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I know the almost homogeneous of cosmic microwave background radiation is claimed to be caused by the rapid expansion of quantum fluctuations in the early universe, but what does density of photons impressions since the big bang got to do with the fluctuations of space? Isn't it more space since the beginning and thereafter then only matter and energy starts to populate?

My thinking: imagine an A4 size paper with 10 pixels of ink dots which represents fluctuations, suppose we apply the dots to an A0 size paper... What happens to the ink dots I mean they are just fluctuations not like they can grow in sizes as textbooks seemed to suggest during the rapid expansion stage.

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    $\begingroup$ apply ink dots to a4 and stretch the a4 into a0 $\endgroup$
    – Kosm
    Aug 9, 2020 at 8:32
  • $\begingroup$ @Kosm: I meant to say that each individual pixels remain unchanged but now there are more pixels than ever, it's a terrible habit of mine to connect unrelated things but the gist is there I think ;D $\endgroup$
    – user6760
    Aug 9, 2020 at 9:55
  • $\begingroup$ or you can think of each pixel stretching out. $\endgroup$
    – Kosm
    Aug 9, 2020 at 9:58
  • $\begingroup$ @Kosm: but the ink dots fade away in no time and new ink gets printed, so no point in scaling the pixel because that's what quantum fluctuations do right? $\endgroup$
    – user6760
    Aug 9, 2020 at 10:11
  • $\begingroup$ @Kosm: I think I'm almost there, did the frequency of the quantum fluctuations being stretched changed or its amplitude? $\endgroup$
    – user6760
    Aug 9, 2020 at 10:33

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CMB carries information about the recombination period when the universe cooled down enough for the neutral atoms to form (mostly hydrogen), allowing light to travel freely. This means that CMB fluctuations reflect the density fluctuations of matter particles (mostly electrons and protons) at that time. In turn, the density fluctuations of matter originated from gravitational overdensities caused by the fluctuations of the inflaton field, according to the inflationary paradigm. The evolution of the inflaton fluctuations/perturbations are studied by cosmological perturbation theory, you can look it up if you are interested.

To summarize my comments on these fluctuations: quantum fluctuations of the inflaton field are constantly being produced, and as they are produced during inflation, wavelength of each such fluctuation is stretched out (amplitude remaining the same, proportional to the Hubble function) by the expanding background, thus frequency decreases. Once inflation nears its end, the Hubble function quickly decreases, and so does the amplitude of each subsequently produced fluctuation.

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