# Were the "Dark Ages" really dark?

1. Nov 19, 2015

### Gerinski

The period from the end of recombination until the birth of the first stars is commonly referred to as "the Dark Ages" since no starlight existed yet.

But the cosmic bath of photons at that time which today we call the CMB was still hot, not yet having redshifted. It is usually said that temperature at the end of recombination (CMB) was around 3000 K, which would mean that the universe would have been glowing bright yellow, and as it expanded further the bright yellow would gradually become more orange, then redder and gradually fainter until it finally went into the infrared and therefore truly dark.

If so, the Dark Ages may be an inappropriate name, at least for the initial period?

By which age would the cosmic radiation have transitioned from red into the infrared so the skies would really look dark?

TX

2. Nov 19, 2015

### Orodruin

Staff Emeritus
During the Dark Ages the CMB would already have cooled off significantly (if you take the Dark Ages to span 150-800 million years after the BB, the temperature would be around 50 K at the onset). However, I do not believe this is the reason for referring to it as the Dark Ages. Rather, it is the fact that it appears dark to us which is the culprit in the naming - if you try to look at that epoch in an instrument, there is essentially nothing to be seen - the CMB is background and the rest is foreground. Essentially all you can try to search for is the 21 cm hydrogen line.

3. Nov 30, 2015

### Allday

To help you answer your last question, the temperature of the CMB scales with redshift like so $T = T_0 (1+z),\, T_0=2.725 {\rm K}$

4. Dec 6, 2015

### nikkkom

Yes, I think you are right, Universe was filled with yellow-reddish light at about 300 thousand years. But first stars formed much later - some 100 *million*, not thousand years after BB. A few million years after BB, gas temperature fell below 500K and CMB peak was firmly in infrared, so it become "pitch black" (for human vision).