1. Based on the laws of conserved quantities, decide whether the following reactions can occur. You can assume that sufficient energy is always available, so you only have to consider charge, lepton number, and baryon number. Explain your answers.

  2. What happens when the temperature of the Universe drops below 6 x 109 K, the threshold temperature for pair production of electrons and positrons? What is the wavelength of the photons that are capable of this pair production?

  3. Given that muons ((mu)- or (mu)+) have masses which are about 200 times that of the electron, what temperature would be needed for pair production of muons? How old was the Universe when the temperature was this high?

  4. Explain why the fraction of baryons in the form of neutrons shrinks from roughly 50% to about 13% during the first 3 to 4 minutes of the Universe's history. Explain why the fraction is not even smaller by the time 3 to 4 minutes has elapsed.

Evans's 324 | Astronomy Department