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6-Primordial nucleosynthesis (αβγ theory (Successes (abundance of He…
6-Primordial nucleosynthesis
Evidence for evolving universe
Hubbles law
recessional velocity (and hence redshift) of galaxies increases linearly with their distance
Olbers paradox
Radio sources
Observations of radio sources of different strengths (2C and 3C survey)
radio source number vs strength means universe has evolved from a denser place in the past
rules out steady state
CMBR
Remnant heat from very hot and dense initial universe
Universe should glow in the gamma ray part of the spectrum
which would subsequently cool as the universe expands
αβγ theory
Proposed early universe was hot an dense
assumed the early universe consisted only of neutrons
as temp fell neutron decay to protons became possible, successive neutron capture creates heavier elements
proposed neutron capture as a process for all elemental abundances
Chemical equations
At each step process is controlled by the balence between the rate of production and the rate of destruction
gives reasonable agreement with trend of observed abundances
In neutron capture the rate at which the reaction is occuring depends on the relative velocity of the particles and target nuclei and is given by maths.
Successes
abundance of He agrees with observations
by splitting the elements into 15 "groups" by atomic weight and using average cross section for each group give a reasonable fit to abundance data
Failures
predicted abundances for heavier elements wrong
problem getting past A=4 due to lack of stable elements at A=5,8
Results helped calculations of thermonuclear fusion
Cross sections
probability of a particular process occurring per target nucleus per incident particle
maths
Big Bang underlying principles
Universe expanded 14 Gyr ago from singularity
at v high temps elementary particles can be created from thermal energy
after big bang universe expands and cools
as temps fall below threshold temp for particle production annihilation rate>creation rate
Assuming thermal equilibrium
normal laws of physics apply
small matter-anti-matter asymmetry
Gravitation described by general relativity
Homogeneous and isotropic, robertson walker metric
expansion governed by the field equations of GR