What average distance could a photon travel before being sca

What average distance could a photon travel before being scattered by an electron just before recombination? (show your work!)

Solution

The electrons does not allow the photons to travel much distances before the recombination. As the result of Thomson\'s scattering of photons with the electrons, the mean free path of the photons is very much less than the Hubble\'s length, which gives the distance travelled by the photons to reach us at present. After recombination, the number of electrons preveting the photon\'s travel will become less than the previous existing number that is the mean free path of the photons will become larger than the Hubble\'s length.