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7-Primordial nucleosynthesis (Key events after big bang timeline (10s…
7-Primordial nucleosynthesis
Key events after big bang timeline
10-43s
kT=10^19GeV
Plank era, quantum gravity, prior to this all forces one, gravity decouples first, many exotic particles
10^-35
Inflation starts, strong nuclear force decouples
kT=10^15GeV
10^-10s-10^-4s
T=10^15 K - 10^12 K
Free electrons, quarks, photons, neutrinos all strongly interacting
10^-4-10^1s
T=10^12-10^10K
Free electrons, protons, neutrons, photons, neutrinos all strongly interacting
10s
T=10^10
Neutrinos decouple from the cosmic plasma (cross-section falls dramatically)
kT=1.1MeV
Pair production of e+ and e- ceases
100s
kT=0.8MeV
baryon:photon ratio is frozen, n:p~0.2 after some reactions cease
next 300s
Thermal energy still high enough to photodissociate atoms
Neutron decay continues n:p ratio changing
Next 10^3 s
Primordial nucleosynthesis starts, ions not atoms yet
~10^3s to 400000 years
T~10^8 or 9 K - T=3000K
"Dark ages" Universe is season of free nuclei, electrons and photons. Photons Thompson scatter off electrons so universe remains opaque to photons. Physics is less well established
380000 years
T=3000K
Photons can no longer ionise, and decouple. "last scattering surface" origin of CMBR
n:p ratio
3 main reactions which determined their number in the early universe
reaction 2 is endothermic in a left-right direction and requires energy to proceed
at t<1s, kT>1MeV T>10^10 K reactions 1 and 2 maintain protons and neutrons in thermal equilibrium
when kT>> mn-mp, protons and neutrons nearly equal
When mn-mp decomes significant compared to kT, n:p given by the boltzmann factor
at t<1s, kT>0.8 MeV T>10^10 K
Reaction rates become slow compared to the expansion rate of the universe
neutrinos decouple (weak interaction rate slow compared to expansion rate)
e+ e- pair creation suppressed as photon energies drop
n:p freezes out
reaction 3 is the only one which still occurs after this
maths:
When kT is too high primordial nucleosynthesis cannot start due to dissociation and reaction 3 continues in left-right direct, neutron decay
after further 300s primordial nucleosynthesis starts
76% hydrogen, 24% helium
Modern day abundances
Can give important cosmological information like the baryon density of baryon to photon ratio
concordance with CMB important check on theory