The Cosmic Microwave Background

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The Cosmic Microwave Background

• Based partly on slides Joe Mohr
• (University of Chicago)

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• History of the Universe
• superluminal inflation,
• particle plasma,
• atomic plasma,
• recombination,
• structure formation

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(No Transcript)
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Outline

• 3. COBE and WMAP
• Nature of the bkgd radiation
• Uniformity of background
• Detecting our motion
• Seeds of structure formation
• 4. Review

• 1. Introduction
• Relic radiation
• Penzias and Wilson at Bell Labs
• 2. Blackbody radiation
• Electromagnetic spectrum
• Lamps, stars and people
• Effects of expansion

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Implications of an Expanding Universe

• Reactions to an expanding universe
• Gamow predicts (1940s) hot, dense early phase
• Novikov predicts (1962) relic radiation from hot,
  dense phase
• Dicke was interested in finding a radiation
  background
• Arno Penzias and Robert Wilson
• Study radio emission at 7cm
• Bell Labs in New Jersey
• Discover background in 1965
• temperature is 3 Kelvin
• isotropic
• Dicke explained significance

Penzias and Wilson with radio horn
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Effects of Expansion on Light

• As the universe expands, light wavelengths
  stretch with space. Photons gravitationally red
  shifted or simply stretched with the expanding
  space.

• Temperature is directly proportional to
  wavelength. The effective temperature of a
  blackbody spectrum decreases as the wavelength
  stretches.

• Galaxy velocities Doppler shifts or universal
  expansion?

rm1/a3 rr1/a4 where acharacteristic scale size
of universe
Sphere courtesy Wayne Hu
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Extrapolation into the Past
Present

• Present day
• Universe cold (3K) with low matter density
• one hydrogen atom per 10 cubic meters
• 400 million CMB photons per cubic meter
• CMB photons and matter rarely interact -
  transparent
• typical matter in form of atoms and molecules
• Recombination or last scattering surface
• universe hot (3,000K) and a billion times denser
• photon energy high enough to ionise atoms and
  molecules
• plasma of e-, p and a (plus trace He3,
  deuterium, Li and Be)
• CMB photons coupled to matter through collisions
• Pre-recombination
• universe even hotter and denser
• CMB photons coupled to matter through collisions
• Early universe hot enough for pair creation,
  neutrino opacity and many particle processes.

13 Gyr
0.5 Myr
Time
lt1 yr
Early Universe
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A Pictorial History of the CMB
Last Scattering Surface where recombination of
electrons and protons takes place.
Blackbody light
Edge of Observable Universe- distance
light could have traveled over age of universe.
The Observable Universe
Observer
Blackbody light
Blackbody light emitted in the surface of last
scattering travels in all directions. We only
see that portion which happens to set off in a
direction that leads it into one of our
detectors.
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Blackbody Radiation

• Every opaque object emits blackbody radiation
• Blackbody spectrum
• Continuous spectrum, depends only on temperature
• Hotter bodies brighter, bluer, shorter l
• Cooler bodies dimmer, redder, longer l

Planck radiation law
Stefan-Boltzmann Law
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Blackbody Radiation Contd

• Stefan-Boltzmann Constant 5.67 x 10-8 Wm2T-4
• 10K 0.56mW/m2
• 300K 450W/m2
• 1000K 56kW/m2
• 104K 560MW/m2

At peak
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Cosmic Background Explorer (COBE)

• NASA satellite designed to test nature of cosmic
  background radiation

• Three instruments
• FIRAS- Far Infrared Absolute Spectrophotometer
• measure CMB spectrum
• DMR- Differential Microwave Radiometers
• measure variations in temperature on the sky
• DIRBE- Diffuse Infrared Background Experiment

Image courtesy COBE homepage.
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FIRAS Spectrum of CMB
Theoretical blackbody spectrum 34 observations
over-plotted largest deviation 0.03
T2.728/-0.004 K
Image courtesy COBE homepage.
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Imaging the Globe with the COBE DMR
Image of the world
Images courtesy E. Bunn
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Imaging the Globe with the COBE DMR
Image of the world
Image with COBE angular resolution
Images courtesy E. Bunn
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Imaging the Globe with the COBE DMR
Image of the world
Image with COBE measurement noise
Image with COBE angular resolution
Images courtesy E. Bunn
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Imaging the Globe with the COBE DMR
Image of the world
Image with COBE measurement noise
Image with COBE angular resolution
COBE-like image smoothed to reduce noise
Images courtesy E. Bunn
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COBE DMR Image

• The sky temperature with range from 0-4 Kelvin
• Microwave background is very uniform at 3 Kelvin

Image courtesy COBE homepage.
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COBE DMR Image 1,000X Zoom

• The sky temperature with range from 2.724-2.732
  Kelvin
• blue is 2.724 K and red is 2.732 K
• Dipole pattern in temperature indicates motion
• Doppler Effect at level of 0.005 K
• Solar system is traveling at 400 km/s with
  respect to CMB

Image courtesy COBE homepage.
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COBE DMR Image 25,000X Zoom

• The sky temperature ranging from 2.7279-2.7281
  Kelvin
• blue is 2.7279 K and red is 2.7281 K
• Dipole variation from Solar system motion removed
• Red emission along equator is galactic emission
• Other fluctuations are likely cosmic in origin

Image courtesy COBE homepage.
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COBE DMR Image Galaxy and Dipole Removed
Amplitude of temperature fluctuations is 30mK
/-3 mK in 10 degree patches. (1 part in 105)
Image courtesy COBE homepage.
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WMAP reduced in resolution to COBE
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WMAP All Sky Image 2002 galaxy removed
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WMAP half sky image and examples of fluctuations
on varying scales
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The Angular Power Spectrum of the CMB
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1999 Image Analysis theory and experiment
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Analysis of CMB Images
Angular Power Spectrum
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Gravitational Enhancement

• Before recombination dark matter fluctuations
  with scale size matching the fundamental acoustic
  wave cause increased clumping of baryons and
  photons. Photons from the troughs are red
  shifted.

By the time of recombination the excess density
regions have been heated enough that the phase
is reversed and the temperature fluctuations are
3 times enhanced.
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Second Harmonic Gravitational Suppression
For even harmonics of the acoustic wave, the same
initial condition (cooler troughs) leads to
density increase and heating well before
recombination.
Because of the shorter scale size there is enough
time for pressure (blue arrows) to act to oppose
gravity (white arrows), thus suppressing the
second peak.
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Summary

• Microwave background observed
• Penzias and Wilson at Bell Labs in 1965 with
  sensitive radio telescope
• NASA Cosmic Background Explorer (COBE) satellite
  in early 1990s
• NASA WMAP Microwave Anisotropy Probe 2002
• CMB photons have travelled 13 billion years to
  reach us
• Nature of cosmic background radiation
• precise blackbody spectrum with temperature of
  2.725K
• highly uniform temperature
• small dipole evidence for our motion at 400
  km/s
• anisotropies 1 part in 105 if you examine 10
  degree patches of sky
• -image analysis consistent with detailed
  cosmological model involving acoustic
  oscillations in early universe
• Universe hot and dense enough to behave as
  blackbody in past
• Fluctuations over non-causally connected regions
  implies inflation
• Fluctuations over causally connected regions
  allows determination of mass density, dark
  matter and dark energy
• See Wayne Hu Sciama lecture, animations and Sci
  Am Feb 2004

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The Electromagnetic Spectrum
Particles Photons
Wavelength Frequency
Light
Waves
Energy
electron
l
Photons and electrons scatter off one another
like billiard balls.
photon
Images from Imagine the Universe! site at
Goddard Space Flight Center http//imagine.gsfc.na
sa.gov/docs/homepage.html
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Stellar Spectrum
Simple Model of a Star
Sodium
Intensity
Magnesium
Calcium
Fusion in the center of the star is energy
source.
Atoms in the cooler, lower density sur- face gas
absorb light at specific wave- lengths,
creating absorption lines.
Wavelength l
Hot, dense gas cools by emitting blackbody
radiation. The Sun emits blackbody rad- iation
with an effective temperature of 5,500 K.
Observed stellar spectrum. Note the large number
of absorption lines.
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Infrared Emission from Living Things
Infrared image of a cat. Orange is brighter (and
warmer) and blue is dimmer (and cooler). Note the
warm eyes and cold nose.
Infrared image of a man with sunglasses and a
burning match. Black is dim (cold) and white it
bright (hot).
Images from IPAC at the Jet Propulsion
Laboratory. The cat image comes courtesy of
SE-IR corporation.
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Compton Lectures

• Foundations of the Hot Big Bang Model
• 1 Observing the Expansion of the Universe
• 2 The Cosmic Microwave Background (CMB)
• 3 Creation of the Elements in the Early
  Universe
• 4 The Dark Night Sky, Causality and Geometry
• 5 A Timeline for the Universe
• Current Topics in Observational Cosmology
• 6 Mapping the Large Scale Structures in the
  Nearby Universe
• 7 Observing the Seeds of Structure Formation in
  the CMB
• 8 Detecting Dark Matter with the Chandra X-ray
  Satellite
• 9 Measuring the Size and Geometry of the
  Universe
• 10 Using Shadows in the CMB to Map the Edge of
  the Universe