Cosmology and the Origin of Life
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.
In this animation:
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
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
NEUTRINOES DECOUPLE --> they no longer interact with any other matter in the Universe but instead "freely stream" to fill the Universe. This predicts that in addition to the microwave background there should also be a neutrino background.
A neutrino background would be impossible to directly detect but, if neutrinoes have mass, then we have already indirectly detected it via its gravitational effects (more on this later).
electron+anti-electron --> photon+photon is now favored over the reverse reaction photon+photon -->electron+anti-electron
Thus, after the neutrinoes decouple, the Universe gets an "extra" source of photons (heating) which the neutrinoes never see. Hence the neutrino background is colder (lower energy) than the photon background
Census is now: 76% protons; 24% neutrons or roughly a 3:1 ratio of protons to neutrons
We are now at the end of particle creation from the photon field
The Universe consists of protons, neutrons and electrons plus a photon background and a neutrino background.
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