physics 133: extragalactic astronomy and cosmology lecture 15; march 5 2014
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Physics 133: Extragalactic Astronomy and Cosmology
Lecture 15; March 5 2014
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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.
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Outline:
• How about before CMB decoupling?– What can be predicted and measured?
• Introduction to nucleosynthesis– Neutrons and protons– Deuterium– Heavy elements and cosmography
• Baryon-antibaryon asymmetry• Problems of the standard Big Bang model:
– Flatness Problem– Horizon Problem– Monopole Problem
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What was before this?
We cannot see, yet we have a way to probe into the time before LS
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Scales
• At t<<47,000 (what is this?): at1/2
• T(t)~10e10 k (t/s)-1/2 ,i.e. kT~1MeV (t/s)-1/2
• MeV is the scale of nuclear binding energies!• At t~1s the universe is hot enough to do
nuclear reactions!• At t~1ps, TeV scales (LHC)!
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Basics of Nuclear fusion/fission
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Example of fusion
Other examples?
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Neutrons are unstable
Decay time is 890s
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At first…
• At very early times reaction is in equilibrium (<<1s) [enough e- e+]
• Relative abundances given by Maxwell Boltzmann quation:– N(n)/N(p)=exp(-Q/KT)
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…but then
• Cross section for weak interactions decays very rapidly with temperature
• Eventually interaction rate drops below expansion rate:– FREEZE OUT!
• [Blackboard]
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After freeze out
• Most remaining neutrons get captured by p forming Deuterium [Saha’s 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 it’s the most stable nucleus
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Final outcome
• Nuclear reactions as long as the expanding universe supports them
• By ~5m everything is over!
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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
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BBN + measurement =baryon abundance
• If we measure e.g. D/H we infer – Best value
5.5±0.5e-10
• We know T(CMB), so we obtain n(baryons)! At 10%
• How do we measure D/H?
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Baryon-antibaryon asymmetry
• There are much many more photons than baryons
• There are much many more baryons than antibaryons
• What happened?
• [Blackboard]
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Problems with classic Big Bang
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Flatness Problem
• At present time the universe is very close to flat
• You can consider this as a coincidence… however.. [blackboard]
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Horizon Problem
• The CMB is VERY ISOTROPIC!
• Perhaps too isotropic..• Regions too far on the sky
are not causally connected, because their distance is larger than the horizon at the last scattering surface [blackboard]
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Monopole ProblemGrand unified theories
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Phase transitions and symmetry breaking
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Monopole ProblemGrand unified theories
• When strong and electroweak forces break apart, magnetic monopoles are predicted to be left over with E~1e12 TeV
• Monopoles would dominate the energy density of the Universe!
• Monopoles have never been seen!
• [Blackboard]
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Summary I
• 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
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Summary II:
• Inspite of its great successes the classic Big Bang model has three major problems:– It’s too flat– It’s too isotropic– There are no magnetic monopoles
• The currently favored solution is called “inflation”• Next time we will see how inflation solves the three
problems
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The End
See you on monday