Lecture14 Early Universe

Lecture14 Early Universe - The Early Universe Hawley &...

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The Early Universe Hawley & Holcomb, Chapter 12
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Radiation Era • In the present universe, the energy in radiation compared to the energy in matter is one part in 3,300. • At the last scattering surface, or “recombination” or “decoupling” was at redshift z~1,000, the universe was 3 parts matter and 1 part radiation. • At redshift z ~3,300, the energy density in radiation equals the matter energy density in matter. This epoch is called “the matter-radiation equality.” z > 3,300 is the radiation era z < 3,300 is the matter era • The timeline: – Matter-radiation equality was at ~30,000 years – The matter-radiation decoupling epoch, which we also call recombination or the last scattering surface, was at ~380,000 years – Today is at ~13,700,000,000 years
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Nucleosynthesis: The Origin of Helium About 24% of the mass of atoms in the universe is helium, 76% is hydrogen. One place where helium can be made is by fusion of hydrogen into helium in the centers of stars. But the amount of helium in the universe is much larger than the helium that could have been made in stars. – The fusion in stars is an inefficient process. – Stars could fuse only 2% of all hydrogen in the universe into helium, which is only a twelfth of the total amount. During the first three minutes in the history of the universe, the temperature reached 1 billion Kelvin. The entire universe was a fusion reactor, similar to the center of the Sun. The observed amount of helium, plus a smattering of deuterium and lithium were produced. This epoch is called “Big Bang Nucleosynthesis.”
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Helium Synthesis in Detail “D” is the deuteron, a nuclear bound state of a proton with a neutron (add an electron and make the atom deuterium, which also called heavy hydrogen) Possible reactions transforming neutrons, protons, and deuterons into each other include: – n + p D + photon (fusion) – D + photon n + p (photo-dissociation) Given enough deuterons, helium-4 can be made in a two step process, similar to the proton-proton chain. (Here, “T” is the tritium, containing one proton and two neutrons.) – D + D T + p – T + T 4 He + n When T > 1 billion Kelvin, photo-dissociation process also occurs frequently and destroys the formed deuterons. – For there to be a substantial residual amount of helium, the temperature must drop below 1 billion Kelvin, when the photo-dissociation slows down rapidly. This happens when the age of the universe is ~100 seconds.
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Equilibrium and “Freeze Out” (What determines the amount of atoms, etc., left over from the Big Bang?) A B A pair of reactions transforming A
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This note was uploaded on 09/14/2009 for the course AST 309 taught by Professor Johnlacy during the Spring '08 term at University of Texas.

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Lecture14 Early Universe - The Early Universe Hawley &...

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