1. Apr 28, 2017

### newphy

If intelligent life had evolved on some planet 6.8 billion years ago (half the time to the big bang 13.6 bilion years ago) and they had sent the equivalent of the COBE / Planck satellite to map the cosmic background radiation, where in the electromagnetic spectrum would they have seen the radiation? I assume it would not have redshifted to the microwave region at that time.
Thanks

2. Apr 28, 2017

### kimbyd

The scale factor 6.8 billion years ago was at roughly $a=0.55$, which means the CMB temperature would have been $2.725K/0.55=4.9K$. so the CMB would have a peak at around 290GHz, which would still be well within the microwave range.

Edit:
I guess to be more pedantic, it's actually in the millimeter wave range. But it's closer to the microwave range than it is currently.

Last edited: Apr 28, 2017
3. Apr 28, 2017

### kimbyd

Also, the first stars formed when the scale factor was at around roughly 1/20th its current value, which means the CMB peak frequency would have been at around 90 microns. Thus the CMB has been in the microwave/millimeter wave range for the entire time that stars have been around.