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How the Universe got its Spots
Edmund Bertschinger
MIT Department of
Physics
Outline
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Introduction: the Big Bang
Cosmic Microwave Background Radiation
A sonogram of the early universe
What have we learned?
Outstanding questions
The Big Bang
• The Universe as we know it began 13.7 billion
years ago in an explosion so powerful that
space itself was propelled outwards.
• Even atoms could not exist in this hot fireball.
• 380,000 years after the bang, H atoms formed
• 200 Million years later, the first stars formed
How do we know?
Measuring the Afterglow of the
Big Bang
Wilkinson
Microwave
Anisotropy
Probe
(WMAP)
Launched:
Summer, 2001
(NASA/WMAP
Science Team)
The Cosmic Microwave Background Radiation
From COBE (1994) to WMAP (2003)
NASA/WMAP Science Team
What is the Microwave Background?
The cosmic microwave
background radiation is
the thermal (blackbody)
radiation emitted by the
hot dense gas formed in
the Big Bang.
It was discovered by
Penzias and Wilson in
1965; they received the
Nobel Prize in 1978.
Animation
Our View Before WMAP
WMAP Actual Sky Maps
Top: Foregrounds removed. Bottom: 41 GHz
NASA/WMAP Science Team
A cosmic sonogram
• The Big Bang began with a burst of nearexponential expansion: cosmic inflation
• Quantum mechanical fluctuations in energy
made ripples in the gasAnimation
• Fluctuations in radiation == Sound waves!
– Light as sound?! Yes!
• Cosmic redshift: Doppler shift in an
expanding universe
The Sonogram in
Numbers:
Angular Power
Spectrum
Top: Temperature
fluctuations vs.
angular scale
Bottom: Crosscorrelation of
temperature and
linear polarization
vs. angular scale
What have we learned from these spectra?
• Age, size, and curvature of the universe
– 13.7 Billion yr, consistent with infinite Euclidean space
on scales of 50 Billion lt-yr
• Contents of the universe: bizarre!
– 4% atoms (almost all Hydrogen and Helium)
– 23% cold dark matter (elementary particles as yet
undiscovered on Earth) detected by its attractive gravity
– 73% mysterious “dark energy” (we’re not even sure of
the physics of this substance!) detected by its repulsive
gravity causing the cosmic expansion to accelerate
• Hydrogen was re-ionized about 200 Myr after BB
We are not made of the same type of matter as most of the Universe!
The WMAP results
greatly improve our
estimates of cosmic
parameters.
Four years ago,
cosmologists proposed
an accelerating universe.
WMAP (in green)
strongly confirms this.
We have seen the seeds
of cosmic structure
animation
Outstanding questions
• What is the dark energy?
– Is it the “cosmological constant?”
– What new physics acts on cosmic scales to
produce this substance?
• What is the dark matter?
• Can we now fully understand the formation
of galaxies?