• Do your course evaluations.

• 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

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.

ϒ + ϒ

• 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.

AB

D

E

C

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.

AB

D

E

C

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?

1 2 3 4 5

20% 20% 20%20%20%1. temperature2. volume3. Density4. Red shift5. 1 & 3

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• 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.

AB

D

E

C

F

What is the difference between a free electron and an electron bound to an atom?

1 2 3

33% 33%33%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

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What wavelengths of light can a free electron absorb and emit?

1 2 3

33% 33%33%1. Only wavelengths that

correspond to a transition

2. Infrared wavelengths3. All possible

wavelengths

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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?

1 2 3

33% 33%33%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.

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• 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…

1 2 3

33% 33%33%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?

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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.”