Assume that the density of baryonic matter in the Universe is 3×10−27kgm−3 and that the distance to the edge of the Universe is given by cτ where τ is the age of the Universe, 13×109 years and c is the speed of light. Given that a typical galaxy has a mass 1011M⊙, estimate the number of galaxies in the observable Universe. Estimate how many protons there are in the observable Universe, stating all your assumptions.