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The Cosmic Microwave Background:
A Love Story
Dr. Andrew Rivers
Northwestern University
Big Bang Test 1: Redshift-Distance Relationship
Hubble Law Expansion
Image Credit: Ernest Norcia
Observed redshifts of galaxies due
to an expansion of the universe.
Image Credit: Freedman et al. (2001)
Expansion rate (Hubble Constant)
+ distance gives origin time.
Big Bang Test 1b: Age of Universe Matches Oldest Stars
HR Diagram Turn-off Pt.
Image Credit: NASA and H. Richer
Oldest Stars in M4 (Milky Way Globular Cluster).
Stars in cluster form together.
Brightest stars turn off first, fainter later.
Location of turn-off point=star cluster age
Result: Ages of oldest star clusters
from turn-off consistent with Big Bang
expansion age from Hubble constant.
Looking out in space is equivalent to looking
back in time. Is it possible to look far enough
out in space to observe the Big Bang itself?
The Andromeda galaxy is 2.3 Million
Light-years away. We see it as it was 2.3
Myr in the past.
Most distant galaxy in the universe? 13.3
Billion light years away. Light from 420
million years after Big Bang.
A Mysterious Observation
Penzias & Wilson were
studying Milky Way Radio
emission.
Calibrated signal in microwave
spectrum.
Could not account for “noise”
observed.
Image Credit: Ted Thai/Time Life Pictures/Getty Images)
Robert Wilson and Arno Penzias and the Bell Antenna
they used to discover the microwave background.
Observed Characteristics of Noise
• Independent of the direction they
pointed the telescope.
• Corresponded to an “antenna
temperature” of 3.5 K
• Not due to atmosphere (would be
greater if pointed toward horizon)
• Did not vary with time of day or
year!
– Not from Milky Way or Solar System
– Cosmic?
About 1% of the fuzz between
analog TV channels was the noise
observed by Penzias & Wilson
Foundation 1: Big Bang Theory:
A Hot Past
Big Bang universe was more dense and hotter in the past.
The Universe has cooled from
this hot dense state.
Foundation 2: Photon-Matter
Interactions
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Opaque (a fog)
Photons of light interact strongly with free (ionized)electrons and protons
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Transparent
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Photons of light rarely interact with stable atoms (paired electrons & protons)
Prediction of the Big Bang: Formation of Stable Atoms
Early Universe: At high enough
temperatures, photons would
ionize atoms.
Early Universe: soup of
free protons, electrons
and photons (and a few
Helium nuclei).
Look-Back Time
Later Universe: Universe cools enough for stable atoms to form (de-ionization).
• As the universe expands it cools
• Eventually the temperature drops
enough so that protons can bind
with electrons  electron capture
Stable, neutral
Hydrogen and
Helium atoms
Observing the Opaque to
Transparent Boundary
Looking out in space=looking
back in time.
We see through the universe until the
last-scattering-surface boundary.
De-ionization
Look-Back Time
Plasma
Opaque
Transparent!
Last Scattering Surface
Analogy: the sun
becomes transparent
at the photosphere
What does the universe look like
380,000 years after the Big Bang?
What would we see?
Radiation released at ~3000K
(all released at same Temp)
Predict: Blackbody Radiation
Doppler shift
Remember red shift: the
radiation is travelling through
the universe to us, while the
universe is expanding.
Image Credit: David Koerner, Northern Arizona University
Predict: Light extremely
redshifted compared to release.
Observed Spectrum of CMB: A near perfect Blackbody
Blackbody spectra are
produced by one-temperature,
opaque and non-reflecting
objects.
Peak of blackbody indicates
the temperature of the universe
today.
Near perfect Blackbody observed by COBE
What will happen to this
Blackbody curve as the
universe continues to expand?
CMB Peak
Cosmic Microwave Background
Distribution in Space
What does the universe look like
380,000 years after the Big Bang?
Cosmic Background
Explorer (COBE)
A baby picture of the
universe, as seen by COBE
Everything that will ever be is built on this.
Is something hiding?
Big Bang Test 2: Cosmic Microwave Background
Big Bang Theory: expanding,
cooling universe goes from opaque
to transparent when atoms form
Observation
Near perfect Blackbody
Looking out into the universe and back in
time, we can see the last scattering
surface (opaque boundary).
Doppler shift: observed radiation will be highly
redshifted because of universe expansion.
Smooth distribution= isotropic &
homogeneous universe
Subtract the average to see if there is hidden
structure (small variations from the mean).
The Dipole Anisotropy due to the motion
of the Earth with respect to the CMB
What happens when we subtract out
the Dipole? Is anything else hiding?
Subtract the Dipole
The Milky Way has some emission in microwaves. This emission is
small compared to the CMB, but shows up here.
Spots on the Microwave Background:
the universe is not entirely smooth!
Recall:
Average, dipole and
MW subtracted
COBE reveals small variations in the background: seeds of structure
The self-construction of the universe
Small temperature variations in CMB imply slight
variations in density: seeds of structure formation.
Early
universe
CMB
Today’s
universe
Galaxies
/clusters
Gravity acts on initial perturbations building structures. Dark
matter (HDM, CDM varieties) drives structure formation
A (nearly) Perfect Universe: Observations of WMAP & Planck
Artist Rendition of WMAP Satellite moving to L2 point
http://map.gsfc.nasa.gov/
WMAP & Planck: Precisely Measure Cosmological Parameters
By actually measuring
features of the CMB, we
can determine history &
fate of the universe
Parameter 1:
Hubble Constant
Comparing results from COBE and higher res. WMAP
Spots seen by three telescopes
Where do the spots come from?
• Quantum mechanical
energy fluctuations during
inflationary period.
• These fluctuations pulse
through the universe as
sound waves.
• When the universe become
transparent sound waves
are frozen in (light and
matter are decoupled)
The Observed Universe of fluctuations
Is composed of these peices
Image Credit: Clem Pryke, University of Chicago
Image Credit: Max Tegmark, MIT
The relative strength (power) of these
fluctuations is determined by the cosmic
parameters of our universe.
Largest spots on the CMB are from larger pulsations that only
oscillated once before the universe became transparent.
WMAP Satellite Full-sky map of CMB fluctuations
(http://map.gsfc.nasa.gov/)
Sound wave resonances in
the early universe
WMAP Power spectrum distribution of fluctuations in the CMB.
http://map.gsfc.nasa.gov/
Observation: not all spots
are equally common
Prediction: Oscillations of right
frequency so that they reach a
maximum compression at time of
transparency “resonate”
Universe as musical
instrument: fundamental
and higher harmonics.
Relative height of peaks reveals
cosmic parameters.
Image Credit: ESA and Planck
collaboration
Planck Power spectrum distribution of fluctuations in the CMB.
The recipe of the
universe
The Answer: From the
Cosmic Microwave
Background (WMAP),
we can intuit the
percentages of normal
matter, dark matter and
dark energy.
The New Recipe
The Planck universe indicates slightly more normal and dark matter than
previous measurements and a slightly older universe (13.8 Billion years).