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Cosmology and
extragalactic astronomy
Mat Page
Mullard Space Science Lab, UCL
9. The very beginning
Slide 2
9. The beginning
• This short lecture:
• The hot big bang
• Timeline
–10-43 seconds to 380,000 years.
Slide 3
• If the recession of galaxies really is because
the Universe is uniformly expanding, then at
some point everything must have been
infinitesimally small.
• How far can we trace it back towards the
• We are limited by quantum uncertainty.
• After 10-43 s (the ‘Planck time’) the
wavelengths of typical particles were similar
to their Schwarzschild radii. We have no
physics (i.e. quantum gravity) to probe earlier
than this.
Slide 4
Radiation dominated era
• Between 10-36 and 10-33 seconds:
– a period of rapid, homogenising expansion
which smoothed out primordial
irregularities and imperfections
– decoupling of the electromagnetic and
nuclear forces
• Radiation and matter in equilibrium
– Pairs of hadrons and leptons < > radiation
Slide 5
• After 10-4 seconds:
– Temperature dropped too low to produce
hadrons. Most hadrons annihilated, many
of the remaining protons combined with
electrons to make neutrons
• After 1 second:
– Temperature too low to produce leptons.
Most leptons annihilated, neutron
production ceased
• After 1 minute:
– Temperature low enough for atomic nuclei
to form. Cosmogenic nucleosynthesis of
helium and other light elements
Slide 6
• After production of particles had
ceased, the majority of the energy
density of the Universe was in radiation.
• After 10,000 years, the expansion had
caused the temperature to drop enough
for the density of matter to exceed the
density of radiation.
Slide 7
Matter dominated era
• Temperatures of radiation and matter still
coupled through electron scattering.
• After 380,000 years of expansion the radiation
field became too cool to ionize hydrogen.
– Protons and electrons combined to form atoms. With
little electron scattering, the Universe became optically
thin. Radiation and matter decoupled, the radiation
from this ‘surface of last scattering’ is now redshifted
by about 1000, and is seen as the cosmic microwave
background today .
Slide 8