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This is a novice question. It is theorized that Cosmic Microwave Background (CMB) is remnant of radiation from very early time of universe creation. As these were photons that have been traveling, they must be traveling at speed of light or very close. Universe is certainly expanding much slower rate than this. Then how is it that all these photons haven't passed us by now? We are receiving them as if universe expanded much quicker than speed of light and only now these photons are catching up.

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The Big Bang happened everywhere, so every point in the universe is a source of the CMB. There was not a single pint in the universe where it happened. The CMB radiation we are detecting today comes from regions of the universe that were about 13.8 billion light years away at the moment the CMB was emitted (those points are a lot farther away now).

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  • $\begingroup$ The soup which produced the CMB was, in fact, much closer when these photons were emitted. The expansion of space carried the photons — which were on their way towards us — away from us, so there is a race going on between the two opposing effects (proper motion of the photons and expansion of space). Every time a CMB photon reaches us, it finally won this race, and by that time it really will have traveled 13.8 billion light years. $\endgroup$ – Alfe Nov 28 '19 at 0:48

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