Cosmology and the Origin of Life
Starting the Thermal Countdown: From the weird to the semi-understandable:
At very early times:
At this time Gravity and the Strong Force have the same magnitude. That is:
At this time Gravity and the Strong force now have separated but the Weak force and the Electrostatic Force still have the same magnitude.
When these forces are decoupled the Universe is said to go through a phase transition. The symmetry is broken in a specific manner so that the forces have now separated and are defined by a characteristic ratio of their strengths. This defines the physics of our Universe.
Important Simplification in the Early Universe: Thermal Equilibrium:
Unfortunately we now have to do this:
When the radiation energy density exceeds the matter density
the universe is said to be RADIATION DOMINATED
When the matter energy density exceeds the radiation density
the universe is said to be MATTER DOMINATED
When the matter energy density exceeds the radiation density the universe is said to be MATTER DOMINATED
MOST IMPORTANT POINT:
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.
In this animation:
At this time the Universe is still so dense that it is opaque to neutrinos.
Remember the opacity analog:
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 neutrinos have mass, then we have already indirectly detected it via its gravitational effects (more on this later).