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The Big Bang Evidence
The Science of Creation
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Cosmological Principle
• Isotropy – The view from
here is the same in all
directions. (observation)
• Homogeneity – We live in
an average place the same
as any other. (statistics)
• This is the cosmological
principle.
• Implies:
– Universe has no edge!
– Universe has no center!
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Olbers’ Paradox
• Why is it dark at night?
• In an infinite and
unchanging universe:
Every line of sight
should end at the surface
of a star.
• Either:
– Universe has edge.
– Universe has a beginning.
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Concept Test
•
Olbers’ paradox asks why the night sky is dark,
when every line of sight must eventually fall on a
star. Which of the following reasons would best
explain the darkness at night? It is because the
universe is:
a.
b.
c.
d.
e.
Infinite and mostly empty.
Clumpy, so not every sightline intercepts a star.
Expanding, so distant stars are red-shifted.
Young, so there are only stars to a finite distance.
None of the above.
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Scientific Evidence
• A scientific model must do two things:
– Explain what is seen.
– Predict what will happen accurately.
– Or, predict what can be seen before one thinks to look.
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Cosmic Microwave
Background
• The Big Bang:
– Massive amount of energy.
– Very high temperature  thermal radiation
• As Universe expands:
– Temperature cools  thermal radiation gets redder.
or
– Cosmological redshift  same thing.
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Origin of the CMB
Once cool enough
for atoms,
radiation could
move freely
through space.
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The Temperature of Space
• After 500,000 yrs: T=3000K
lpeak a 1/T
lpeak = near infrared.
• The Universe was red.
• After 16 billion yrs:
Universe 1000x bigger.
lpeak = 1000x longer.
lpeak = microwaves (radio).
T = 3K!
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Where do you look?
•
•
•
•
Recall Lookback time.
Universe is a time machine.
Look beyond the youngest galaxies.
Look everywhere.
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What We See
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Cosmic Microwaves
• Big Bang theory predicted cosmic microwave
background.
• In 1965: discovery of radio waves (microwaves)
from sky.
• Prediction confirmed.
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COBE
• Early 1990s:
NASA Cosmic
Background
Explorer
• Measure the
intensity of
radiation at many
wavelengths.
• 3K thermal
emission!
l peak
const.

T
lpeak = (3 x 10-3 m/K) / 3 K = 0.001m = 1mm
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COBE cont.
• Also measure the (temperature) intensity and
wavelength in various directions.
• Look for fluctuations out of which galaxies came.
V = 600 km/s
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Large Scale
Structure
WMAP
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Concept Test
•
The cosmic background radiation is visible in
every direction because:
a.
b.
c.
d.
We are at the center of the universe.
It is just now passing by us at the speed of light.
It pervades all space.
It has reflected in every direction over the age of the
universe.
e. All of the above.
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Evidence of Eras
• Can’t see what happened before origin of CMB.
• Can estimate given change of temperature and
density with time.
• Look for Proof.
– CMB (Age of Nuclei)
– Ratio of H/He and Li (Age of Nucleosynthesis)
– Weak Bosons (Electroweak Era)
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A Good Theory?
• The Big Bang is a very successful theory.
• Makes predictions:
–
–
–
–
CMB.
Hydrogen, Helium, Lithium abundances.
Formation of structure.
Expansion of Universe.
• Are other Creation theories:
– Scientific?
– Successful?
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