• Do your course evaluations.
• http://www.pa.uky.edu
• I will add 5 points on your final exam if you
complete the evaluation.
• Quasars are supermassive black holes at the
centers of galaxies, which are accreting
material.
• The accretion disk is what gives off the
luminosity.
• When galaxies merge, material can be
deposited in the vicinity of the super massive
black hole, causing an accretion disk to form
• At very large distances (very long ago) galaxies
were forming through mergers. This is why
we only see quasars at large distances.
• The straight line represents a mass-less
universe with no gravity. In other words, the
expansion began and now the universe is
coasting. Growing in size but at a constant
rate.
• When the data falls below the sloping straight
line it means that the universe was moving
more slowly in the past and the expansion is
speeding up.
• How can this be?
• Gravity is only attractive. It can only work to help
slow the expansion of the universe.
• If the expansion is speeding up, then there has to
be a repulsive force that is acting to force the
expansion to grow more rapidly
• Gravity can’t do this.
• This repulsive force has been named “Dark
Energy”
• It fits into Einstein’s equations just like his old
cosmological constant. But this isn’t keeping the
universe static. It is forcing it to grow more
rapidly.
Using relativity we can model what will happen
in the future as well.
Past
Future
Today
Today
Our location in
the Universe
A distant galaxy or
quasar
Today
Our location in
the Universe
Today
A distant galaxy or
quasar
• The model shows that the galaxy is, today,
much farther away than it was when the light
left the galaxy billions of years ago.
• Also, notice that what ever direction you look,
when you look out into space, you are looking
back toward the earlier universe.
• All directions point back toward the center of
expansion.
• Also, there is no observable edge of the
universe. You are located at the edge of the
universe. Any direction you look, is backward
in time, toward the center of expansion.
What is this expansion?
• Remember back to the gravitational red shift that is
caused when light near the event horizon of a black
hole moves away from the black hole.
• The red shift is caused by the clock near the event
horizon running slow compared to the observer far
from the black hole. The effect is caused by the
slowly running clock.
• The expansion of the universe is similar. The spacetime metric is growing as a function of time. What
that means is that today, in our portion of the
universe, a “meter” is larger than it was in the past.
• Suppose we used the wavelength of light as our
“meter” stick.
Distant
past
Intermediate
Past
Today
• When we look into the distant past and up to today,
we see that the “meter” stick grew in size.
• I use quotes around “meter” stick, because a real
meter stick (say made of wood and held together by
chemical bonds) would not have changed in size.
• The space-time metric for the universe is growing as
a function of time. Light is a wave and is not held
together by forces. When space-time grows, so does
the distance between peaks in the wave.
• Since the time when the Earth formed, it has not
grown in size with the expansion of the universe.
Earth’s matter is held together by its gravity.
• But the distance between galaxies that are not held
together by gravity, has grown as time has gone by.
• In our Local Group of galaxies, we do not see
the expansion of the universe between our
galaxies because we are bound by gravity.
• In relativity, we would say that we all agree on
the length of a “meter”. Just like different
observers far from a black hole can all agree
on the wavelength of light. These observers
have clocks that run at the same rate.
• The Local Group of galaxies all have the same
“meter” stick. So we all agree on the distance
and we do not see the universe expanding
between us.
Final word on expansion
• The galaxies are not moving apart from each
other in the universe.
• It is the universe that is growing.
• Remember, space-time is what we mean by
the universe.
• When space-time grows, when the meter
stick changes length, then the distance
between objects is seen to grow.
• Long ago, the distance between objects was
much less than it is now. The universe was far
more dense.
The growth of the space-time metric does not
have to be grow at a linear rate with time.
• In a closed, finite universe the growth is rapid at first,
then slows to a stop and then the metric begins to
shrink.
• In an open universe, the growth of the metric slows
over time but never stops growing.
Think back… What causes space-time to warp?
30
30
1. High velocities
2. light
3. mass
1
2
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22
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33%
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10
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1
33%
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2
33%
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3
20
In all cases, the universe was exceedingly dense
long ago.
A
B
C
D
E
F
A. The Plank Era
• Time is about 10-43 seconds after Big Bang
• It is hypothesized that all four forces of nature
were combined into one at this time. (gravity,
electromagnetism, weak nuclear and strong
nuclear)
• Currently we can not describe this era.
Physics does not have a unified model for
combing gravity with the other three forces.
• During this time the universe is nearly infinitely
dense.
• Notice, I did not say infinitely small. Small would
suggest that the universe could be viewed from
the outside. There is no outside.
• The space-time metric is so compressed that
there is virtually no distance between locations.
• As an analog, think about the twin paradox and
how the traveling twin thought the distance to
Alpha Centauri was only 0.6 light years. Not the
4.5 light years that we measure.
A
B
C
D
E
F
B. Inflation
• At the time between 10-35 seconds and 10-32
seconds, gravity breaks free from the other
forces of nature.
• This break releases an enormous about of
energy into the universe and causes the
universe to expand at a rate far faster than the
speed of light.
• The temperature drops from 1028 to 1016
degrees.
A
B
C
D
E
F
C. Particles begin to form
• At around 10-6 seconds, particles begin to
form.
• Here I have a series of questions for you.
• After the inflation era (B) how was light
changed?
After the inflation era (B) how was light
changed?
30
30
1. It had a longer
wavelength
2. It had a shorter
wavelength
3. It disappeared
1
2
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22
0
33%
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4
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8
9
10
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1
33%
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2
33%
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20
What does a longer wavelength mean in terms
of Energy?
1. The light had more
energy
2. The light had less
energy
3. The energy stayed
the same
33%
1
33%
2
33%
3
Particles, made of matter begin to form. Where
do these particles come from?
30
30
1. The first supernova
explosions
2. The expanding
universe
3. The light (E=mc2)
1
2
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C. Particles begin to form
• At around 10-6 seconds, particles begin to
form.
• During this time quarks and anti-quarks where
forming from radiant energy.
•
ϒ +ϒ
quark + anti-quark
• It is important that the gamma-ray light must
have energy sufficient to make these particles.
•
E = mc2
• The process is also reversible.
quark + anti-quark
ϒ +ϒ
But if this were the only thing happening then then
the quarks and the light would be in balance.
But the light is rapidly losing energy due to the
expansion of the universe.
The quarks annihilate each other, but a slight
asymmetry allows produces more quarks than
anti-quarks.
• For every 1 billion quark/anti-quark pairs that
are produced, there are 3 extra quarks.
• At the end of this era, for every 999,999,997
quark/anti-quark pairs, there are 3 lone
quarks. With the exception of these extra
quarks, all the rest is turned back into light.
• But the light looses energy do to the
expansion of the universe, and no more
quarks can be produced.
• Since most of the quarks/anti-quarks
produced light, the matter is only 3 parts per
billion, compared to the photons of light.
A
B
C
D
E
F
D. Baryons begin to form
• At a time of about 1 second, the remaining
quarks begin to form neutrons and protons
out of quarks.
• This could not happen earlier, because the
light had so much energy that it would break
apart any quarks that combined.
• Electrons also begin to form out of a similar
process as the quarks.
A
B
C
D
E
F
E. Nucleosnythesis
• After the protons and neutrons form, they begin
to collide and make Deuterium. (That’s an
isotope of hydrogen that has a nucleus with one
proton and one neutron.
• At first, these nuclei can not survive, because the
light has so much energy it splits them back
apart.
• At around 3 minutes after the Big Bang, the
expansion causes the light to loose energy to the
point that it can no longer break the nuclei apart.
• During the next few minutes, the reactions
that occur are similar to that in the Sun,
except the Deuterium is made by combining
free protons and free neutron.
• Even in this Era, where there are many free
particles with high energy, it takes a bit of
time to fuse Helium out of protons and
neutrons.
• But in the early universe, time is not
something that we have a lot of.
• By the time that the universe is 10 minutes
old, the expansion of the universe has stopped
nucleosynthesis.
• This only provided enough time to make
helium and very trace amounts of Lithium.
• The new universe will end up with a
composition of about 75% hydrogen and 25%
helium and a miniscule amount of lithium.
• Now think back to what reaction rates in the
Sun depend on.
What do nuclear reaction rates
depend on?
30
30
1.
2.
3.
4.
5.
temperature
volume
Density
Red shift
1&3
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2
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20%
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20%
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20%
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20%
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20
5
• Reaction rates depend on the temperature
and the density.
• Temperature because nuclei must have
sufficient kinetic energy to over come the
repulsion force between protons.
• Density, because these tiny particles have to
have head-on collisions in order to “stick”.
• So knowing the exact ratio of Hydrogen to
helium, or measuring the exact amount of
lithium in the universe, can tell us about the
density of the universe. And density decides
the shape of the universe.
A
B
C
D
E
F
F. Electrons combine with nuclei
• After 10 minutes, the universe is filled with
protons, helium nuclei, electrons and light.
• The photons of light still have huge amounts
of energy. If an electron binds to a nucleus, it
is immediately ionized by light.
• Let’s think back to the stellar spectroscopy.
Why do O-type stars have weak
hydrogen absorption lines?
30
1. They have very little
hydrogen
2. They are too hot for
hydrogen to hold on
to its electrons
3. They are very young
stars.
1
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33%
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33%
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2
30
33%
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20
• In order for electron to really stay bound to
the nucleus we need temperatures that are
less than an A-star would have. This is around
10,000 degrees.
• So the light in the universe has to be stretched
to a wavelength (by the expansion) that gives
the photons less than the ionization energy of
hydrogen.
• This happens after about 300,000 years of
expansion.