1. Feb 25, 2004

### Antonio Lao

Cosmologists all agree that the present state of the cosmic background radiation is in the microwave region of the electromagnetic spectrum. This corresponds to a blackbody radiation temperature of about 3K. Since our naked eyes are sensitive only to the visible spectrum, the night sky does appear dark to all of us.

But if we assume a continuous decreasing change of EM energy since the Big Bang, the night sky did appeared in the visible region in the distant past.

I am looking for a simple formula that will calculate the epochs where the night sky appears as red, orange, yellow, green, blue, indigo, and violet. Anybody has any idea?

2. Feb 25, 2004

### Antonio Lao

Using the following equation relating the age of the universe to the temperature:

$$t = \frac{10^{20}}{T^2}$$

and the blackbody temperature of red light is 447 K and blue light is 506 K.

The CBR is red when the universe is 16 million years old.
The CBR is blue when the universe is 12 millions years old.