Certificate programme in Science: Astronomy Core Module 4 Cosmology

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Certificate programme in Science Astronomy (Core
Module 4)Cosmology

• Dr Lisa Jardine-Wright,
• Institute of Astronomy, Cambridge University

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Lecture Overview

• Jones Lambourne
• Chapters 6.3 6.4
• Big Bang Nucleosynthesis
• Periodic Table
• Particle Physics
• Nuclear fusion
• Stars
• Early Universe
• Timeline of the elements
• BBN the expansion rate of the Universe

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Periodic Table
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Periodic Table
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Periodic Table
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Periodic Table
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Particle Physics

• Atomic Masses
• mp 1.673 x 10-27 kg
• mn 1.675 x 10-27 kg
• me 9.11 x 10-31 kg
• Isotopes
• 3He2 4He2
• Atoms of the same name have the same number of
  protons - same atomic number
• Isotopes have different number of protons but
  the same atomic number

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Particle Physics

• Radioactivity
• Isotopes want to decay to more stable isotopes.
  http//ie.lbl.gov/education/isotopes.htm
• Half-life
• The half-life of an isotope is the amount of time
  it takes for half of the atoms to decay into a
  more stable form.
• Naturally abundant isotopes exist around us
  because their half-lives are longer than the age
  of the earth.
• Uranium 238 (238U) has a half-life of 4.5 billion
  years so it is naturally abundant.
• Most isotopes have short half-lives and must be
  produced in the laboratory to study or use. For
  example, cobalt 60 (60Co) has a half-life of 5.3
  years and is made in a reactor.
• 60Co is used for radiation therapy of cancer
  patients.
• Over 3500 isotopes are known, and most are merely
  laboratory curiosities.

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Particle Physics

• Radioactivity
• Beta decay
• Neutron converted to a proton plus and electron
  and neutrino
• Alpha decay
• Emission of a 4He nucleus
• Gamma decay
• Emission of a gamma ray

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Herman, Alpher Gamow

• Primoridial Nucleosynthesis
• G Can element abundances be explained with BB
  theory?
• What about atoms of weight 5 and 8?
• BBN H He

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Burbidge, Burbidge, Fowler Hoyle

• Stellar Nucleosynthesis
• High densities and pressures can produce elements
  heavier than Helium
• Couldnt however produce enough helium

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Present Day Thinking

• Hydrogen Helium
• High densities and pressures can produce elements
  heavier than Helium
• Couldnt however produce enough helium
• Carbon
• Produced in stellar evolution (triple helium
  reaction
• Lithium Beryllium
• Cosmic ray collision resulting in carbon
  dissintegration

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Timeline of the Elements

• Tlt1 s
• Neutrons and protons in thermal equilibrium
• Until expansion causes the temperature to drops
  to
• T 9.3 Billion K
• Protons and neutrons freeze out (not enough
  energy to maintain the reactions) at a ratio of
  1 neutron for every 6 protons

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Timeline of the Elements

• Tgt1 s
• Neutron decay reduces the number of neutrons
• Half-life of a neutron 650s
• Without further reactions within stable nuclei
  the Universe would be pure hydrogen.
• T100s
• Neutrons preserved in nucleus of deuterium atom
  (2H)

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Timeline of the Elements

• Tgt100 s
• Temperature of the Universe now 1 Billion K

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Timeline of the Elements

• Tgt100 s
• Temperature of the Universe now 1 Billion K
• Faster reactions as no photo emission

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Timeline of the Elements

• Tgtgt100s
• Net effect
• Temperature eventually drops so low that the
  reaction stops
• Neutrons are preserved in the Helium Nuclei

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Big Bang nucleo- synthesis model
Time
ratio1/5 (t100s)
Deuterium production starts to preserve neutrons
ratio1/7 (t700sec)
value frozen at observed abundance
1MeV1.15x109K
k
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Light Element Abundances

• Helium
• We can use this reaction to help us calculate the
  ratio of helium to hydrogen in the Universe (by
  mass)

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Gamow Criterion

• 2H production is the first and crucial step for
  production of further light elements.
• If too much 2H is formed the neutrons are locked
  up and no higher elements form.
• If too little is formed there isnt enough 2H to
  form further elements.
• If there are too many photons in the Universe, 2H
  photodissociates.
• Number of baryons to photons is important

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Gamow Criterion

• Time 2H production started is also important.
• The longer the deuterium is around the more
  Helium will be produced.
• Measure the amount of helium tells us how long
  deuterium was produced
• We know what temperature the Universe is at when
  2H can be produced which tells us the expansion
  rate of the Universe

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Big Bang nucleosynthesis the ingredients

• Theory uses well-understood particle physics,
  nuclear reaction rates, weak-interactions, photon
  interactions.
• Observation 1 the Hubble constant, H0.
• Observation 2 the primordial abundances of 2H,
  3He, 4He and 7Li relative to H.
• Observation 3 the number-density of photons in
  the CMB.

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The nucleosynthesis argument

• To make the theory agree with the observed
  abundances the ratio of the number of baryons to
  the number of photons when the universe is 100
  sec old has to be in the range

• The baryon number-density is given by

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Abundance versus ?
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The nucleosynthesis argument - 2

• The critical density (see last lecture) is

• from the CMB the number-density of photons is
  N?4.11?108 photons/m3

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The nucleosynthesis argument - 3
Observations tell us that the Hubble constant is
in the range 0.6 lt h lt 0.8 Therefore assuming
that
we have
Observations (and theories) indicate that
0.1lt?lt1.0 so there must be some other matter
dominating the gravity of the universe.
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Abundance versus Baryon density
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Summary

• Measurements of the relative abundances of the
  light elements today when combined with
  measurements of the cosmic microwave background
  prove extremely powerful estimators of the
  conditions of the early Universe.
• If there were 100 times fewer photons to baryons
  in the early Universe dark matter would no longer
  be required.