# Questions tagged [bao]

Baryon Acoustic Oscillations (BAO) are periodic density fluctuations in the density of matter in the universe. BAO is observed in the large-scale structure of the universe (e.g. distribution of clusters and quasars) in addition to the power spectrum of fluctuations in the Cosmic Microwave Background (CMB).

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### Standing barionic waves in CMB require that the geometry - i.e. the spatial perimeter condition - does not change for a sufficiently long time

Standing barionic waves in CMB require that the geometry - i.e. the spatial perimeter condition - does not change for a sufficiently long time. How long should the standing wave of a byryon ...
1 vote
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### Sound speed of Baryonic Acoustic Oscillations

I've read in scientific papers, that at the time of decoupling the baryonic sound speed was 5 km/s. Can anyone tell me how this is deduced or point to textbook/papers that do a derivation? I want to ...
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### $\Lambda$CDM's observations and the universe's matter content

It's known that the current value of the universe's total density parameter $\Omega_0=1$. According to the $\Lambda$CDM model, the current density parameter of baryonic matter $\Omega_P \sim 0.04$, ...
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### Multipole ${\it l}$ value for the second acoustic peak in the CMB power spectrum

So according to the Planck paper https://arxiv.org/pdf/1502.01582.pdf page 20, the first peak is ${\it l}$=220, but the second peak is ${\it l}$=537. Not quite the double of the first peak, why? If ...
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1 vote
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### Issues with baryon acoustic oscillations

I have a difficulty understanding the various explanations of baryon acoustic oscillations that I've been reading. Even on this site, I haven't been able to find answers that directly address the ...
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### How do BAOs provide evidence for dark matter?

So far my understanding of BAOs is that they are a relic of the old universe formed by the freezing of acoustic density waves in baryonic matter as the universe entered the recombination epoch. These ...
1 vote
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### BAO : Relation between redshift, Hubble constant and radial

From this link https://en.wikipedia.org/wiki/Baryon_acoustic_oscillations#Measured_observables_of_dark_energy , I can't get this relation : $$c\Delta z = H(z)\Delta \chi\quad\quad(1)$$ with $z$ ...
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### What causes structure formation: Baryon acoustic oscillations or inflation?

Formation of structures such as galaxies and galaxy clusters require anisotropies in the cosmic plasma which (as I understand) were due to the anisotropies or density inhomogeneities created by Baryon ...
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### Is the baryonic acoustic oscillation a sound wave?

I was told by a friend that a baryon acoustic oscillations are sound waves with a wavelength so large that they can travel through a vacuum. Is there any truth to this? I read what Wikipedia has to ...
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### Different cold dark matter density parameters from early- and late-Universe observations

Edit: @John Rennie, thank you for your good and concise review of how the parameters of the $\Lambda$CDM model evolve with time. However, the answer I was looking for was a different one (sorry for ...
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### Is BAO a standing or moving wave?

The sound horizon is the distance that a wave of plasma can move from the end of Inflation to Recombination (roughly 300,000 years). In several papers and talks, this is described as a moving wave (...
1 vote
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### Are there any public program that can calculate baryon acoustic oscillations (BAO) acoustic peak from the data Lyman alpha spectra of BOSS?

Are there any public program that can calculate baryon acoustic oscillations (BAO) acoustic peak from the data Lyman alpha spectra of Baryon Oscillation Spectroscopic Survey (BOSS)? I am confused in ...
1 vote
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### Baryon Acoustic oscillations - simple confusion about length scales

I am confused about length scales and baryon acoustic oscillations. I am clearly missing something very simple. The length scale found in the $SDSS$ and $2dF$ data is quoted as being $150Mpc$. ...
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### What determines the size of features on the CMB?

I have been learning about baryon acoustic oscillations. I am however confused about the size of the features as seen on the CMB. It is claimed that the largest structures have a size given by the ...
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### baryon acoustic oscillation

I have one question about baryon acoustic oscillation. I understand why we should have the baryon-photon fluid sound wave before recombination: Suppose we have a spherical overdense region. This ...
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### Galaxy bias and baryon acoustic oscillations (BAO)

I have a doubt with the concept of galaxy bias and how it affects baryon acoustic oscillations, it is supposed to mean that by measuring the distribution of galaxies we are not measuring the actual ...
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### Why is the Baryon Acoustic Oscillations (BAO) calculated like this?

In the Wikipedia, it says that, when calculating the Baryon Acoustic Oscillations (sound horizon), we measure $150\text{ Mpc}$, saying that the sound horizon is the "Physical Length of sound horizon ...
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