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Unformatted text preview: Fluctuations and Infation Hawley & Holcomb, chapters 15 and 16 The Problems of Cosmology So Far (Outstanding problems to be discussed: Dark Energy and Dark Matter) Horizon Problem Why is the universe homogeneous and isotropic on scales much larger than the Hubble horizon? Flatness Problem Why is the density of energy in the universe so close to the value that implies a at universe? Fluctuation Problem Why were the density and temperature uctuations that we can observe in the cosmic microwave background anisotropy, and that seeded all structure in the universe, so small (one part in 100,000)? Baryon asymmetry problem The early universe contained both matter and antimatter in very nearly equal amounts. How did the universe end up containing just a little bit more matter than antimatter (one to few parts in one billion), to end up being lled only with matter after most of the original matter annihilated with the antimatter? The Flatness Problem Revisited General relativity (Einsteins equation, the Friedmann equation) relates: Energy density in the universe Expansion rate of the universe Curvature of the universe For the universe to be nearly at, energy density and expansion rate need to be in specic, nelytuned relation to each other. At present, the density of the universe is within 1% of the critical density that gives a at universe. The departure of density from the critical value increases with time. Being within 1% of the critical density today means: Being within about 105 of the critical density at matterradiation equality, Being within about 1014 of the critical density during Big Bang nucleosynthesis Therefore, the initial density of the universe had to be extremely close to the critical value, to result a nearly at universe today. (Note: the universe may be exactly at, instead of being nearly at, but that would in itself require an explanation.) Infation (A theoretical construction that solves the horizon, fatness, and fuctuation problems) Infation is the hypothesis that there was a period early in the history oF the universe (prior to Big Bang nucleosynthesis) when the expansion oF the universe was accelerating, that is, when the distance between any two points stretched at a rapidly increasing rate. To understand infation, we need to learn: How is it possible that the expansion oF the universe can accelerate? How does acceleration solve the outstanding problems (horizon, fatness, fuctuation)? How can we test the acceleration hypothesis? How Does Expansion Accelerate General relativity relates expansion rate to energy density and curvature General relativity also relates the rate of change of the expansion rate (=acceleration) to the energy density and pressure Pressure can be positive or negative Examples of positive pressure: a pressure cooker, the atmosphere, the interior of a star, etc.Examples of positive pressure: a pressure cooker, the atmosphere, the interior of a star, etc....
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 Spring '08
 JohnLacy
 Cosmology, Dark Energy

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