are not a "regular, periodic" feature, they are a statistical correlation between density fluctuations on a certain length scale. In the ripples on a pond analogy, there are many overlapping ripples, which look very messy by eye, not a like a regular periodic pattern. This misconception seems to have slipped in from an edit in 2012 https://en.wikipedia.org/w/index.php?title=Baryon_acoustic_oscillations&oldid=480347117, and has affected descriptions in other places.

There was also a line at the end of the opening section which described the accelerated expansion of the universe as "apparent". This seems meant to imply that the universe is not expanding, or not accelerating. Perhaps the editor was influenced by tired light theories. However, the scientific consensus (from supernovae, BAO, and CMB) is that the universe is undergoing accelerated expansion. Discussion of alternate theories is not appropriate for the opening of an unrelated article. Bensaliw (talk) 21:46, 25 October 2018 (UTC)

History and Big Picture?
Would it be beneficial to add some more history? One of the chat bots says that BAO was predicted by Rashid Sunyaev and Yakov Zel'dovich in the late 60's, and confirmed by the "Sloan Digital Sky Survey (SDSS), particularly its Data Release 7 (DR7) in 2009. This landmark discovery was made by a team of researchers led by Daniel Eisenstein, David H. Weinberg, and others." I do not have the expertise to judge these claims. No doubt Fritz Zwicky had this figured out in the 1930's. Just kidding. Was 2005 the first detection as implied by the article?

I'm asking because as a lay follower of Cosmology this topic came out of the blue for me about 15 years ago and it would be interesting to see how it percolated before its confirmation. Thanks. Bluepost22 (talk) 13:41, 5 April 2024 (UTC)
 * Yes, Eisenstein et al. 2005 was the first detection of BAO (though 2dFGRS published at about the same time). The article basically says that in "Detection in other galaxy surveys", but there are several sections here that really should be completely rewritten: there's no narrative flow. - Parejkoj (talk) 00:12, 17 April 2024 (UTC)