How is the temperature of the Cosmic Background Radiation calculated to be 2.7 K?
Although we can measure the temperature of the cosmic background radiation today and determine it is 2.7 K,( see above plot with a 2.726 K curve fitted to the best data) the actual calculation from Big Bang theory as to what its temperature today should be, is rather complicated. In fact, to my knowledge, it is something of a free parameter in Big Bang cosmology which can be estimated to within a few degrees, but not exactly predicted. The reason is that to forecast what its temperature ought to be, you have to know the details of your cosmological model, and have an independent estimate of what epoch you are living in in the future of such a model. We do not know these ‘scaling’ parameters with very high precision, and this is why we cannot independently determine what the Cosmic Microwave Background (CMB) temperature ought to be. We can, however, place limits on it by using the ages of the oldest stars, the current expansion rate, the current average density of the universe, the prim
