Keydar

Ah, okay. So I take that to mean that when the CMB was visible the universe was not exactly habitable? I'm not certain you understood my question. I'm talking about whether or not the universe was habitable when the background radiation was in the visible spectrum. Whether or not an eye would evolve to perceive the background is not specifically relevant to my question. We didn't evolve eyes to observe stars, but nonetheless we can do so as a byproduct of our eyes capabilities to detect light. Basically, I'm wondering if the night sky over some planet long ago (maybe even an early Earth) was not exactly black as it appears today, but slightly reddish?

I understand that the expansion of the universe has caused CMB radiation to redshift into the microwave range. What I am wondering is how long ago was the background radiation in the visible light range, before it redshifted into the infrared? Also, when it was in the visible range, did it have enough intensity to be visible with the naked eye? Was the universe habitable at this time? What I am getting at, in general, is was there a time in the universe, when life forms similar to humans could have observed background radiation with the naked eye?