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Baryonic acoustic oscillations and density perturbations

  1. May 28, 2014 #1
    I am just trying to understand the connection between these two things. As far as I understand it:

    BAO - Before decoupling the baryons were in thermal equilibrium with photons and as a result this gave rise to a pressure force. But given in regions of high dark matter density baryons are drawn in. So there is a competition between the outward pressure and this gravitational force giving rise to acoustic oscillations. After decoupling these then "clump" together in different regions of densities and begin to build large scale structures.

    Primordial density perturbations (PDP) - Quantum mechanical fluctuations before inflation lead to small perturbations, when inflation occurs there is a super luminous expansion where virtual particles become real particles as the expansion is to fast for them to annihilate, this gives rise to oscillations which then become sub-horizon (connecting casually disconnected regions).

    But are the above two anything different, they both lead to large scale structure. I understand that baryons won't have existed in the very early universe so clearly there is a difference in what is being described, but is it true that:
    PDP leads to BAO which leads to Large Scale structure?

  2. jcsd
  3. May 29, 2014 #2


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    What causes the initial dark matter inhomogeneities that set up the BAO? That's where the PDP come in -- the quantum fluctuations generated during inflation are the seeds of structure formation, including the BAO.

    Your take on PDP is a little off. First, the fluctuations occur during, not before, inflation. The exponential expansion causes the wavelengths of these fluctuations to grow larger than the causal universe (it's not "superluminal expansion" -- this is a common misconception). The result is a spectrum of fluctuations across a wide range of length scales. Once outside the horizon, the quantum fluctuations become classical curvature perturbations. You mention particles and antiparticles -- yes, this is the frequently seen cartoon that helps visualize this process, but it's not likely to be physically correct. Instead of particle creation during inflation, we have an amplification of quantum fluctuations of the inflaton field (yes, in normal QFT field fluctuations about the vacuum *are* particles, but during inflation the field is never in the true vacuum). The way to picture these fluctuations is as differences in the value of the inflaton field across space (the field essentially executes Brownian motion). Regions of the universe with different values of the inflaton field have different energy densities, and hence, expand at different rates. When the universe reheats, these different regions of the universe will have different densities -- these are the seeds of BAO and structure formation.
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