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What exactly are Baryon acoustic oscillations?

  1. Jul 26, 2010 #1
    What exactly are Baryon acoustic oscillations? How do we detect them? How are they used to build a "ruler" of cosmological parameters? How are BAOs related to the late time ISW effect? and to dark energy?
    Lots of questions, hopefully someone here can give me some answers or at least some comment on ths interesting stuff that brings together data from CMB power spectrum and optical surveys like SDSS or spectroscopical like the LRGs.

  2. jcsd
  3. Jul 27, 2010 #2


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    Re: BAOs

    In the early universe, before the emission of the cosmic microwave background, the normal matter was a plasma. This means that the protons and electrons were separated from one another, which in turn meant that the normal matter felt pressure due to interaction with radiation (this pressure went to essentially zero once the electrons combined with the protons to produce neutral atoms).

    Because you could have pressure in the normal matter in the early universe, sound waves propagated. We see those sound waves in the cosmic microwave background, in fact. But perhaps more crucially, these sound waves got imprinted in the later density of the universe: you get more galaxies clumped together at distances where the matter that later made up these galaxies had enough time to make one bounce.

    So we basically get a measurement of BAO by looking at the typical separation between galaxies. You can see a plot of the effect here:

    As you can see, at around 120Mpc/h or so way from any galaxy, you end up with a bunch of extra galaxies. This distance expands as the universe expands, and thus can be used to measure how the universe has expanded through time by surveying lots and lots of galaxies at different redshifts.
  4. Jul 27, 2010 #3
    Re: BAOs

    Good explanation, chalnoth . In the figure, the y axis represents the correlation: CMBR map of ripples from WMAP and map of galactic redshifts, I suppose, is this right?
    And can someone elaborate on the relation between BAO and dark energy?

    Thanks a lot
  5. Jul 27, 2010 #4


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    Re: BAOs

    In that particular image, there is no CMB information. That is purely talking about the distribution of galaxies. The x-axis is a distance, and the y-axis is related to the typical number of galaxies you find separated by that distance.

    The primary way in which baryon acoustic oscillations constrain dark energy is by constraining the total matter density and the spatial curvature.

    Spatial curvature is essentially measured by checking to see if the angles of a triangle add up to 180 degrees, or some number greater or less than that. With Baryon Acoustic Oscillations, the correlation between a length scale in nearby galaxies and a length scale we see on the CMB allows us to sort of draw a tremendously large triangle that gives us a very accurate measurement of the curvature.

    BAO also constrains other parameters, but curvature is the big one. This sort of removes a degree of freedom from other data, which, in turn, gives us a good handle upon how much dark energy there is out there. For example, from WMAP we get a very good measurement of the total matter density (comes out to around 27% the required density to get a flat universe), from WMAP+BAO we get a very good measurement of spatial curvature, and find that it's basically flat. So the remaining 73% we call dark energy.
  6. Jul 27, 2010 #5
    Re: BAOs

    Ok ,thank you, I think I get it. Constraining the flatness will make sure the proportions we obtain from WMAP of dark matter plus baryonic matter and dark energy add up to the critical density smoothly.

    And I just read that the late time ISW effect is what we should be seeing on WMAP to independently of SNIa, confirm the existence of Dark energy.
    So I guess one way to highlight this is to correlate the WMAP CMBR anisotropies with galactic densities of the local skies, like they do in this paper http://arxiv.org/pdf/astro-ph/0603690.
  7. Jul 27, 2010 #6


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    Re: BAOs

    True. It is a bit of a difficult signal to puzzle out of the data, though, because it a zero-mean signal (the effect from the overdense regions tends to cancel out the effect from the underdense regions). This means that the signal is only really visible on large scales, where there aren't enough underdensities/overdensities to average out to below the measurement accuracy. And at large scales the statistical power, for the same reason, is rather low.

    But, evidence so far seems to be in the direction that the ISW effect is real.

    Yes, this sort of observation is our best chance of really nailing the ISW effect, but it is further complicated by systematic errors related to galaxies (galaxies themselves are exceedingly complicated beasts).
  8. Jul 28, 2010 #7
    Re: BAOs

    I guess it gets further complicated, I found some papers stating absence of cross-correlation WMAP-SDSS and thus no evidence of ISW effect:



    But actually the subtle statistical reasons to discern between these opposing views on the correlation are beyond me.
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