What Was the Temperature of the Universe When a Typical $\lambda$

Essay

What was the temperature of the universe when a typical CMB photon was no longer able to spontaneously create a proton and antiproton pair? Note that a photon has an energy equal to E = hc/ $\lambda$ , the CBR at any point in time has a blackbody spectrum that peaks at $\lambda$ _peak = (2,900 $\mu$ m K)/T, and the mass of a single proton is 1.7 *10^-27 kg.