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I am trying to understand how it is possible to make predictions about the energy density of early universe using the freidman equation if the expansion rate of the universe has not been constant throughout history. As I understand it there are three main variables in the freidman equation, the energy density, the hubble constant squared (the expansion rate of the universe) and the shape of space (K). We know today that space is flat (or very close to flat) but in order to calculate anything with the other two variables one of them must also be known. I thought that the expansion rate was first thought to accelerate, decrease, and is now accelerating again and that as the universe has expanded that radiation particles has become redshifted and lost energy changing the energy density. With these two unfixed variables how can you say anything certain about either one? As you can probably guess this is all very new to me so maybe I am missing something very obvious but maybe I would be thankful if someone can try and clue me in...?