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Cosmology • Cosmology: the study of the universe as a whole and single system • Physical Cosmology: the scientific study of the universe as a whole and single system • Newtonian Cosmology: the model of the universe proposed before the Big Bang • Big Bang Cosmology: the model of the expanding universe consistent with general relativity Newtonian Cosmology • Four assumptions about the average properties of the universe on the largest physical scales: – – – – The universe is static The universe is eternal The universe is homogeneous The universe is infinite in volume • Assumptions are treacherous! Obler’s Paradox • Adopting the assumptions of the Newtonian cosmology results in a paradox: • Why is the night sky dark instead of light? – Every line of sight will have a star or other source of light Assumptions Challenged • Edgar Allen Poe noted that universe may have been “created” and thus is not eternal – The distance light travels over the life of the universe is our “horizon”. • The static assumptions will prove to be wrong: Edwin Hubble (1929) • We will examine our assumptions carefully General Relativity • According to solutions from General Relativity, the universe cannot be static. • The Cosmological Principle contains the two assumptions about the universe on the largest scales: – The universe is homogeneous – The universe is isotropic • These assumptions make it possible to calculate evolution of the universe • LeMaitre and Friedman argued for an expanding spacetime for the Universe Big Bang Theory Uniform Expansion of the Universe The Hubble Law: vr = H0 D Galaxies Recede Using Redshifts to Map Galaxies Large Scale Structure Mass Density of the Universe Spacetime Curvature Three Possible Geometries The Cosmological Constant • Einstein introduced this term to the models of LeMaitre and Friedman so that the Universe would not expand • Called it his “the worst mistake”. • It now is represented by the term Quintessence (or Dark Energy) • Interpreted as an energy fluctuations in the vacuum of spacetime Expansion History and Fate Looking Back in Time The Cooling Universe The Early Universe Particle Physics in the Early Universe