Cosmology I

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Cosmology I II

• Expanding universe
• Hot early universe
• Nucleosynthesis
• Baryogenesis
• Cosmic microwave background (CMB)
• Structure formation
• Dark matter, dark energy
• Cosmic inflation

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UNITS, NOTATION
Energy mass GeV Time length 1/GeV
c h kB 1
Metric signature (1,-1,-1,-1)
Planck mass MP 1.22 ? 1019 GeV Newtons
constant G 1/ MP 1 eV 11000 K 1 s 1/MeV
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Quantities, observables

• Hubble rate expansion rate of the universe H
• Energy density of particle species x ?x E/V
• Number density nx N/V
• Relative He abundance Y 4He/(H4He)
• Baryon number of the universe (nB-nB)/n?
• Scattering cross section ? 1/energy2, (decay)
  rate ? energy ?n

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(cont)

• CMB temperature T(?) T0 ?T(?) (CMB power
  spectrum)
• Galaxy-galaxy correlators (Large scale
  structure LSS)
• Distant supernova luminosities

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The starting point

• expansion of the universe is very slow (changes
  adiabatic) H ltlt scattering rates
• Thermal equilibrium ( some deviations from this
  is where the interesting physics lies)
• Need statistical physics, particle physics, some
  general relativity

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History of cosmology

• General theory of relativity 1916
• First mathematical theory of the universe
• Applied by Einstein in 1917
• Problem thought that universe Milky Way ?
  overdense universe ? must collapse ? to
  recover static universe must introduce
  cosmological constant (did not work)

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Theory develops

• Willem de Sitter 1917
• Solution to Einstein equations, assuming empty
  space (exponential) expansion (but can be
  expressed in stationary coordinates)
• Alexander Friedmann 1922
• Solution to Einstein eqs with matter no static
  solution
• Universe either expanding or collapsing

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Observations

• Henrietta Leavitt 1912
• Cepheids luminosity and period related ?
  standard candles
• Hubble 1920s
• 1923 Andromeda nebula is a galaxy (Mount Wilson
  100 telescope sees cepheids)
• 1929 redshifts of 24 galaxies with independent
  distance estimates ? the Hubble law v Hd

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• Georges Lemaitre 1927 primeaval atom
• Cold beginning, crumbling supernucleus (like
  radioactivity)
• George Gamow 1946-1948
• Hot early universe (nuclear physics the Sun)
• Alpher, Gamow, Herman 1948 relic photons with a
  temperature today of 5 K
• Idea was all but forgotten in the 50s

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Demise of the steady state

• Fred Hoyle 1950s
• steady state theory the universe is infinite
  and looks the same everywhere
• New matter created out of vacuum ? expansion
  (added a source term into Einstein eqs.)
• Cambridge 3C galaxy survey 1959
• Radiogalaxies do not follow the distribution
  predicted by steady state theory

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Rediscovery of Big Bang

• Penzias Wilson 1965 Bell labs
• Testing former Echo 6 meter radioantenna to use
  it for radioastronomy (1964)
• 3 K noise that could not be accounted for
• Dicke Peebles in Princeton heard about the
  result ? theoretical explanation redshifted
  radiation from the time of matter-radiation
  decoupling (recombination) CMB
• Thermal equilibrium ? black body spectrum
• Isotropic, homogenous radiation however,
  universe has structure ? CMB must have spatial
  temperature variations of order 10-5 K

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Precision cosmology

• COBE satellite 1992
• Launch 1989, results in 1992
• Scanned the microwave sky with 2 horns and
  compared the temperature differences
• Found temp variations with amplitude 10-5 K,
  resolution lt 7O
• Balloon experiments end of 90s
• Maxima, Boomerang first acoustic peak discovered
• LSS surveys
• 2dF etc 90s ongoing Sloan Digital Sky Survey
  (SDSS)

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• WMAP 2003
• High precision spectrum of temperature
  fluctuations
• Determination of all essential cosmological
  parameters with an accuracy of few
• Big bang nucleosynthesis 1980s ?
• H, He, Li abundances (N?, ?)
• Planck Surveyor Mission 2007 (Finland
  participates)

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Surprises/problems

• Dark matter (easy)
• Dark energy ( cosmological constant, very hard)
• Cosmic inflation (great, but how?)
• Baryogenesis (how?- Standard Model not enough)