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I General questions about the CMB

  1. Apr 19, 2017 #1
    Hi everybody! I am (like many) very fascinated by the cosmic microwave background, but there are many aspects of it that I do not understand yet. I'm currently in 4th semester of my physics studies, and just starting quantum physics now. Some of those questions might be stupid or (mis)leaded by the interpretation of the information I read, but I think that asking them might help me clarify a few things. :)

    I've numbered the questions so that it is easier to answer:

    1. The CMB is often -if not always- presented as an almost isotopic background "glow" dating from around 400000 years after the Big Bang. How is it possible that we are still able to observe the photons that could escape at the time of photon decoupling? I read that their wavelengths were stretched by the expansion of the universe, therefore they are in the microwave range today. But what I don't get is: when the photons were freed, why didn't they simply escape at (nearly) the speed of light, so faster than matter, which I guess would mean that they would be far away now? Probably a thinking mistake from me, possibly because I treat photons as particles and not as EM waves here.

    2. The shape of the CMB map we have is spherical. Did that observation come from observing the sky in all directions? Can we approximate the size of the sphere at the time when the light could escape? Can we say "where" the Big Bang happened, relatively to the position of the galaxies today? Is the last question senseful? :)

    3. Why are we able to map the radiation of this precise time of our universe and not other periods afterwards? Is it because the energy emitted at other times was weaker and the wavelengths have been stretched so much that there is quasi zero observable energy (according to Planck's relation, energy is inversely proportional to wavelength)? Will the CMB become virtually unobservable in some distant time because the wavelength will keep on stretching as long as the universe expands?

    4. About this page: http://thecmb.org/ (3D model of the CMB)
    Do the (slight) anisotropy correspond to the model of the universe as we observe it today? Can we even compare? When you switch the channel (bottom left button) to GHz, there appears to be an equator separating the sphere. What does that show and why does it look so anisotropic then?

    I think that answers to those basic questions would already give me a better understanding of how to consider the CMB... Thanks a lot in advance for your answers, I'm looking forward to reading you!

    Last edited: Apr 19, 2017
  2. jcsd
  3. Apr 19, 2017 #2


    Staff: Mentor

    Because they were emitted everywhere in the universe. So we will continue to observe them because as time goes on, we see CMB photons passing us that were emitted further and further away.

    What particular map are you looking at? We do observe CMB radiation coming to us from all directions, if that is your question.

    The "time" of the CMB emission is really the time at which our universe became transparent to electromagnetic radiation (because the temperature became low enough for electrons and ions to form neutral atoms). In other words, before this time, radiation could not travel very far without being absorbed. After this time, radiation could travel indefinitely without being absorbed. So it's not that the radiation just appeared in the universe at that time; it's that that time was when the radiation became able to travel indefinitely.
  4. Apr 19, 2017 #3


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    It looks to me like you're thinking of the big bang as matter and radiation escaping from a single point. The standard cosmological models are homogeneous and isotropic (on large enough scales). What it means, is they treat the universe as either infinite or finite and closed (similar to the surface of a sphere), with matter and radiation spread uniformly throughout the volume. You arrive at the big bang = hot and dense state, when you make all distances shrink sufficiently.
    In such an universe you receive CMBR once emitted from ever further regions of space, centred around the observer.
    A picture is worth a thousand words, as they say:
    How can we still see CMBR.png
    As seen above, the spherical shape of CMBR comes from the fact that you're receiving radiation emitted from around you. You can't say where BB happened, since there was no centre. Alternatively, it happened everywhere.

    Because decoupling happened only once. All the CMBR radiation we see was emitted at one point in the history of the universe. Again, it's one point in time, but all points in space.
  5. Apr 19, 2017 #4
    Hi and thanks @PeterDonis and @Bandersnatch for your very interesting answers! If I may, I'd like to ask @Bandersnatch a few specifications about what you are saying.

    Yes I actually do think of the Big Bang as you describe it, i.e. matter escaping an infinitesimal volume. I'd like to emphasize that that's the way I understand what I read about Big Bang theories, but it is not necessarily my opinion (I have none so far). I also do not think of the Big Bang as the spectacular explosion repeatedly presented in related documentaries. But does your sentence mean that I have been misunderstanding the theory? I thought that it says (more or less): when going back in time and considering the expanding evolution of our universe until now, it seems as if the density of matter used to be extremely high in an extremely small space. We have no observation of the universe prior to the time of photon decoupling available, but it seems that the universe was even smaller before and taking the limit led me to think that it was a "single point event". Would you mind pointing out what doesn't seem quite right to you about this perspective?

    Again this blows my mind. :) Maybe it is because I (quite naively) tend to imagine the Big Bang as an event happening in some space, whereas you seem to say (forgive me if I'm wrong) that the universe at the time of the photon decoupling is all there is, and that the universe expanding also expands this space where it happened. In other words, at the time of today being in the universe also means being within this volume of space where the Big Bang happened. I am a bit afraid of having widely misunderstood here. :)

    Thanks a lot for your answers, I appreciate it. Did you guys see that I edited the first post to include a 4th question?

  6. Apr 19, 2017 #5
    The differences of temperature between one region and another are in fact tiny.
    They are exaggerated in this and other maps because if they were done in a linear scale way, there would be nothing there to see with with human eyes.
    Similarly the coloration of regions is adjusted into the visible range as a visual aid, whereas the actual CMB wavelengths are not in the visible range.
  7. Apr 19, 2017 #6


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    Yes, that's pretty much it.
    The model goes like this: you start with the universe as it is today. Call it t=0. It might be infinite or not, but let's assume that it is for now. You measure any set of distances. From redshift observations you know that these distances are growing. If you now extrapolate backwards in time, you see all distances shrink, which leads to higher densities and temperatures as you go back in time, since in any constant volume there's progressively more matter. At some point you get sufficient density that all matter must have existed as plasma - that's the moment decoupling happened. Extrapolating further back you get even higher densities, which at some finite point in the past t~= -13.7 Gy reach infinity (so you know the model is no longer applicable there).
    Notice that you did not need to measure any of the distances against some 'edge' or centre of the universe. You can see the expansion by measuring any arbitrarily chosen set of distances of arbitrary length. Even an infinite distance would contract in the same way.

    You might find these helpful:
    http://www.mso.anu.edu.au/~charley/papers/LineweaverDavisSciAm.pdf - a popular article on BB misconceptions (first page blank)
    https://arxiv.org/pdf/astro-ph/0310808.pdf - same authors and topic, but a more advanced treatment

    There's also quite a few nice articles written by PF members to be found in the 'Insights' tab that might be worth reading:

    I'm not sure I understand the question. Do you mean the little dot-like anisotropies? Those are imprints of inhomogeneities in the plasma filling the universe at decoupling. Look up 'Baryon acoustic oscillations' or BAOs.

    Can you think of a structure that looks like a fuzzy band stretching across the whole sky, maybe bulging out a bit in one place, which could potentially obscure certain wavelengths of radiation?
  8. Apr 19, 2017 #7
    Hi and thanks for your answer. I am aware that the anisotropies are tiny, but still the GHz maps (whether the wavelengths have been shifted or not) have this white light splitting the sphere into two (which the temperature mapping does not have). Do you happen to know more about that?

  9. Apr 19, 2017 #8
    @Bandersnatch thanks again for your answers, that pretty much cleared up my confusion (...for now :) ).

    Thanks I will check it out. I think I cannot guess further without knowing more.

    Oh so it is only light pollution from the Milky Way you mean? Crazy!

    And thanks for all those links you've attached, I'm on my way to reading each of them! I appreciate your input very much.

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