1. The problem statement, all variables and given/known data If ne represents the electron density and the interaction between particles is the photons scattering off of the electrons through the Thomson cross section, how low must the electron density be to cross the universe with no scattering? 2. Relevant equations The size of the universe is given by c*(Ho^-1) 3. The attempt at a solution I have no clue how to approach this problem. Obviously the Hubble distance is in Mpc, but I do not know the significance of the Thomson cross section. Can anyone help me to understand this problem?