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Physics 133: Extragalactic Astronomy and Cosmology

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This section extends the work done in the first section, generalising from ... Graph taken from Essential Pure Mathematics, J. K. Backhouse et al., Longman ... – PowerPoint PPT presentation

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Title: Physics 133: Extragalactic Astronomy and Cosmology


1
Physics 133 Extragalactic Astronomy and Cosmology
Lecture 21 March 9 2007
2
Previously
  • CMB fluctuations are generated by fluctuations in
    the gravitational field at the time of last
    scattering
  • The angular scale of the fluctuations gives us
    information on the content of the universe.

3
Outline
  • How about before the surface of last scattering
    of the CMB?
  • What can be predicted and measured?
  • Introduction to nucleosynthesis
  • Neutrons and protons
  • Deuterium
  • Heavy elements and cosmography
  • Baryon-antibaryon asymmetry

4
What was before this?
We cannot see, yet we have a way to probe into
the time before LS
5
Scales
  • At tltlt47,000 (what is this?) a?t1/2
  • T(t)10e10 k (t/s)-1/2 ,i.e. kT1MeV (t/s)-1/2
  • MeV is the scale of nuclear binding energies!
  • At t1s the universe is hot enough to do nuclear
    reactions!
  • At t1ps, TeV scales (LHC)!

6
Basics of Nuclear fusion/fission
7
Example of fusion
Other examples?
8
Neutrons are unstable
Decay time is 890s
9
At first
  • At very early times reaction is in equilibrium
    (ltlt1s) enough e- e
  • Relative abundances given by Maxwell Boltzmann
    quation
  • N(n)/N(p)exp(-Q/KT)

10
but then
  • Cross section for weak interactions decays very
    rapidly with temperature
  • Eventually interaction rate drops below expansion
    rate
  • FREEZE OUT!
  • Blackboard

11
After freeze out
  • Most remaining neutrons get captured by p forming
    Deuterium Sahas equation
  • Maximum number of non-H nuclei is set by
    abundance of n at freeze-out and their decay.
  • Most of the non-H nuclei end up as He because
    its the most stable nucleus

12
Final outcome
  • Nuclear reactions as long as the expanding
    universe supports them
  • By 5m everything is over!

13
Critical parameter
  • The yield is dominated by ? (remember
    recombination?)
  • High ? starts BBN early and is more efficient at
    producing He
  • So there are fewer leftovers..
  • Li is more complicated since there are competing
    channels

14
BBN measurement baryon abundance
  • If we measure e.g. D/H we infer ?
  • Best value 5.50.5e-10
  • We know T(CMB), so we obtain n(baryons)! At 10
  • How do we measure D/H?

15
Baryon-antibaryon asymmetry
  • There are much many photons than baryons
  • There are much many more baryons than antibaryons
  • What happened?
  • Blackboard

16
Summary
  • Theory of big bang nucleosynthesis predicts the
    abundance ratios of light elements remarkably
    well
  • So well that it can be used to measure baryon
    abundance.
  • By the way this is another piece of evidence for
    non-baryonic dark matter
  • The dominance of matter over antimatter is
    explained in the standard model by a tiny
    violation of symmetry

17
The End
  • See you on Monday!
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