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• http://www.pa.uky.edu
• I will add 5 points on your final exam if you
complete the evaluation.
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
1
2
21
22
0
20%
3
4
5
6
7
8
9
10
23
24
25
26
27
28
29
30
11
12
1
20%
13
14
2
20%
15
16
3
20%
17
18
4
20%
19
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.
• It is a difficult task to determine the amount
of helium and lithium created in the Big Bang
because these elements are also created
inside stars.
• Studies of the most ancient stars in the
universe, which have almost no processed
materials, set strong constraints on the
amount of lithium formed in the Big Bang.
• That also sets strong constraints on the
density of the early universe, when
nucleosynthesis was occurring.
A
B
C
D
E
F
What is the difference between a free electron
and an electron bound to an atom? 30
30
1. Free electrons act like
particles, bound
electrons act like waves
2. Free electrons act like
waves, bound electrons
act like particles
3. Free electrons are free
and bound electrons
are bound
1
2
21
22
0
3
4
5
6
7
8
9
10
23
24
25
26
27
28
29
30
33%
11
12
13
1
33%
14
15
16
2
33%
17
18
19
3
20
What wavelengths of light can a free electron
absorb and emit?
30
30
1. Only wavelengths that
correspond to a
transition
2. Infrared wavelengths
3. All possible
wavelengths
1
2
21
22
0
33%
3
4
5
6
7
8
9
10
23
24
25
26
27
28
29
30
11
12
13
1
33%
14
15
16
2
33%
17
18
19
3
20
F. Electrons combine with nuclei
• After 10 minutes, the universe is filled with
protons, helium nuclei, free 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
2
21
22
0
3
4
5
6
7
8
9
10
23
24
25
26
27
28
29
30
33%
11
12
13
1
33%
14
15
16
2
30
33%
17
18
19
3
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. This time is called the
Recombination Era. When nuclei and
electrons combined to form atoms.
Ramifications of pre-Recombination
• Before the electrons were bound to the nuclei, they
could absorb and emit all wavelengths of light.
• This means no structures of any kind can be seen
when electrons were free. The free electrons absorb
the light and emit it off into other directions.
• This happens in stars as well. We can only look into
the star (the photosphere) up to a depth where
electrons are free. Then all the light becomes
scattered.
Where’s the Sun? Where did it go?
• When we see an object, like the Sun, we are
seeing the light coming to our eyes. If the
light travels in a straight line, then we see the
object.
• If the light is scattered all around, light still
reaches our eyes, but from all directions.
• This is what was happening in the universe
before the electrons combined with nuclei.
• Once the expansion of the universe dropped
the energy of light below the ionization
energy of the atom, the electrons became
bound.
• Bound electrons act like standing waves.
• They can only absorb and emit very particular
wavelengths of light.
• The other wavelengths of light just pass right by
the atom as if it weren’t there.
• At this point the light from the Big Bang and the
matter from the Big Bang decoupled.
• The universe became transparent.
• This means that most of the wavelengths of light
could travel in straight lines throughout the
universe.
• We could now see structures.
Another complication…
• Objects that are made of normal matter, such
as stars, could not form before recombination.
• Gravity is too weak to be able to pull charged
particles together to form stars.
• The force of gravity is 1042 times weaker than
the repulsive electromagnetic force.
• Only when neutral atoms (# electrons =
#protons) formed, did gravity have any
chance, what-so-ever, in forming structures
out normal matter.
• However, Dark Matter does not interact with
light.
• This means if Dark Matter is made of particles,
those particles must be neutral.
• There is no reason that Dark Matter can’t
begin to clump together, due to its gravity,
before the time of Recombination.
• Remember that there is 10 times the amount
of Dark Matter in the universe, then there is
normal matter.
The Cosmic Microwave Background Radiation
The Cosmic Microwave Background Radiation
(with the Milky Way removed)
• In every direction that we look, there is a light
coming in the form of microwaves.
• It is everywhere.
• This is the radiation that was created in the Big
Bang.
• After Recombination, the light was free to move
throughout the universe without being
continually absorbed and re-emitted.
• We can look at a spectrum of this light. It looks
like a radiating source, with a peak wavelength
that corresponds to the peak in intensity.
We can use Wien’s Law to tell us the temperature
• This radiation comes to us from the moment
when the universe became transparent.
Recombination Era.
• Before we said the electrons will bind to the
atoms when the temperature is around
10,000 degrees (K).
• But the Background Radiation has a
temperature of 2.7 degrees (K).
• Why is this?
Why is this…
30
30
1. The universe must have
been colder than
scientists thought
2. The universe has
expanded a lot since
the time of
recombination
3. What in the hell is
Wien’s law?
1
2
21
22
0
3
4
5
6
7
8
9
10
23
24
25
26
27
28
29
30
33%
11
12
13
1
33%
14
15
16
2
33%
17
18
19
3
20
The orange and yellow are areas where the
temperature is slightly higher.
• This is where the normal matter was clumped
together at the time of Recombination.
• But this can not happen, unless Dark Matter is
present to contain the normal matter.
• The clumpy regions seen in the Background
Radiation are the clumps that turned into
galaxies and galaxy clusters, after
Recombination.
• The process was already started by Dark
Matter before neutral atoms formed.
• From here, galaxy fragments formed and merged
to form the galaxies we see today.
• Stars formed in these early fragments and began
to turn the Hydrogen and Helium into the other
elements in the universe.
• When these processed elements where shot back
out of exploding stars, new stars formed with
those elements present inside them.
• Stars where able to have planets which formed
out of the dusty material around the star.
• WE ARE STAR DUST
Quiz #12 (The last one)
• Explain why it is different to say,
“The Big Bang was an explosion in the
universe.”
Compared to
“The Big Bang was an expansion of the
universe.”