Cosmology
---------------The Beginning of Time
© Sierra College Astronomy Department
Summary for Final
• Last week of classes 12/10 to 12/13
• First hour (Wednesday):
– Planetarium Sky Quiz (30 pts)
– Final, Part 1 (Deep Sky Object Quiz, 20 pts)
• Second hour (Wednesday):
– Final, Part 3: SCANTRON Test (Form 882, #2 pencils),
70 questions, cumulative  60 from Review Questions
for Final and 10 questions on SGA and Planispheres
(70 pts)
• During your Third Hour
– Final, Part 2: Group effort on questions relating to 3rd
hour (20 pts)
• All extra credit due by 12/14 at NOON.
• Review Session: Friday, 7 December, 2-4 pm,
here
Lecture 15: Cosmology: The Beginning of Time
The Big Bang
• Hints of a Beginning
– Up until the early 20th century, the origin of the Universe
was mostly covered by the domains of philosophy and
theology.
– Then Einstein’s General Theory of Relativity gave science
a framework to discuss the Universe’s origin.
– Observations also mounted that gave credence to a
scientific investigation into origins:
• An expanding Universe seen in the redshifts of galaxy
clusters
• Change in galaxies as a function of distance (time)
• Observations of the H/He ratio
• The cosmic microwave background
– Observations and theory lead us back to the time of
the Big Bang – a time that, as yet, can only be
theoretically described.
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The Big Bang
• This is the simplistic name given
to the event which began the universe
(given as a joke by Fred Hoyle)
– Calvin and Hobbes’s Name:
The Horrendous Space Kablooie!
• Has become the dominate theory
of the universe’s origin
• It is not an event that occurred at a single place
• But it did fill an entire volume
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Lecture 15: Cosmology: The Beginning of Time
The Big Bang
• An Expanding Universe in Reverse
– Traveling back in time before the formation of the first stars and
galaxies, the Universe is filled with particles of matter and radiation.
– As the size of the Universe continues to decrease, the temperature and
density of matter and radiation increase.
– Eventually, we reach a time of such extreme conditions that particles of
matter and radiation are continuously transforming into other particles of
matter and radiation.
• In particular, matter and anti-matter pair annihilation forms gamma-ray
photon pairs.
• The reverse process of matter and anti-matter pair creation from gamma-ray
photon pairs also occurs.
– Theoretical physics can take us back, with reasonable confidence, to an
age of the Universe of about 10-10 second; earlier times require “missing
physics”.
– The “soup of matter and radiation particles” can be broken into seven
eras.
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Lecture 15: Cosmology: The Beginning of Time
The Seven Eras of the Early Universe
• The Planck Era
– Time: Less than about 10-43 second
– Physics of this time period is unknown.
• Quantum Mechanics is a highly successful theory of the very small.
• General Relativity is a highly successful theory of gravity (mass)
and the very large.
• A theory of very large mass on small scales does not yet exist
(Superstring Theory?)
– Planck Era is expected to experience very large energy
fluctuations (from Heisenberg Uncertainly Principle).
• From E = mc2, these large energy fluctuations translate into large
gravity (mass) fluctuations.
• Large gravity fluctuations mean large fluctuations in curved
spacetime structure.
• Quantum Mechanics is based on a flat spacetime – hence the
problem with using our current physics in the Planck Era.
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Lecture 15: Cosmology: The Beginning of Time
The Seven Eras of the Early Universe
• The GUT Era
– During the Planck Era, it is believed that the four distinct forces
of today (gravitational, electromagnetic, strong, and weak) were
indistinguishable and essentially explained by one force law.
– The beginning of the GUT Era (at 10-43 second) is marked by the
splitting of gravity from the other three forces, which are
described as one force (the GUT force) by Grand Unified
Theories (hence the acronym GUT).
– At the end of the GUT Era, the strong force separates from the
electroweak force.
• This occurs at a temperature of about 1029 K when the Universe is
10-38 second old.
• The freezing out of the strong force releases a tremendous amount
of energy which drives inflation, which lasts about 10-36 second and
increases the size of the Universe from that of an atom to that of the
Solar System.
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Lecture 15: Cosmology: The Beginning of Time
The Seven Eras of the Early Universe
• The Electroweak Era
– This era lasts until the temperature has decreased to 1015
K when the Universe’s age was 10-10 second.
– At the end of the Electroweak Era, all four forces are now
distinct.
– The temperature at the end of the Electroweak Era is
about 100 million times hotter than the center of the Sun.
• Experiments have been conducted at these energies
– Detected the electroweak W and Z (or weak) bosons, carriers
of the electroweak force
– This detection confirms the electroweak theory, at least for
temperatures as high as 1015 K.
• These successful experiments are what give scientists
confidence about predicting the state of the Universe from
10-10 second to today.
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Lecture 15: Cosmology: The Beginning of Time
The Seven Eras of the Early Universe
• The Particle Era
– At the beginning of the Particle Era, spontaneous
creation/annihilation keep the number of photons and particles
roughly in balance.
– As the temperature of the Universe lowers, less and less
photons have sufficient energy to create particles.
– The photon-to-particle ratio increases with time
• The variety of particles settles into the more stable forms: protons,
neutrons, electrons, neutrinos, and perhaps WIMPs.
• A slight imbalance of matter over antimatter results in a Universe of
matter rather than antimatter.
– The current ratio of photons-to-protons is about 109:1
– Consequently, for each 1 billion protons and antiprotons that annihilated
each other, one proton was left over at the end of the Particle Era.
– The Particle Era ended at 0.001 second and a temperature of
about 1012 K.
• Some of the protons and neutrons from this point will eventually
make their way in the bodies of humans.
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Lecture 15: Cosmology: The Beginning of Time
The Seven Eras of the Early Universe
• The Era of Nucleosynthesis
– The Nucleosynthesis Era starts at 0.001 second (faster
than a blink of an eye) and lasts until the Universe is about
3 minutes old.
– The temperature drops from 1012 to 109 K.
– This period is noted for fusion and breakup of nuclei
containing protons and neutrons.
– The end of the Nucleosynthesis Era
• Fusion ceases even though temperature is higher than
center of Sun (density is now too low)
• 75% of the ordinary (baryonic) mass in the Universe is
hydrogen (a proton), 25% is helium, and a trace percentage
is deuterium and lithium.
• Except for small percentage heavier elements forged in stars,
composition of the Universe has not changed much since the
end of the Nucleosynthesis Era.
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Lecture 15: Cosmology: The Beginning of Time
The Seven Eras of the Early Universe
• The Era of Nuclei
– At the beginning of this era, the Universe is a hot plasma of
hydrogen, helium, and free electrons.
• This basic picture stays the same for 370,000 years as the Universe
grows and cools.
• Matter and photons are coupled (collide frequently, changing their
directions of travel).
• Any ionized hydrogen or helium that acquire an electron are quickly
ionized again.
– At the end of the Era of Nuclei, the temperature reaches 3,000 K.
• Hydrogen begins to capture electrons to form neutral atoms that will
remain so for very long periods of time.
• The Universe becomes transparent as photons are now free to pass
by nuclei without being absorbed (the photons are no longer
energetic enough to lift electrons to higher atomic orbits).
• These free-flowing photons are now seen as the cosmic microwave
background.
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Lecture 15: Cosmology: The Beginning of Time
The Seven Eras of the Early Universe
• The Era of Atoms
– Once the Era of Atoms begins, the formation of
neutral atoms is in full swing.
– Matter begins to clump around concentrations of
dark matter.
– As the clumping becomes significant,
protogalactic clouds begin to emerge and the Era
of Galaxies begins with an age for the Universe
standing at about 1 billion years.
• We are still in the Era of Galaxies – some 14 billion
years after it all started.
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Lecture 15: Cosmology: The Beginning of Time
Evidence for the Big Bang
• Summary of Evidence for Big Bang
– Red-shifted clusters of galaxies
– Theory predicts that the billions of years of
expansion would redshift the initial radiation into
3 K (microwave) cosmic background radiation,
and this is in fact observed.
– The H-He elemental ratio is also predicted and
matches present observed values.
– Age of the universe (inverse Hubble constant) is right
order of magnitude.
– Distant galaxies look younger.
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Lecture 15: Cosmology: The Beginning of Time
Evidence for the Big Bang
• Cosmic Background Radiation (CMB)
– Arno Penzias and Robert Wilson in 1965 were
calibrating a sensitive microwave antenna
– They found unexpected “noise” in every
direction and they tried everything to eliminate
it
– In the paper they mentioned this noise at the
end, almost as a footnote
– After taking to other astronomers they
discovered that the noise was the predicted
background radiation from the Big Bang
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Lecture 15: Cosmology: The Beginning of Time
Evidence for the Big Bang
• Cosmic Background Radiation (CMB)
– The radiation is coming from the time when the
universe became transparent to radiation
– This happened 380,000 years after the Big Bang at
the end of the era of nuclei, when atoms started to
form.
– The temperature of the universe at that time was
about 3000 K.
– But since the universe has expanded by a factor of
1000, the photons emitted from that time have been
stretched out by a factor of 1000, which corresponds
to a thermal temperature of about 3 K.
• See Cosmic calculations 17.1
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Lecture 15: Cosmology: The Beginning of Time
Evidence for the Big Bang
• Dectecting the CMB with COBE and WMAP
– The COsmic Background Explorer (COBE) was used
to verify that the background radiation was indeed at
2.73 K.
– Careful study revealed that the background radiation
had slight variation from 1 part in 100,000.
– The Wilkinson Microwave Anisotropy Probe (WMAP)
looked at variations in the CMB in even greater detail.
– These variations are essential as this enable the
universe to clump up into galaxies.
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Lecture 15: Cosmology: The Beginning of Time
Evidence for the Big Bang
• Abundances of elements
– The classic Big Bang theory predicts that about 25%
of the universe (by mass) should be helium.
– Studies of the Milky Way and other Galaxies show
that the helium abundance at least 25%
• For the Milky Way it’s 28%.
• This slight enhancement is due to the conversion of
hydrogen into helium.
• This is a great verification of the theory.
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Lecture 15: Cosmology: The Beginning of Time
Evidence for the Big Bang
• Formation of light elements in the Early Universe
– During the first 3 minutes all the helium nuclei formed from the
Big Bang
– When universe was 1012 K protons and neutrons were in equal
numbers
– As the universe cooled to 1011 K, the neutron production rate
started to fall
• Neutrons converted into protons, but protons no longer converted
back to protons
– Nuclear fusion occurred and neutron and protons fused into
deuterium (heavy hydrogen) and helium
• This helium did not survive since the numerous gamma rays
present broke it apart
Helium
production
– When the universe cooled to 1010, helium fusion was sustained.
At this time the proton-neutron ratio was about 7:1.
• This allow about 25% of the material to fuse into helium
– Before the universe cooled to 109 some lithium was formed too
© Sierra College Astronomy Department
Helium
ratio
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Lecture 15: Cosmology: The Beginning of Time
Evidence for the Big Bang
• The Density of Ordinary Matter
– The Big Bang model allow us to estimate the density of ordinary
(baryonic) matter in the universe
– Deuterium is the key: it was made and then used to make
helium. The amount remaining in the universe tells us something
about the density of baryons in the era of nucleosynthesis.
– The amount of deuterium suggests the density of ordinary matter
is 4% of the critical density,
• This is confirmed also by looking at helium-3 and lithium-7
– Since the universe appears close to 25% of the critical density,
about 83% of the rest of the matter seems to be composed of
extraordinary (non-baryonic) material.
Abundance
Vs. time
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Lecture 15: Cosmology: The Beginning of Time
Big Bang and Inflation
• The Flaws of the Big Bang: The classic Big
Bang theory works back to when the universe
was at 1015 K (10-10 sec old).
• It does not explain:
– Where does structure come from?
• Why did the universe clump up?
– Why is the large-scale structure so uniform?
• The distance reaches of the universe are remarkably similar
– Why is the density of the universe close to the critical
density?
• If matter, dark matter and dark energy are added up, their
combine density is nearly the critical density. Why?
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Lecture 15: Cosmology: The Beginning of Time
Big Bang and Inflation
The Inflationary Model: The basic principle of the
inflationary model is that the universe expanded
dramatically (by a factor of 1030) in less than 10-36 sec
• Tiny quantum fluctuations which pervade the present
universe, would have a far more significant role in the
tiny universe
– When the universe inflated, these fluctuations got magnified
– This ultimately caused a non-uniform density throughout the
universe
– As the universe cooled this allowed things to clump up and form
into galaxies
Abundance
Vs. time
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Lecture 15: Cosmology: The Beginning of Time
Big Bang and Inflation
• Uniformity:
– This question arises when considering why the CMB
is so nearly uniform
• Parts of the universe (which we are seeing when they are
380,000 years old) are billions of light-years apart, how could
they have exchanged information?
– Inflation answers this question by suggesting that
these two regions were in contact before inflation
occurred
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Lecture 15: Cosmology: The Beginning of Time 3 poss.
Big Bang and Inflation
geometry
Density: Balancing the universe
• The universe is close the critical density which is triangles
another way of saying the universe is flat.
• Rapid inflation will make everything flat Balloon
To flat
The Curvature of Spacetime
• In general relativity, a universe with negative
curvature (like the surface of a saddle) does not Positive
curvature
curve back on itself and is infinite.
• Alternatively, a universe with positive curvature
negative
(like the surface of a sphere) curves back on itself,
and is finite.
positive
circles
• Between these two is flat curvature.
flat
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Lecture 15: Cosmology: The Beginning of Time
Big Bang and Inflation
Testing inflation
• We might be able to look for curvature of the universe by
measuring “internally” through the sum of a triangle’s angles,
the area of a circle, and the paths of parallel lines.
• Our best evidence of inflation comes from WMAP
– Careful observations of the temperature fluctuations can tell us
about structure in the early universe
– Here are the results:
negative
positive
circles
flat
• The overall geometry is flat √
• The density of ordinary matter is 4.4% √
• The total matter density is 26% and therefore dark matter makes up
about 22%, in good agree with measures of galaxy clusters √
• The flatness of the universe and the density being so close to
critical implies that the remaining 74% is made of some repulsive
force (i.e. dark energy) √
WMAP
• The universe’s age is 13.7 years old, consistent with the Hubble
results
constant and ages of stars in the universe √
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Lecture 15: Cosmology: The Beginning of Time
Observing the Big Bang
Olbers’
paradox
• Suppose we assume the universe to be
static, infinite, eternal, and uniformly filled
with stars (or galaxies)
• If we look in any direction we should see a
star and there the sky should be a bright as
the surface of a star
• This is often referred to as Olbers’ paradox
Thick forest
Thin
forest
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Lecture 15: Cosmology: The Beginning of Time
Observing the Big Bang
One or more of our assumptions must be wrong
• Sky uniformly filled with stars – not so bad
• Infinite – might be true, there seems to be no limit
(except to our instruments – The observable universe)
• Static – doesn’t seem to be true, but will this affect the
dark sky issue?
• Eternal – if the universe has finite age, then all the
light from all the stars will not have had time to reach
us
• Conclusion: The universe is not infinitely old (and its
changing)
• The Visible Universe vs The Universe and the Cosmic
Horizon
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