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Unformatted text preview: Lecture 15 – The Universe Homogeneity : One randomly-chosen large volume of the universe will have the same physical properties as another randomly-chosen large volume of the universe Isotropy : there is no preferred direction in space Universe’s distance is measured in light years or Megaparsecs Special Relativity: one observer is moving with respect to the other at a constant velocity General Relativity : observers experience different accelerations due to gravity Big Bang Predictions: universe had a specific beginning, sky’s dark at night, universe was hot, opaque and dense, universe has evolved, it’s expanding, stars should be 75% hydrogen and 25% helium brightness of each individual star decreases as (1/distance) 2 number of stars at that distance increases as (distance) 2 Redshift: things move away and emit red Hubble’s Law : finding distances using luminosity(known) and brightness(evaluated). Using this technique, Hubble discovered that the more distant galaxies seemed to be moving away from us at faster speeds than the more nearby galaxies. Does not imply that we’re the center of the universe Implies that universe was denser time = 1 / H = Hubble Age, how long it took the universe to get to where it is today = 13.8billion years Doppler Shift: energy of a photon decreases as wavelength increases. Density of radiant energy (light) decreases faster than the density of mass energy. Energy density is in mass/matter CMBR: Primordial light would’ve been a continuous black body spectrum + high energy (gamma rays) Lecture 16- Evolution of the Universe When you go back 5-7 billion years: Universe has 100 billion galaxies (.5-9 billion years) Large galaxies have 10 billion starts or more; small galaxies 1-100 million Elipticals are in large clusters; spirals are in small groups Spiral galaxies were brighter Elipitical were also brighter, by an even larger factor, and were bluer 7-12 billion years ago: elliptical and spiral were rare; less galaxies; almost all were disturbed Universe is too cool to make more photons so instead, because it’s hot and dense enough, it fuses protons and neutrons to make detrium (H, He, Be, Li) Low density universe : expand forever but slower and slower High density universe: expand, decrease in speed and collapse onto itself Critical density universe: expand slower until it halts and does not collapse Most likely, our universe will expand forever Lecture 17 – Life of a Star Clouds are transparent in radio and IR Clouds can be studied w/molecular spectral lines Star formation: cloud collapses protostar forms disk forms planets form Cloud collapse: hydrostatic equilibrium: cloud has inward (gravity) balances w/outward (pressure); gravity wins and cloud collapses Protostar: middle of a collapsed cloud that’s dense and hot; gravitational Disk forms: conservation of Energy, Momentum, Angular Momentum!!! L = mvr- L (angular momentum) is a vector!...
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This note was uploaded on 05/06/2008 for the course CORE 105 taught by Professor Jackson during the Fall '07 term at BU.
- Fall '07