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Density perturbations made simple

  1. Dec 27, 2014 #1

    I know that this question has been answered in other threads, but those threads seemed to be closed now, and what I am looking for is a real "for beginner - real dummies" type answer.

    I understand that there are ripples in the background microwave radiation. The text book (OU) I am reading at the moment, then goes on to start talking about variations in the density of matter. So my first question is (please excuse the basic level of these questions) is there assumed to be a connection between the variations in matter density and CMB? If so, why is that? Do the surveys of the CMB reflect the distribution of galaxies in the universe?

    Also can any one explain in really simple terms, how Fourier series and Fourier transforms get involved with all of this? I kind of understand what Fourier series are.

    Is there any decent literature that someone could point me in the direction of in order to better understand this topic? I generally like the Open University and have had little but praise for them in the past, but the four pages related to this topic are among the most incomprehensible I have ever come across.

    Thanks in advance for any help.

    Cheers and seasons greetings to all you.
  2. jcsd
  3. Dec 27, 2014 #2


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    The CMB variations are the matter density variations at the time (and place) the CMB we see today was emitted. More dense matter was hotter.
    The galaxies we see today were not formed out of the material that emitted the CMB radiation we see today, and there are billions of years in between. If you start with the CMB distribution we see and simulate billions of years of time-evolution in the computer, the result looks very similar to the galaxy distributions we see. One prominent example was the millenium simulation, but there were more and better simulations afterwards.
  4. Dec 27, 2014 #3


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    The short of it is: it makes the math easier.

    If you make use of certain approximations, approximations which are valid for large length scales, you can make the differential equations that describe how these perturbations evolve linear. If you have a linear differential equation, then you can decompose the solution into sinusoidal waves, each of which evolves independently of all of the others. So instead of having a huge number of interacting particles that all impact one another, you reduce the problem to the behavior of individual waves whose behavior is (comparatively) easy to calculate.
  5. Dec 27, 2014 #4

    George Jones

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  6. Dec 27, 2014 #5
  7. Dec 27, 2014 #6
  8. Dec 28, 2014 #7

    George Jones

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    While these articles may be nice, they are not at the level for which the original poster is looking.
  9. Jan 1, 2015 #8
    Thanks that is a great help.
  10. Jan 1, 2015 #9
    thanks - that's clear
  11. Jan 1, 2015 #10
    Thanks George - I will check this out.
  12. Jan 1, 2015 #11


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    I will start from a book recommendation. If I recall well, Dodelson's book is a good one to look into the topic of CMB fluctuations.

    The variations in matter density can take into account weird behaviors in the thermal evolution of the universe, because the distribution then is not taken to be the Fermi-Dirac but a perturbed version of it. CMB is already (a little) anisotropic in temperatures, of [itex]\delta T/T \sim 10^{-5} [/itex]. This small variation can also lead in matter anisotropies.

    Fourier series come into play because they make things more easy to deal with.
  13. Jan 2, 2015 #12


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  14. Jan 3, 2015 #13
    Thanks - I will see if I can find the book you have recommended. Typically, with a course like the one I currently doing, only a small amount of time is spent on each topic before moving on to the next. So it is doubly frustrating if you find yourself not being able to understand anything at all. Cheers
  15. Jan 3, 2015 #14
    I keep seeing the word 'constraints' used - eg. "The fact that constraints on one parameter can correlate with constraints on another is known as 'parameter degeneracies'

    Please excuse again the basic level of my question, but I am confused by what they mean in this context by the word 'constraints' and 'parameters' I keep seeing these two words used over and over again. I am used to using them in ordinary every day speech but I just don't get what they are saying in terms of cosmology. Does constraints mean limiting range? And does parameter mean factor or variable? Cheers
  16. Jan 3, 2015 #15


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    yes. Contraints are either put by theory or experimental data and they are limits. In the parameter space they correspond to an area/volume that your theory can still exist in. An example, is the cosmological contraint on the sum of neutrino mass which comes from asking for neutrinos not to close the universe density. So you know that their masses sum should not exceed eg [itex]12~eV[/itex]. It can be as well [itex]11~eV[/itex] or [itex]0.1~eV[/itex] (in this context).

    Parameters are parameters. You can see them as variables. But if I recall well, variables are dynamical objects, whereas parameters can be just numbers that are put into your theory and are in general unknown. Eg take the coupling of an unknown dark matter particle X to photons. When you search for the interaction of X with photons, and you find nothing, you can say that the coupling constant (the model parameter) is very small , and give some limits.

    Are you OK with the "parameter degeneracies" term?

    These examples can work with almost everything concerning contraints and parameters, you just have to change the language and look into the technicalities.
  17. Jan 4, 2015 #16
    Thanks Chris. That is a great help again. cheers
  18. Jan 10, 2015 #17


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    I would second this article. @resurgance2001, is this the sort of level that you are looking at (it's not clear what course you are studying...)?
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