The Big Bang
Attendance Quiz
Are you here today?
Here!
(a) yes
(b) no
(c) Big Bang! Big Crunch! Nestle’s Crunch! Mmmm…
Final Exam
• The final exam is Thursday, 6/9, from 11:30am to
1:30pm (2 hours), in this room; please arrive early!
• The final exam will be comprehensive, i.e., it will cover
all the material you have studied this quarter
• It will be multiple choice, so make sure to bring a 100question (2-sided) scantron to class!
Homework reminder
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For homework, complete the ranking task “Stellar
Evolution and Lookback Time” on course website
Reselling Your Clicker
• Reminder: you can resell your clicker to the bookstore
after this class
• You may also be able to sell it someone who will use it
another class
Today’s Topics
• The Beginning of Time
• The Big Bang
• Cosmic Background Radiation
• The Helium Problem
• Big Bang Helium Nucleosynthesis
• Inflation
• Discussion/Questions
The Big Bang
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Since the universe is expanding, and the galaxies are moving farther apart with
time, then at some earlier time, they must have been closer together
If we imaging running the “movie” of the universe back in time far enough, the
universe would become denser and hotter (Big Crunch movie)
The Big Bang
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As we run the “movie” further
back, the density and temperature
keep increasing (note axis scales!)
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At high enough temperatures
(> 3000 K), before stars and
galaxies formed, the universe
would be ionized (~400,000 yrs)
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Thus, the very early universe was a
hot, dense, fireball of interacting
particles and photons
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The expansion of the universe
from this early, hot phase is called
The Big Bang - (movie)
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The Big Bang is not an explosion it is just the time at which the
universe started expanding
The Helium Problem
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We have seen how stars of various
masses form all the elements
heavier than hydrogen, from helium
to uranium, through nuclear fusion
and supernovae
However, if one calculates the rate
of helium production over the age
of the universe by all the stars, it
produces an abundance of helium
of only ~10% by mass
Observations of stars and galaxies
show, however, that ~24% of all
the mass of stars is helium
Furthermore, there is almost no
correlation between oxygen and
helium abundance, suggesting they
come from different places
Where did this helium come from?
Big Bang Nucleosynthesis of Helium
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Where else was the temperature high
enough to drive nuclear fusion?
During the Big Bang!
During the first three minutes of the
universe, the temperature was greater
than 109 K, hot enough to fuse H → He
What determined the actual amount of
helium formed? Why wasn’t all the
hydrogen fused into helium?
Very early, things were too hot for any
fusion to stick—when a proton and
neutron fused into deuterium, it was
immediately destroyed by a gamma ray
Above 1011 K, there was so much
energy available, protons and neutrons
converted back and forth freely (even
though mn > mp) so that Nn ~ Np
n + e+ ↔ p + νe
n + νe ↔ p + e−
Big Bang Nucleosynthesis of Helium
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Below 1011 K, the mass difference (in
energy units via E = mc2) was too large
to convert p → n, so neutrons started
decaying into protons
n → p + e− + νe
At the temperature where deuterium
could form (and not be destroyed),
about 1010 K, the neutrons and protons
quickly formed deuterium which then
combined to form 4He
At the time (and temperature) that this
happened, we can calculate (from the
neutron to proton conversion rate), that
there were ~7 protons for each neutron,
a ratio of 14:2
Since all the available neutrons were
incorporated into 4He nuclei, this leads
to 12 protons for each 4He nucleus, a
mass ratio of 12:4 or 75:25, exactly as
observed!
Big Bang Nucleosynthesis
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Even more detailed calculations,
which include other elements, such as
D, 3He, and 7Li, are all consistent
with Big Bang predictions, and
further predict that only 4.6% of the
mass-energy in the universe is
ordinary matter
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Recall that current measurements
suggest that this is correct, with the
remaining ~95% made up of dark
(non-ordinary) matter and dark
energy
The Early Universe
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The average energy of particles in the early universe depends on their temperature
As long as the matter is ionized, the photons of light will interact strongly with the
matter, and will have the same average energy as the particles
After the universe cools below the ionization temperature, the photons of light
flow freely through the universe filling it with light (Interactive Figure 23.6)
The Early Universe
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As the universe expands from this time forward, this radiation is stretched by the
expansion of the universe to longer wavelengths (Interactive Figure 20.24)
Since this is thermal radiation, this is equivalent to the radiation cooling
(Wein’s Law) — longer wavelength = lower temperature
Following the expansion of the universe from 400,000 years to today, astronomers
as far back as the 1950s, had predicted that, if the Big Bang had occurred, the
universe should be filled with thermal radiation of about 3 K temperature
Cosmic Background Radiation
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In 1965, two physicists at Bell Labs were
looking for sources of interference with
microwave communications
After identifying all other sources of
radiation, they found a leftover source of
“noise” coming equally from all
directions
They considered this “noise” to be an
embarrassment, and buried their results at
the end of a long paper on their work
In fact, they had discovered the radiation
left over from the Big Bang — the
Cosmic Background Radiation
More detailed observations of this
radiation agree very precisely with
thermal radiation of T = 2.73 K in
exceptionally good agreement with the
predictions of the Big Bang
COBE satellite
Penzias and Wilson
Wilkinson Microwave Anisotropy Probe
(WMAP)
COBE satellite
ΔT
~ 10 −5
T
Wilkinson Microwave Anisotropy Probe (WMAP)
COBE satellite
Contents of the Universe
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WMAP’s results confirmed that only 4.6% of the matter in the Universe is
regular matter while 23% is made up of dark matter.
The remaining 72% is dark energy.
The Big Bang and Inflation
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What caused the universe to be so close to exactly flat (Ω = 1)?
Also, the relative lack of structure in the cosmic background radiation is a puzzle
because at the time the CBR was scattered for the last time (when the universe
was 400,000 years old), widely separated parts of the universe would not have had
time to “communicate” and hence equalize their temperatures
The Big Bang and Inflation
• The relative lack of structure in the cosmic background radiation is a
puzzle because at the time the CBR was scattered for the last time
(when the universe was 400,000 years old), widely separated parts
of the universe would not have had time to “communicate” and
hence equalize their temperatures
Inflation
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However, in 1981, Alan Guth proposed
that in the first 10−35 sec of the universe,
as it cooled, an initially unified set of
three of the four forces of nature (EM,
weak nuclear, strong nuclear), known as
the Grand Unified Theory (GUT) force,
split into the strong nuclear force and the
electroweak force
This split released an enormous amount
of energy (like the latent heat of a phase
transition) causing a rapid expansion (by
a factor of 1030) in the size of the
universe, called inflation
During this expansion, regions that had
been in close contact were carried to
their present distant separation,
explaining the uniform temperature of
the CBR (Interactive Figure 23.14)
Note the timescale!
Triumphs of the Big Bang and Inflation
Together, the Big Bang and Inflation explain
a large number of facts
1. Existence and uniformity of CBR
2. 4He abundance of 25% by mass
3. The universe is 13.7 billion years old
4. The total mass-energy of the universe is
almost exactly the critical density of the
universe
5. The density of “ordinary” matter
(protons, neutrons, electrons) is 4.6% of
the total mass-energy of the universe
6. The total matter density (ordinary and
dark matter) is 28% of the total massenergy
7. There exists another form of massenergy, dark energy, which accounts for
72% of the mass-energy of the universe
Exam review
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Take out your exam scantrons
We will spend the rest of class allowing you to
review your exams and ask me questions about them
You can also ask me any questions you like about
the tutorials or anything else we have covered in
class