The purpose of this problem is to determine how changing the value of the baryon-to-photon ratio, η, affects the recombination temperature in the early universe. Plot the fractional ionization X as a function of temperature, in the range 3000 K <T< 4500 K; first make the plot assuming η=4×10−10, then assuming η=8×10−10. How much does this change in η affect the computed value of the recombination temperature Trec, if we define Trec as the temperature at which X=21?