When the temperature was 10 MeV

t ~ 10^-2 s

• There are lots of photons compared to the number of protons and neutrons.
  • At such a high temperature, blackbody radiation has lots of photons.
  • We knew before that we needed a large number (10⁹) of photons per proton.
  • The density of protons and neutrons at this time was not enormous, roughly comparable to the density of water.
• There are lots of electrons and antielectrons.
  • The number of electrons and antielectrons should be about the same as the number of photons.
  • There is a reaction to keep them in equilibrium.
    
  • If there are not enough electrons and antielectrons, then some more are made by
    • photon + photon --> electron + antielectron
      
  • If there are too many electrons and antielectrons, then they turn into photons
    • electron + antielectron --> photon + photon
      
• There are lots of neutrinos and antineutrinos.
  • The number of neutrinos and antineutrinos should be about the same as the number of photons.
  • There is a reaction to keep them in equilibrium.
  • If there are not enough neutrinos and antineutrinos, then some more are made by
    • electron + antielectron --> neutrino + antineutrino
      
  • If there are too many neutrinos and antineutrinos, then get wiped out by
    • neutrino + antineutrino --> electron + antielectron
• Actually, there are a three kinds of neutrinos, and there should have been lots of all three kinds.
  • These neutrinos should still be around.
  • But so far we can't detect them.
  • Maybe one kind has enough mass to provide the dark matter.
• Quarks are bound into protons and neutrons.
  
  • At a temperature bigger than about 1000 MeV, protons and neutrons would have been dissociated into quarks.
  • That is because with more than 1000 MeV of energy, you can rip protons and neutrons apart.
  • That happened at an earlier time.
  • Now it is too cold for free quarks.
• There are no nuclei (He, Li, Be, ...)
  
  • At a temperature bigger than a few MeV, nuclei get ripped apart.
  • Now still too hot for nuclei.
• The number of protons is about the same as the number of neutrons.
  • There is a reaction to keep them in equilibrium.
  • If there are too many protons, then they turn into neutrons
    
    • p + antineutrino --> n + antielectron
    • p + electron --> n + neutrino
  • If there are too many neutrons, then they turn into protons
    • n + neutrino --> p + electron
    • n + antielectron --> p + antineutrino