What was the temperature of the universe when a typical CMB photon was no longer able to spontaneously create an electron and positron pair? Note that a photon has an energy equal to E = hc/$\lambda$, the CMB at any point in time has a blackbody spectrum that peaks at a wavelength $\lambda$_peak = (2900$\mu$m K)/T, and the mass of a single electron is 9 * 10^-31 kg. A) 2000 K B) 400,000 K C) 2 million K D) 40 million K E) 2 billion K

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The energy required to create an electron-positron pair is twice the rest energy of an electron, which is about 1 MeV. Using the relation between energy and temperature, the temperature of the universe when photons had enough energy to create such pairs was around 2 billion K.

What Was the Temperature of the Universe When a Typical λ\lambdaλ

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