Transcript Evidence for the Big Bang

The Evidence for the Big Bang
Student Resource Sheet 5
Science and Religion in Schools: Unit 4a
Evidence for the Big Bang
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The Big Bang model is a concise explanation for 3 independent
observations:
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The red shift of light from distant galaxies
The presence of large amounts of helium in the universe
The cosmic microwave background radiation
Each can possibly be explained in other ways, but the Big Bang
model draws all of them together into a convincing story
Unit 4a
The red shift
Spectrum of light from the sun
2  1 
z
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Fraunhoffer lines
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Light from a cluster of galaxies 109 light years away
Unit 4a
Hubble’s original data
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Red shift due to expansion
 Further away, the longer the light
has been traveling
 Universe has expanded more
 Red shift is bigger
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The velocity (speed) is basically how
much the distance to the galaxy has
changed in the age of the universe
Unit 4a
Much improved data
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Distance scale 250 x bigger
Much more convincing!
Implication - the universe had a
start
Unit 4a
Expansion of the universe
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Expansion of the universe is NOT
a rushing to fill empty space
The WHOLE universe is
expanding - there is no empty
space
Unit 4a
Helium abundance
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Universe is 24% He (by mass)
Stars fuse hydrogen into helium
 In 14 billion years, stars have not burned hot enough, or long
enough to make this much helium
 The oldest stars (11 billion years) also have ~ 24% He
so this He cannot have been made in stars!
Big Bang - in early history the whole universe was the same temp
as the inside of a star!
Fusion in the first three minutes of history made the helium
This shows that the universe was hot in earliest times
Unit 4a
Cosmic Microwave Background
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Arno Penzias
Robert W Wilson (Nobel Prize, 1978)
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Persistent microwave noise
independent of time and direction
Relic radiation from Big Bang
Thermal radiation now redshifted so it
is in the microwave part of the
spectrum
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Unit 4a
CMB
profit
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(Cosmic Microwave Background)
Dates to 300 000 years into history
Epoch of recombination (bad name!)
 Before then, all matter ionized
 At this time temp of universe down to the
point at which atoms could form
 All free electrons captured
 Thermal radiation decouples from the
matter
Imprint of matter density on temperature
variations
for fun and
WMAP whole sky map
±200 K
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The density fluctuations ‘seed’ galaxy
formation
Unit 4a