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

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
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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

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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
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Graphic from WMAP website
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An image of the Universe at 380,000 years old
The CMB
(Cosmic Microwave Background)
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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)

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Graphic from WMAP website
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• 400 photons per cubic cm !

Graphic by Wayne Hu, http//background.uchicago.ed
u/whu/beginners/introduction.html
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The History of CMB observations
1965
Discovery
COBE
1992
Graphic from WMAP website
2003
WMAP
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The frequency spectrum

• Universe in equlibrium -gt 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

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frequency spectrum
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COBE residuals (Mather et al. 1994)
COBE residuals (Mather et al 1994)
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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
WMAPWilkinson Microwave Anisotropy Probe
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Launch June 2001
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What WMAP saw
Graphic from WMAP website
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The Isotropy of the CMB

• CMB snapshot of z1000 universe
• z1000 universe was homogeneous
• Leads to 'Horizon problem'
• Horizon size c x time since Big-Bang
• Horizon at z1000 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

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Zooming the colour scale
1 in 1000
Graphic from WMAP website
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Removing the effect of our motion through the
galaxy
Graphic from WMAP website
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Observations in 5 frequency bands23 GHz to 90 GHz
Graphic from WMAP website
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We have to look through our own galaxy
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Dust in our galaxy is the most prominent feature
Graphic from WMAP website
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An image of the Universe at 380,000 years old!
Graphics from WMAP website
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A characteristic scale exists of 1 degree
Graphics from WMAP website
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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

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What are the Cls?

• Qualitatively power in each Fourier mode
• Quantitatively

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Spherical Harmonics
http//web.uniovi.es/qcg/harmonics/harmonics.html
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3 regimes of CMB power spectrum
Acoustic oscillations
Damping tail
Large scale plateau
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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
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Increasing Baryon Density
Graphic by Wayne Hu, http//background.uchicago.ed
u/whu/beginners/introduction.html
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Decreasing Matter Density
Graphic by Wayne Hu, http//background.uchicago.ed
u/whu/beginners/introduction.html
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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 featureposition of 1st peak

• Bouncing fluid causes peak structure
• Curvature of Universe -gt peak locations

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Bouncing fluid

• Photon-baryon fluid oscillates in dark matter
  potential wells

• Large scales oscillate slowest

Graphic by Wayne Hu, http//background.uchicago.ed
u/whu/beginners/introduction.html
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An analogy

• Drop bouncy balls from different heights and wait
  5 minutes
• Lower balls bounce more times
• Highest balls dont even reach the ground
• There is one ball that just touches the ground in
  the time available

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The link with cosmology

• 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

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The first acoustic peak

• Consider scale which had time only to collapse
  under gravity since big-bang
• it is at maximum T gt hot-spot
• Scale collapse speed x time allowed
• sound speed x age of universe at z1000
• 200 (? mh2) Mpc comoving
• 1 degree

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Graphic from WMAP website
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Graphic from WMAP website
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Which way will the peak move?
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Flat ? OpenPeak shifts to the right
Graphic by Wayne Hu, http//background.uchicago.ed
u/whu/beginners/introduction.html
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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

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Plot on linear axes
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Graphic by Wayne Hu, http//background.uchicago.ed
u/whu/beginners/introduction.html
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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 -gt deep well
• ? ? / ? ? T / T ? ? / c2
• Full GR
• factor 2/3 deep wells, t is smaller -gt hotter
• ? T / T 1/3 ? ? /c2

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ISW

• Integrated Sachs-Wolfe effect
• Due to photons travelling through collapsing
  structures
• Boosts power at low l in CTTl

Graphic by Wayne Hu, http//background.uchicago.ed
u/whu/beginners/introduction.html
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WMAP results
Graphic from WMAP website
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CMB cheat sheet
?Omh2
?Age of Universe
?Obh2
? ? OmO?
?ns
?zre
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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

55
CMB polarization

• Measure polarization angle and mag. at each point
  on the sky
• Decompose this into 2 maps E and B
• E is like contour lines on a map
• B is like water going down a plughole
• Can correlate with non-polarized signal
• possible power spectra TT, TE, TB, EE, EB, BB
• Expect TB and EB to be zero due to parity

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From WMAP website
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Graphic by Wayne Hu, http//background.uchicago.ed
u/whu/beginners/introduction.html
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3 regimes of CMB TE polarization
Reionization peak
Acoustic oscillations
Damping
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hu polarization plot
TT
TE
EE
BB
Graphic by Wayne Hu, http//background.uchicago.ed
u/whu/beginners/introduction.html
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CMB TE power spectrumacoustic oscillations

• Dominant effect is velocity of baryons
• Velocities are maximum when density contrast is
  minimum
• Polarization is out of phase with acoustic peaks
• Measures same cosmological parameters as TT.
  Tests assumptions

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WMAP results
Acoustic oscillations
From WMAP website
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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 CTEl, CEEl, CBBl
• The future

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The near future

• Full WMAP data release
• error bars shrink by factor of 2
• EE results
• Small scale data from ground based
• CBI, VSAE, VSASE, ACBAR
• Polarization experiments
• Boomerang, CBI, Pique, Clover, QUest

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From Planck website
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From Planck website
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Planck the ultimate (2008)

• MAP is cosmic variance limited for llt?
• so Planck cannot improve here
• beam size ? cf MAP ?
• max l ? cf MAP ?
• write the above on a Planck cls plot

Graphic by Wayne Hu, http//background.uchicago.ed
u/whu/beginners/introduction.html
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Graphic from WMAP website
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Planck the ultimate (2008)

• MAP is cosmic variance limited for llt?
• so Planck cannot improve here
• beam size ? cf MAP ?
• max l ? cf MAP ?
• write the above on a Planck cls plot

Graphic by Wayne Hu, http//background.uchicago.ed
u/whu/beginners/introduction.html
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The holy grail, BB

• Why bother measuring EE?
• isocurvature modes
• develops technology for BB
• Primordial BB ? primordial gravity waves
• Predicted by some inflation models
• predicted not to exist by ekpyrotic models
• Severe difficulties
• signal expected to be small
• contamination by gravitational lensing

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Learning Outcomes

• Explain origin of the CMB
• Sketch frequency and temperature fluctuation
  spectra
• Explain 1st peak origin and position
• Explain how polarisation of CMB can arise
• List some upcoming experiments
• Next lecture Cosmic Shear, The new CMB