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UCL Graduate Lectures An Introduction to Cosmology Sarah Bridle 19 October 2004

UCL Graduate Lectures An Introduction to Cosmology Sarah Bridle 19 October 2004. Lect 1: Global contents and dynamics of the Universe Lect 2: Dark matter clustering and galaxy surveys Lect 3: The cosmic microwave background radiation Lect 4: Gravitational lensing.

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UCL Graduate Lectures An Introduction to Cosmology Sarah Bridle 19 October 2004

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  1. UCL Graduate LecturesAn Introduction to CosmologySarah Bridle19 October 2004 Lect 1: Global contents and dynamics of the Universe Lect 2: Dark matter clustering and galaxy surveys Lect 3: The cosmic microwave background radiation Lect 4: Gravitational lensing

  2. This lecture:The Cosmic Microwave Background • The origin of the CMB, spectrum, history • WMAP satellite and maps • The CMB power spectrum, CTTℓs • Understanding main features, esp 1st peak position • Polarization • The future

  3. This lecture:The Cosmic Microwave Background • The origin of the CMB, spectrum, history • WMAP satellite and maps • The CMB power spectrum, CTTℓs • Understanding main features, esp 1st peak position • Polarization • The future

  4. The History of the Universe Universe is hot Electrons are free Light scatters off electrons Until ~380,000 years after BB Universe is cooler e- and p+ form hydrogen Light travels freely

  5. Graphic from WMAP website

  6. An image of the Universe at 380,000 years old The CMB (Cosmic Microwave Background)

  7. Why Microwave? • Universe was ~ 3000° K at 380,000 yr • Full of visible light (~1μm) Universe is expanding • Causes light to change wavelength • Visible light becomes microwaves (~1cm)

  8. Graphic from WMAP website

  9. 400 photons per cubic cm ! Graphic by Wayne Hu, http://background.uchicago.edu/~whu/beginners/introduction.html

  10. The History of CMB observations 1965 Discovery COBE 1992 Graphic from WMAP website 2003 WMAP

  11. The frequency spectrum • Universe in equlibrium -> Black body • Observe perfect black body at 2.73K • Can relate present day no. photons, protons, 13.6eV to get Trecombimation. • From TCMB today, get zrecombination

  12. frequency spectrum

  13. COBE residuals (Mather et al. 1994) COBE residuals (Mather et al 1994)

  14. This lecture:The Cosmic Microwave Background • The origin of the CMB, spectrum, history • WMAP satellite and maps • The CMB power spectrum, CTTℓs • Understanding main features, esp 1st peak position • Polarization • The future

  15. The WMAP Satellite Graphic from WMAP website WMAP=Wilkinson Microwave Anisotropy Probe

  16. Launch June 2001

  17. What WMAP saw Graphic from WMAP website

  18. The Isotropy of the CMB • CMB = snapshot of z~1000 universe • z~1000 universe was homogeneous • Leads to 'Horizon problem' • Horizon size ~ c x time since Big-Bang • Horizon at z~1000 is ~ 1° on sky • Sky at 0° and 180° not yet 'causally connected' • 'Inflation' invoked to solve • Rapid expansion expands horizon scale to greater than observable universe size

  19. Zooming the colour scale… 1 in 1000 Graphic from WMAP website

  20. Removing the effect of our motion through the galaxy Graphic from WMAP website

  21. Observations in 5 frequency bands23 GHz to 90 GHz Graphic from WMAP website

  22. We have to look through our own galaxy

  23. Dust in our galaxy is the most prominent feature Graphic from WMAP website

  24. An image of the Universe at 380,000 years old! Graphics from WMAP website

  25. A characteristic scale exists of ~ 1 degree Graphics from WMAP website

  26. This lecture:The Cosmic Microwave Background • The origin of the CMB, spectrum, history • WMAP satellite and maps • The CMB power spectrum, CTTℓs • Understanding main features, esp 1st peak position • Polarization • The future

  27. Statistical properties • Spherical harmonic transform • ~Fourier transform • Quantifies clumpiness on different scales

  28. What are the Cℓs? • Qualitatively: ~power in each Fourier mode • Quantitatively:

  29. Spherical Harmonics http://web.uniovi.es/qcg/harmonics/harmonics.html

  30. 3 regimes of CMB power spectrum Acoustic oscillations Damping tail Large scale plateau

  31. Cosmological Parameters • Universe content:b, DM, f, , w(z) • Universe dynamics:H0 • Clumpiness:8, ns(k) • Primoridial gravity waves:At, nt • When the first stars formed:zre • Other: WDM, isocurvature, non-Gaussianity... Each parameter has an effect on the CMB

  32. Increasing Baryon Density Graphic by Wayne Hu, http://background.uchicago.edu/~whu/beginners/introduction.html

  33. Decreasing Matter Density Graphic by Wayne Hu, http://background.uchicago.edu/~whu/beginners/introduction.html

  34. This lecture:The Cosmic Microwave Background • The origin of the CMB, spectrum, history • WMAP satellite and maps • The CMB power spectrum, CTTℓs • Understanding main features, esp 1st peak position • Polarization • The future

  35. Understand main feature:position of 1st peak • Bouncing fluid causes peak structure • Curvature of Universe -> peak locations

  36. Bouncing fluid • Photon-baryon fluid oscillates in dark matter potential wells • Large scales oscillate slowest Graphic by Wayne Hu, http://background.uchicago.edu/~whu/beginners/introduction.html

  37. An analogy • Drop bouncy balls from different heights and wait 5 minutes • Lower balls bounce more times • Highest balls don’t even reach the ground • There is one ball that just touches the ground in the time available

  38. Balls bouncing 5 minutes Bouncing Original height of ball that only just reaches the ground Photon-baryon fluid oscillating Age of universe at recombination Peaks in CMB plot Position of first peak The link with cosmology

  39. The first acoustic peak • Consider scale which had time only to collapse under gravity since big-bang • it is at maximum T => hot-spot • Scale = collapse speed x time allowed ~ sound speed x age of universe at z~1000 ~ 200 (Ω mh2) Mpc comoving ~ 1 degree

  40. Graphic from WMAP website

  41. Graphic from WMAP website

  42. Which way will the peak move?

  43. Flat  OpenPeak shifts to the right Graphic by Wayne Hu, http://background.uchicago.edu/~whu/beginners/introduction.html

  44. Secondary peaks • Plot is ~ FT of (T() -mean(T)) • Second peak = collapse, expand to max • Third peak = collapse, expand, collapse • etc.. • Expect peaks to be equally spaced in l

  45. Plot on linear axes

  46. Graphic by Wayne Hu, http://background.uchicago.edu/~whu/beginners/introduction.html

  47. Largest scales: the Sachs-Wolfe effect • Gravitational potential wells due to DM • Follows large scale matter power spectrum • Photons climb out of potential wells • gravitational redshift: cold -> deep well •  /  ~  T / T ~  / c2 • Full GR • factor 2/3 ~ deep wells, t is smaller -> hotter • T / T = 1/3  /c2

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