Hello! I am reading some basic stuff in cosmology and I am a bit confused by the microwave background. As far as I understand, it is the radiation produced at the beginning of the universe, when the matter density was not that high so the photon can escape and travel freely. So the microwave background we measure on earth, the one having 2.7K, is the one coming from a circle (assuming the universe would be flat for simplicity) of radius 13.9 light years around us and due to the redshift, the initial energy of these photons is reduced such that they have 2.7K. Is this right? Now we have this picture and as far as I understand it represents the whole visible universe. Now I am not sure I understand how it was done. You just look at this circle of 13.9 billion years radius, see the temperature at each point and then you run the time backwards, to see where each point would fit 13.9 billions years ago?