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Observational Cosmology 2.The Cosmic Background
My goal is simple. It is complete understanding
of the universe, why it as it is and why it
exists as all.    Stephen Hawking.
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2.1 The Isotropic Background

• Is the Universe really homogeneous isotropic ??
  - Olbers Paradox revisited

WHY IS THE SKY SO DARK ?
Heinrich Olbers 1826 (Thomas Digges 1576)
The Sky should be the average surface brightness
of a star !!!
Solution The Universe has a finite age ? Not all
the light has had time to reach us yet !
This is the optical Olbers Paradox. BUT What
if Mr Olber had microwave eyes ?
The sky would be uniformly bright at l5cm At a
constant temperature of 2.73K
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2.1 The Isotropic Background

• Is the real Universe really homogeneous and
  isotropic ??

Actual Temperature Distribution
1 / 1000 Temperature variation
1 / 100, 000 Temperature variation
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2.1 The Isotropic Background

• The discovery of the microwave Background

• 1964 Penzias Wilson -
• Bell Laboratries Satellite Telecommunications at
  microwave wavelengths 7.35cm
• Found a value of 3.5K higher temperature than
  expected when turning antenna to blank sky
• Serendipitously discover the 2.73K microwave
  background radiation

These photons are the redshifted relic or ashes
of the Big Bang Originally high energy gamma
rays, these primordial photons have cooled to be
2.73K 2mm microwaves today
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2.2 The Origin of the Microwave Background

• Recombination and Decoupling

BIG BANG

• matter in thermal equilibrium with the
  radiation. photons and electrons to interact via
  Thompson scattering

• Temperature drops then pe-?H ? recombination

• Eventually interactions stop allowing the photons
  to flow freely on scales of the horizon ?
  de-coupling

• Era at which any photon last scattered off any
  electron surface of last scattering

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2.2 The Origin of the Microwave Background

• The Surface of Last Scattering

After Recombination and Decoupling the photons
are no longer bound to matter and can stream
freely Photons from the Big Bang fill the
universe and we observe them as the 2.7K
microwave background. These photons are the
redshifted relic or ashes of the Big Bang Last
time photons interacted ? Surface of Last
Scattering This also means that we can not
observe the Universe when it was younger than
400,000 years
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2.2 The Origin of the Microwave Background

• The Relic Background

• Today Energy density in Baryons is 800 times
  energy density in photons
• But Number density of Baryons to photon is 1
  in 109

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2.2 The Origin of the Microwave Background

• The Physics of Recombination

But even at lower temperatures sufficient photons
with appropriate ionization energy
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2.2 The Origin of the Microwave Background

• The Physics of Recombination

Between temperatures of To5000 ? 2000,
Ionization fraction drops 1 ? 0
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2.2 The Origin of the Microwave Background

• The Physics of Recombination

Decoupling Optical Depth
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2.2 The Origin of the Microwave Background

• The Physics of Recombination

Epoch of Recombination (kTQ) To 3740K z
1370, Dz 200 T 240kyr, Dt 70kyr
Epoch of Decoupling (GH) To 3000 z 1089,
Dz 195 T 379,000yr, Dt 118ky
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2.3 Observations of the CMB

• Temperature Fluctuations

Observations of CMB ? Fluctuations in Temperature
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2.3 Observations of the CMB

• Temperature Fluctuations - Isotropy and
  Homogeneity

Early Universe was highly homogenous

• 1989 COBE
• Cosmic Microwave Background Explorer
• Diffuse Infrared Background Experiment
• DIRBE 0.001mm lt l lt 0.24mm
• Far Infrared Absolute Spectrometer
• FIRAS 0.1mm lt l lt 10mm
• Differential Microwave Radiometer
• DMR l 3.3, 5.7, 9.6mm

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2.3 Observations of the CMB

• Temperature Fluctuations - The Dipole Anisotropy

At the level of 10-3 Observe Dipole
Anisotropy One half of sky seemingly blue shifted
to higher temperatures One half of sky seemingly
red shifted to lower temperatures
Net motion of COBE wrt frame of reference in
which CMB is isotropic
Doppler Effect? 1) increases energy of photons
seen in direction of motion 1bcosq Doppler
Effect? 2) dn, interval of frequencies also
increased 1bcosq (bv/c10-3)
There is no quadrapole moment

• COBE - Earth correction 8 kms-1
• Earth - Sun correction 30 kms-1
• Sun - Galactic Centre correction 220 kms-1
• Galaxy - Local Group 80 kms-1
• ? Local Group moving towards Hydra at
  v63020kms-1 0.002c

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2.3 Observations of the CMB

• Temperature Fluctuations

• Early Universe was highly homogenous
• Planck Time quantum fluctuations
• Inflation amplified fluctuations ? macroscopic
• Fluctuations frozen until zdec
• Fluctuations in the density (dr/r)3(dT/T)

Cl(q) Correlation function (mean product over
all points seperated by q) Value of Cl(q) as a
function of q (0lt q lt180o) gives a complete
statistical description of the CMB
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2.3 Observations of the CMB

• Temperature Fluctuations

Cl(q) is scale dependent The value probed will
depend on resolution of instrument
Individual Cl s probe structure on different
angular scales given by q180o / l l 0 the
monopole l 1 the dipole (due to our motion wrt
CMB) l gt1 fluctuations imprinted on SLS
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2.3 Observations of the CMB

• Horizons and Fluctuations

• Particle horizon the distance light can have
  traveled from t 0 to any given time t
• Event horizon the distance light can travel
  from any given time t to t8 (or tmax).
• Hubble Distance (Hubble Sphere) the distance
  beyond which recession velocity exceeds the speed
  of light.

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2.3 Observations of the CMB

• Horizons and Fluctuations Large Scale
  Fluctuations qgt1o

The Horizon Distance at recombination and
decoupling (Surface of Last Scattering SLS)
Scales of qgt1o different origin to scales
qlt1o Spherical harmonics q180o / l qgt1o
Corresponds to llt180 qlt1o Corresponds to lgt180

• Scales of qgt1o outside horizon
• fluctuations from inflation
• Gravitational effect of primordial density
  fluctuations

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2.3 Observations of the CMB

• Horizons and Fluctuations Sachs-Wolfe Effect

• Scales of qgt1o outside horizon
• fluctuations from inflation
• Gravitational effect of primordial density
  fluctuations

Poisson eqn
At surface of last scattering

• Photon a local potential minima (bottom of well)
  has to climb out ? lose energy ? Redshift

• Photon a local potential maxima (top of well)
  falls in ? gain energy ? Blueshift

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2.3 Observations of the CMB

• Horizons and Fluctuations Small Scale
  Fluctuations qlt1o

• Scales of qlt1o are inside the horizon ? baryons
  photons
• Baryons and photons fall into DM potential well

• At decoupling
• Baryon/photon fluid in max compression ? high
  r,T
• Baryon/photon fluid in max expansion ? low r,T

Generally q10 (l180) corresponds to potential
wells in which Baryon/photon fluid had just
reached max compression at time of decoupling
(fundamental mode of oscillation).
These potential wells had sizes of dH,SLS (seen
as qH today)
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2.3 Observations of the CMB
Different angular scales probing different
Physical processes

• Horizons and Fluctuations

Savage 2003
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2.3 Observations of the CMB

• CMB Experiments

Different angular scales probing different
Physical processes.
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2.3 Observations of the CMB

• CMB Experiments

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2.3 Observations of the CMB

• WMAP

• Wilkinson Microwave Anisotropy Probe (2001 at
  L2)
• Detailed full-sky map of the oldest light
  380,000yr old in Universe.
• It is a "baby picture" of the 380,000yr old
  Universe
• Probe the CMB fluctuation Spectrum below the
  horizon scale
• q 900 - 0.2 (l2-1000)

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2.3 Observations of the CMB

• WMAP

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2.3 Observations of the CMB

• Resolving the Different Cosmological World Models

• Relative heights and locations of these peaks
• ? signatures of properties of the gas at this time

Open Universe - photons move on faster
diverging paths gt angular scale is smaller for
a given size
Peak moves to smaller angular scales (larger
values of l)
THE UNIVERSE IS FLAT
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2.3 Observations of the CMB

• Resolving the Different Cosmological World Models

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2.3 Observations of the CMB

• Polarization measurements

CMB photons may be polarized
Stokes vector S(I,Q,U,V) characterizies the
intensity and polarization of light.
VIRCP-ILCP
Unpolarized light QUV0 polarized light,
Q2U2V21 CMB Polarization V0
UI45-I-45
QI0-I90

• Inflation ? Gravitational wave background

• CMB SLS gravity wave amplitude ? B (curl) mode
  component to CMB polarization
• The smoking gun of inflation
• Extend observations from 380,000yrs ? 10-35 s
  after Big Bang !!
• Combination of Scalar, Vector Tensor fields
  carry information on temperature anisotropies,
  acoustic peaks, cosmological parameter.
• Information on epoch of re-ionization

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2.3 Observations of the CMB

• Polarization measurements

100mK
Temperature
E (Tensor)-modes
4mK RMS
B (curl)-modes
300nK
1 degree
B-mode amplitude is Determined only by the energy
scale of inflation. Characterized by Tensor to
scalar ratio lt 0.71 (WMAP
Hu et al. astro-ph/0210096
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2.4 Background Light Components

• Backgrounds or Foregrounds? (signals or noise?)

The total integrated background light comes from
many sources

• Cosmic Microwave Background Radiation CMBR 3K,
  peaks at 5cm
• Our Atmosphere Sunlight scattered through
  atmosphere
• Zodiacal Light Dust in plane of Solar System
  illuminated by Sun peaks at 60mm
• Galactic emission from dust, peaks at about 100mm
• Emission from hot gas, Synchrotron free-free
  radio emission
• Extra galactic contributions from Radio Sources,
  Galaxies
• S-Z Compton scattering of CMBR photons by
  relativistic e- in cluster gas

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2.4 Background Light Components

• Backgrounds or Foregrounds? (signals or noise?)

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2.4 Background Light Components

• Infrared Cirrus

• Extended whispy neutral interstellar dust in the
  Milky Way heated by the interstellar radiation
  field.
• Cirrus emission peaks at far IR wavelengths
  (100µm) but was detected in all 4 IRAS bands
• The galactic cirrus is a function of galactic
  latitude and is serious for wavelengths longer
  than 60µm.

B100 Contours at 1 and 2 MJy/sr
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2.4 Background Light Components

• Confusion to extragalactic sources

• Extragalactic Background
• The superposition of sources below the flux
  limit / resolution of the instrument

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2.4 Background Light Components

• Contributions to the Extragalactic Background

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2.4 Background Light Components

• Backgrounds or Foregrounds? (signals or noise?)

Bouchet 1999
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2.5 Summary

• Summary

• The CMB is strong vindication for the Hot Big
  Bang Theory
• The CMB
• Isotropic to one part in 105 - An ideal Black
  Body
• Shows a Dipole distortion due to the motion of
  the Earth wrt CMB frame
• After Dipole Subtraction shows fluctuations on
  30mK
• The epoch of recombination and decoupling define
  the Surface of Last Scattering (SLS)
• The SLS is the last time the CMB interacated
  with matter
• The SLS is a fossil of the 380,000yr old
  Universe
• Primoridial density fluctuations are imprinted
  on the SLS
• The Fluctuations in the CMB has 2 origins
• On scales gt 1 degree ? Primordial Fluctuations
  from Inflation (Sachs Wolfe effect)
• On scales lt 1 degree ? acoustic oscillations in
  the baryon-photon plasma
• Decomposing the CMB fluctuations into spherical
  harmonics
• Plot the fluctuation power as a function of
  angular size
• Discriminate between different world models
• WMAP - THE UNIVERSE IS FLAT !
• Foreground (contamination)
• Zodiacal Light

BUT.
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2.5 Summary

• Summary

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2.5 Summary

• Summary

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Observational Cosmology 2. The Cosmic Background
Observational Cosmology 3. Structure Formation
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