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Understanding the Cosmic Microwave Background: A Brief Overview of Cosmology

This presentation outlines the essential features of cosmology with a focus on the Cosmic Microwave Background (CMB). Detected in 1965 by Penzias and Wilson, the CMB is a relic radiation that provides insights into the early universe. We discuss its significance as a nearly perfect black body and how tiny temperature anisotropies offer invaluable information about the cosmos. The presentation covers the fundamental principles of the universe's expansion, the Friedmann equations, and how our observations align with the cosmological principles of isotropy and homogeneity.

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Understanding the Cosmic Microwave Background: A Brief Overview of Cosmology

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  1. Cosmology : a short introduction Mathieu Langer Institut d’Astrophysique Spatiale Université Paris-Sud XI Orsay, France Egyptian School on High Energy Physics CTP-BUE , Egypt 27 May – 4 June 2009

  2. 0. What do we see ? (depends on wavelength…)

  3. Cosmic Microwave Background (detected 1965, Penzias & Wilson, Nobel prize 1978) (COBE data, 1996)

  4. First detection 1965 at 7.35 cm Penzias & Wilson Nobel Prize 1978

  5. What Penzias & Wilson would have seen, had they observed the full sky The Milky Way Cosmological interpretation: Dicke, Peebles, Roll, Wilkinson (1965)

  6. Cosmic Microwave Background (detected 1965, Penzias & Wilson, Nobel prize 1978) (COBE data, 1996)

  7. The Cosmic Microwave Background : a “perfect” black body

  8. The Cosmic Microwave Background : a “perfect” black body

  9. CMB : tiny anisotropies COBE, 1991-1996 First detection of anisotropies (Nobel prize 2006: Smoot & Mather)

  10. CMB : tiny anisotropies, huge information -200 µK < ΔT < 200 µK First fine-resolution full-sky map (0.2 degrees) WMAP: 2003, 2006, 2008 (Launched June 2001)

  11. CMB anisotropies : angular power spectrum From temperature maps… …to power spectra…

  12. …to cosmological parameters and cosmic pies : Age : 13.7 billion years

  13. Panoramic view of the entire near-infrared sky Blue : nearest galaxies Red : most distant (up to ~ 410 Mpc) Distribution of structure on large scales (2MASS, XSC & PSC)

  14. Notice : isotropy & homogeneity!

  15. Hubble’s law, expansion of the universe V = H0 D H0 = 71 ± 4 km/s/Mpc (from WMAP + Structures) (Hubble, 1929) Rem : 1 parsec ~ 3.262 light years ~ 3.1×1013 km

  16. Ambitious cosmology…

  17. Our understanding of the universe…

  18. 1. How do we understand what we see?

  19. Cosmological principle Universe : spatially homogeneous & isotropiceverywhere  Applies to regions unreachable by observation Copernican principle Our place is not special  observations are the same for any observer Isotropy + Copernicus  homogeneity  Applies to observable universe Fundamental principles

  20. Friedmann-Lemaître-Robertson-Walker metric Maximally symmetric space-time equivalent to where

  21. Coordinates : Scale factor a(t): Redshift & Expansion : Scale factor, expansion, Hubble’s law

  22. Hubble’s flow : 2 observers at comoving coordinates x1 & x2 Physical distance : Separation velocity : Proper velocities Galaxy moving relative to space fabric  x not constant Velocity : Scale factor, expansion, Hubble’s law  scatter in Hubble’s law for nearby galaxies

  23. Einstein equations : geometry  energy content Friedmann equations : dynamics of the Universe Dynamics : Einstein, Friedmann, etc. Stress-energy tensor: Expansion rate Variation of H

  24. Critical density : put k = 0 today (cf. measurements!) Density parameters : Equation of state : for each fluid i : pi = wiρi Dynamics and cosmological parameters and today: • Photons : p = ρ/3  wr=1/3 • Matter : ρ = mn, p = nkTρ wm = 0

  25. Friedmann equations expansion variation acceleration Matter-Energy conservation : Dynamics of the Universe so clearly (Rem: only 2 independent equations)

  26. Evolution of a given fluid : Conservation equation gives Summary : * assume wi constant, * integrate Rem : C.C.  wΛ= -1

  27. Matter-radiation equality Expansion history wrt. dominant fluid Universe Expansion History (from WMAP)  for zzeq : Universe dominated by radiation 

  28. Acceleration wrt. fluid equation of state of dominant fluid Deceleration Acceleration Universe Expansion History Matter and radiation OK Observed accelerationrequires exotic fluid withnegative pressure!

  29. Back to the CMB… time, age density, z, T radiation & matter in thermal equilibrium radiation & matter live separate lives

  30. CMB : Primordial Photons’ Last Scattering 380 000 years time, age (Planck) density, z, T radiation & matter in equilibrium via tight coupling radiation & matter are decoupled, no interaction CMB z =1100

  31. The CMB : a snapshot of the Baby Universe

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