When CMB radiation was emitted, the universe was only 350,000 years old or so, and so why wasn't it absorbed at that time? The universe is postulated to have been expanding at that point but I think much much less than than the speed of light.


2 Answers 2


Since the fact that photons were not absorbed by neutral atoms was already addresses, I will address your last sentence, and your comments on the size of the universe and the fact that photons were traveling faster then the speed of the expansion itself.

I understand you are confused because you hear phrases like "the universe was much much smaller at the time the CMB was emitted".

Perhaps Brian Greene was talking about the size of the observable universe at the time when the CMB photons started traveling towards us. That happened 379,000 years after the big bang at a redshift of 1098 which means the universe was about 84.6 million light years in diameter which, per WolframAlpha, is about half the diameter of the local super cluster of galaxies or about 840 times the diameter of our galaxy.

Size of universe after inflation?

But it is very important to understand this is all about the observable universe. The whole universe might very well be infinite and might always have been. The big bang did not happen at a certain point in space, it happened everywhere at the same time. The universe did not have an edge, what the photons would reach.

So to conclude: the Big Bang is the zero time limit of the FLRW metric, and it's a time when the spacing between every point in the universe becomes zero and the density goes to infinity. It should be clear that we can't associate the Big Bang with a single spatial point because the distance between all points was zero so the Big Bang happened at all points in space. This is why it's commonly said that the Big Bang happened everywhere. In the discussion above I've several times casually referred to the universe as infinite, but what I really mean is that it can't have an edge.

Did the Big Bang happen at a point?

  • $\begingroup$ If it happened everywhere and the universe was always infinite / very large, how could there have been a period of opacity when there is a finite and relatively small amount of mass? $\endgroup$ Dec 2, 2019 at 3:52
  • $\begingroup$ @RithwikSudharsan the universe itself was infinite (endless, no edge), but the spacing between the matter/energy particles was zero. It is very hard to grasp, but what happened was that the spacing between the matter/energy particles started to become bigger, space itself expanded. And what is very confusing, is that when they say there was a finite and relatively small amount of mass, they talk about the part of the universe that is equal to the currently observable part. The whole universe might very well always have been infinite, and the amount of matter/energy too in it. $\endgroup$ Dec 2, 2019 at 4:06
  • $\begingroup$ @RithwikSudharsan opacity was caused because photons of all wavelengths were scattered off free electrons. It became transparent when the electrons and protons combined into neutral atoms. "The universe was opaque before the recombination, due to the scattering of photons (of all wavelengths) off free electrons (and, to a significantly lesser extent, free protons), but it became increasingly transparent as more electrons and protons combined to form neutral hydrogen atoms. $\endgroup$ Dec 2, 2019 at 4:11
  • $\begingroup$ @RithwikSudharsan While the electrons of neutral hydrogen can absorb photons of some wavelengths by rising to an excited state, a universe full of neutral hydrogen will be relatively opaque only at those absorbed wavelengths, but transparent throughout most of the spectrum. " $\endgroup$ Dec 2, 2019 at 4:11

Around 350,000 years after the Big Bang, charged nuclei and electrons joined to form neutral atoms. Neutral atoms have much less interaction with photons and are essentially “transparent” to the CMB compared with the “opaque” plasma earlier.

  • $\begingroup$ Right, I get that, but the universe didn't expand at many times the speed of light, the photons were moving much faster than the expansion of space at that time (I believe). So why weren't they absorbed by matter, and if there wasn't nearly enough clumped matter to absorb them, what happened when those photons reached the "edge" of the universe at the time, which was very small? $\endgroup$ Dec 2, 2019 at 1:18
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    $\begingroup$ You can picture a CMB photon as going right through a neutral atom most of the time, without being absorbed, because they have a low probability of interaction. As for the edge of the universe, it doesn’t exist. The universe is not a big explosion exploding into empty space. $\endgroup$
    – G. Smith
    Dec 2, 2019 at 1:57
  • $\begingroup$ So, because the universe was much smaller when CMB photons were emitted, what happened when they reached this edge that doesn't exist? Reading more about it now, it seems like the universe might have been huge right after inflation, so I guess the universe was actually really big when CMB was emitted $\endgroup$ Dec 2, 2019 at 3:50
  • $\begingroup$ "What happened when they reached this edge that didn't exist?" The same thing that happened when the were swallowed by the giant pink unicorn that didn't exist either. $\endgroup$
    – WillO
    Dec 2, 2019 at 6:02

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