AST 205 1st Edition Lecture 25Outline of Past Lecture (#25 from 4/21) Topic: Particle Physics I. PhotonsII. WeinbergIII. Goal of Particle PhysicsIV. Matter Dominated UniverseLecture (#26 from 4/23)Fate of The Universe The universe is expanding. Will it expand forever or is there enough matter to eventually halt the expansion and begin contraction?- Contraction implies the Big Crunch - To understand the fate of the universe, we revisit Einstein’s theory of GR- RμV – ½ RGμv = 8πGTμV- R is proportional to T R curvature (shape of universe) T matter & energy Think of someone who shoots a gun. Does the bullet fall back to Earth or does it travel away forever? If the initial velocity (kinetic energy) is greater than the gravitational potential energy, the object travels to infinity - Vescape on earth = 7 min. per sec. The expansion of the universe is the same way - If the expansion velocity is to small, the universe will collapse and end as a “Big Crunch”- If the expansion velocity is great enough, the universe will forever expand.- There is a tug of war between the kinetic energy and the gravitational potential energy - We know the kinetic energy via the Hubble’s constant- To obtain the gravitational potential energy, we need to know the matter density of the universe. Slight problem: we do not know the amount of dark matter - Calculations show that gravity will win out over expansion (kinetic energy) if the matter density exceeds the critical value Pcritical = 10-26kg/m3 critical matter density - We want to compute the matter density of the universe, or Puniverse Has two contributions: ordinary matter (baryonic matter) and dark matter Puniverse = Pmatter + Pdark-matter Expect P-universe to be roughly P-critical - Then, only 5% of Puniverse comes from Pmatter, thus the other 95% must be dark matter Some of the dark matter is likely to be super-symmetric particles and/or scalar particles Some clearly something else… Quiz
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