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Cosmology I & II

Cosmology I & II. Expanding universe Hot early universe Nucleosynthesis Baryogenesis Cosmic microwave background (CMB) Structure formation Dark matter, dark energy Cosmic inflation. UNITS, NOTATION. Energy = mass = GeV Time = length = 1/GeV. c = ħ = k B = 1.

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Cosmology I & II

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  1. Cosmology I & II • Expanding universe • Hot early universe • Nucleosynthesis • Baryogenesis • Cosmic microwave background (CMB) • Structure formation • Dark matter, dark energy • Cosmic inflation

  2. UNITS, NOTATION Energy = mass = GeV Time = length = 1/GeV c = ħ= kB = 1 Metric signature = (1,-1,-1,-1) Planck mass MP = 1.22  1019 GeV Newton’s constant G = 1/ MP 1 eV = 11000 K 1 s ~ 1/MeV 2

  3. 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/(H+4He) • Baryon number of the universe (nB-nB)/n • Scattering cross section  ~ [1/energy2], (decay) rate  ~ [energy] ~ n ¯

  4. (cont) • CMB temperature T() = T0 + T() (”CMB power spectrum”) • Galaxy-galaxy correlators (”Large scale structure” = LSS) • Distant supernova luminosities

  5. The starting point • expansion of the universe is very slow (changes adiabatic): H << scattering rates • Thermal equilibrium (+ some deviations from: this is where the interesting physics lies) • Need: statistical physics, particle physics, some general relativity

  6. 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)

  7. 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

  8. 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

  9. 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 50’s

  10. 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

  11. 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

  12. 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 < 7O • Balloon experiments end of 90’s • Maxima, Boomerang: first acoustic peak discovered • LSS surveys • 2dF etc 90’s; ongoing: Sloan Digital Sky Survey (SDSS)

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

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

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