system
Given the following post, respond as if you're commenting on Reddit:
human
When was the cosmic background radiation in the visible spectrum?

And how long ago did it fade out of visible?
gpt
The CMB was in the visible spectrum when it was emited. The universe was at roughly 3000 K (For comparison: The surface of our sun is 5800 K) at that point, which means it would seem red to us.  From this point on it was shifted to the current value of approximately 2.7 K.