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I was reading through this answer about the cosmic microwave background radiation, which implied that we receive it as microwaves because it's been shifted to that wavelength by the expanding universe. Did I understand that correctly?

If so, wouldn't that mean that the rate of expansion would change the wavelengths perceived? Based on our understanding, has the CMB ever not been microwaves? Will it eventually stop being microwaves? If/when it is visible light, would we be able to see it?

Telastyn
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    If microwaves are stretched by the expanding universe, they aren’t going to become visible light in the future. Visible light has a shorter, not longer, wavelength. – G. Smith Apr 01 '21 at 22:55
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    This answer shows the approximate colour of the cosmic background radiation during the era when the universe first became transparent. – PM 2Ring May 13 '21 at 02:05

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Yes you read the other answer correctly. The expansion does change the wavelength and not just the perceived wavelength. Each individual photon gets stretched as its traveling through a stretching spacetime. The CMB was released back when the entire universe was filled with a hot opaque plasma which emitted light as a black body radiation in a similar way as stars do. There was originally a lot of visible light. It will eventually cool off further than microwaves if the universe expansion continues which at this point most theories predict it will.

PM 2Ring
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shai horowitz
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  • As a point of reference, the Universe was about the same temperature as the Sun (and so the CMB would have had a similar spectrum) around 100,000 years after the Big Bang, give or take an order of magnitude. – Michael Seifert May 13 '21 at 01:08
  • @MichaelSeifert True, although the cosmic (non-microwave) background couldn't travel very far in that era, since the universe was opaque. – PM 2Ring May 13 '21 at 02:01