 Here's a projection of the celestial dome, as seen by the Wilkinson Microwave Anisotropy probe, factoring out all local and local group motion. The mapping preserved the relative sizes of the surface objects. The key observation is that the light fits the black body radiation curve perfectly. Its mean wavelength is around 2 mm, and its peak intensity has a wavelength of 1 mm. That's in the microwave range. This gives us the temperature of the radiation today. It is 2.725 degrees. We know that at decoupling it was 3,000 degrees. So the temperature has been reduced by a factor of 1,100. We also know that the ratio of the current temperature to the temperature at decoupling is equal to the ratio of the current scale factor to the scale factor at decoupling. So the universe has expanded by a factor of 1,100 times since decoupling. The black body radiation formula also gives us the number density of CMP photons. There are over 400 million of them in every cubic meter of space throughout the cosmos. This is a thousand times more than all the photons from all the starlight ever created by all the stars and all the galaxies for all the billions of years that stars have been shining.