When the temperature was 1 MeV

t ~ 1 s

• Photons.
• Neutrinos and antineutrinos.
• Electrons and antielectrons.
• A comparatively very small number of protons and neutrons.
• Number of protons = number of neutrons.
  • Energy difference between a proton and a neutron is small, only about 1 MeV.
  • Weak interactions establish an equilibrium
    • p + electron <--> n + neutrino.
    • p + antineutrino <--> n + antielectron.
• No nuclei.
  • If one forms, it is is quickly broken up in, say, a collision with a high energy photon.

Now at T ~ 1 MeV three things happen at approximately the same time

(1) Neutrino's decouple

• The reaction e⁺ + e^- <--> neutrino + antineutrino is not fast enough at the lower density and lower energy.
• Other neutrino interactions are similarly too rare to have much effect.
• The neutrinos and antineutrinos remain, but they no longer interact significantly with the rest of matter.
• The neutrinos should still be here.
• They should form a neutrino background radiation with a temperature of 1.96 K.
• They are thus a little cooler than the photon background radiation with a temperature of 2.73 K.
• We can see the photon background, but the neutrino background remains so far undetected.

(2) e⁺ and e^- annihilate

• The energy it takes to make an electron plus an antielectron is about 1 MeV.
• Even after the neutrino reactions have become ineffective, the reaction e⁺ + e^- <--> photon + photon would keep the number of electrons equal to the number of photons.
• But with the temperature dipping, the photons no longer have so much energy.
• The reaction photon + photon --> e⁺ + e^- stops happening.
• The reaction e⁺ + e^- --> photon + photon continues.
• In less than a second, almost all of the electrons and antielectrons are gone.
• Almost all, but there were a few more electrons than antielectrons.
• When all of the antielectrons are gone, the few extra electrons remain behind.

(3) Neutrons start to turn into protons

• As the temperature dips, the reactions
  • p + electron --> n + neutrino.
  • p + antineutrino --> n + antielectron.
  that turn protons into neutrons slow down because neutrons have about 1 MeV more energy than protons and it is hard to find that much energy any more.
• But the reactions
  • n + neutrino --> p + electron.
  • n + antielectron --> p + antineutrino.
  continue with full intensity.
• The ratio of neutrons to protons starts to drop.
• Just as the ratio of neutrinos to protons is dropping to zero, the neutrino reactions are becomming ineffective.
• The ratio becomes almost frozen at (number of neutrons)/(number of protons) ~ 1/6.
• Over the next couple of minutes, this ratio will slowly decrease to about 1/7.