1. The problem statement I am asked to find the time of the matter-radiation equality. I have already find the red shift of this era, so I just need to find the expression of time in terms of red shift. From the Friedmann equation I can derive 2 relations between time and the red shift, one to the matter dominated era and another to the radiation dominated era. I suppose that I will have to use the second one because from the begining until the equality of the densities it was a radiation dominated universe. My problem is that I saw, in the Kolb and Turner book, that they use the matter dominated expression. What am I missing??