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The Big Bang
Where do we come from, where are
we going?
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Goals
•
•
•
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What’s the cosmological principle?
Where did things begin?
Where will things end?
What’s the temperature of space?
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Hubble’s Law
• Recall: All galaxies are
moving away from us.
• The farther away the
faster they go.
• V = Ho x D
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Large Scale Structure
Farther away we look, further back in time we see!
You Are Here
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Cosmological Principle
• Isotropy – The view from
here is the same in all
directions.
• Homogeneity – We live in
an average place the same
as any other.
• This is the cosmological
principle.
• Implies:
– Universe has no edge!
– Universe has no center!
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Olber’s 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.
• Universe is neither.
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Expanding Universe
• If galaxies are all moving away, then at some point
they were all much closer.
• Hubble’s Law implies the Universe is expanding.
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Age of the Universe
• Since all galaxies are moving away from us, how
long has it been since all galaxies were together?
time = distance / velocity
velocity = Ho x distance
time = distance / (Ho x distance)
time = 1/Ho
• Measured Ho ~ 65 km/s/Mpc
T ~ 15 billion years
• Recall: Recent HST result says the oldest white
dwarfs are 13 billion years old!
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Age Disagreements
• Until recently, much disagreement on the value of
Ho and therefore, the age of the Universe.
• Need to know the distance to some galaxies in
order to know the slope of velocity versus
distance.
• Different methods yielded different distances.
• Some values of Ho yielded an Universe younger
than some of its stars.
• HST has helped solve the problem: Cepheids.
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The Big Bang
• Big Bang: the explosion out of which the Universe
began expanding.
• Into what did the Universe expand? Nothing.
• Where was the Big Bang? Everywhere.
• Where is the center of the Universe? Nowhere.
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The End of the Universe
• Will the universe expand
forever?
• Depends on the density of
the Universe.
• Too big: Big Crunch
– Closed Universe
– Bound Universe
• Too small: Big Freeze
– Open Universe
– Unbound Universe
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Critical Density
• Dividing line
is the critical
density.
Wo is the ratio
of measured
density to
the critical
density.
• If Wo > 1 then closed, Wo < 1 then open.
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Density of the Universe
• Add up all the mass we see and Wo = 0.01
• But we know there is some dark matter in galaxies
and clusters.
• How much?
• Think ~10 x more dark matter than “light” matter.
• Cosmologists think Wo < 0.3
• Result: Open Universe  Big Freeze!
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Are we Slowing down?
Slowing
Accelerating
Dark Energy
• In our experience,
things slow down over
time.
• Is the Universe slowing
down at all?
• Use supernovae as
“standard candles.”
– Get distance to distant
supernovae.
• Plot distance versus
velocity.
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Observational Clues
• Instant of the Big Bang:
– Very Dense
– Very hot
• Universe one giant blackbody.
– Temperatures  gamma rays
• As time goes on the Universe should cool.
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3 Kelvin
• Recall: lp a 1/T
• Since longer
wavelengths mean
cooler blackbodies
then space should
be cooling.
• Today, it should be
~3 K
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COBE
• Satellite measured the intensity of energy from space
at a range of frequencies.
2.7 K
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Isotropy
• Amazingly uniform temperature in all directions.
• Deviations < 0.0002 Kelvin
10 x more energy
in background
than everywhere
else.
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