# I At which time did the CMB become dark?

1. Feb 13, 2017

### Gerinski

At the time the CMB was emitted it was a glowing yellowish-white radiation at 3000 K. From there as space expanded it became redder and redder eventually falling into what for humans is non-visible infrared.
At which age of the universe did the CMB background turn from red to infrared?
Thanks

2. Feb 13, 2017

### Q.B.

To estimate this, you need to know:
-the dependance of the temperature $T$ of the CMB with time/scale factor $a$ of the universe, which is roughly $T \propto \frac{1}{a}$
-the evolution of the scale factor with time, from the Friedman equation which gives for a matter-dominated universe $\frac{a}{a_0}=(\frac{t}{t_0})^{2/3}$
Then you can get the $t$ you look for by choosing the temperature you want in the following equation: $t = t_0(\frac{T_0}{T})^{3/2}$ where $t_0$ would be present time (13.8 billion years) and $T_0$ would be the present temperature of the CMB, 2.73 Kelvin degrees.

3. Feb 13, 2017

### Staff: Mentor

There is no hard threshold, it just got darker over time. Based on those arbitrary categories, thermal radiation becomes visible somewhere around 525°C, or ~800 K. Today the temperature is 2.7 K, so we are talking about z=300. Based on this cosmology calculator, this was around 3 million years after the Big Bang.

"Cherry red" at 1000 K corresponds to z=370, 2.2 million years after the Big Bang.

4. Feb 13, 2017

### Gerinski

Thanks a lot.

5. Feb 14, 2017

### Chronos

Photons do not play well in plasma, largely due to a phenomenon known as Compton scattering. This same effect makes it difficult to see anything below the atmosphere of a star. For futher discussion, see; https://ned.ipac.caltech.edu/level5/Sept05/Gawiser2/Gawiser1.html, ORIGIN OF THE COSMIC BACKGROUND RADIATION.

6. Feb 14, 2017

### Staff: Mentor

Everything discussed in this thread happened after the plasma recombined enough to make scattering negligible. That is the whole point of the CMB. The hydrogen got re-ionized much later (>100 million years), but at that time it was so spread out that it didn't make the universe opaque any more.

7. Feb 14, 2017

### Staff: Mentor

Moderator's note: some off topic posts have been removed.