system
Given the following post, respond as if you're commenting on Reddit:
human
Was CMBR visible light in earlier periods of the universe?

If Cosmic microwave background radiation was once gamma but red-shifted to microwave, does this mean that at some point the photons we are receiving would have been within the visible spectrum of light in earlier period in the universe?

If so when?
gpt
Yes. The cosmic microwave background is an almost perfect blackbody radiation and its origin lies in the recombination phase, a time when free protons and electrons cooled down enough to form neutral hydrogen. At this time, the gas was around 3000 K hot, so the light was primarily red. Before that, that plasma was opaque, so no light from earlier times can reach us.