Cosmology and the Origin of Life

Remember the ratio of photons to baryons in the Universe now is one billion to one.

Hence, at early times when the Univere was hot and the average energy per photon was large -> the universe has to be radiation dominated!

Remember, the threshold for particle creation only depends on the temperature. If the rest mass energy () of a particle is higher than the average energy of a photon that particle can't be created.

Epoch #1:

At this time the Universe is still so dense that it is opaque to neutrinoes. So we can have


neutrino + neutron <------> proton + electron

The above process keeps the # of protons and the # of neutrons in the early Universe constant. This is extremely important and will result in a testable prediction later.

Remember, a free neutron (one that is not bound in an atomic nucleus) will decay into a proton + electron (+ anti-neutrino) in about 900 seconds. AT t=0.01 seconds, none of the neutrons have decayed. Since all of matter has a neutron component, the Universe must find a way to prevent neutrons from decaying. Fortunately it did, but that's later.

During this epoch, the proton-to-neutron ratio is 1.0

Epoch #2:

Since neutrons are slightly heavier than protons (by the mass of an electron) then the n --> p cycle in the previous neutrino reaction is favored over the p --> n cycle

Now we have 62% protons and 38% neutrons where at 0.01 seconds we had 50% protons and 50% neutrons. The proton-to-neutron ratio is 62/38 or 1.6

Epoch #3:

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