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Lecture 3

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PCS 181
Margaret Buckby

Types of Universe y Open Universe Density of matter Is lowgravity is weakUniverse expands forever y Closed Universe Density of matter is highgravity will halt the expansionUniverse will recontract If this happens repeatedly we will have an oscillating Universeof mattergalaxies barely manage to avoid recollapse y Flat Universe Universe contains the socalled critical density Future of an Open or Flat Universe Time after Big Bang Situation 12 1 1 x 10yearsHydrogen and helium are depletedno new stars can form 142 1 x 10 years Existing stars run out of fuel 173 1 x 10 years All stars will have experienced 100disruptive encounters flinging their planets into space 184 1 x 10 yearsEvaporation of stars and collapse of galaxies into the central Black Holes 30 5 1 x 10 yearsProtons decaype v a positron and neutrino eey a photona burst of radiative energy 1006 1 x 10 yearsHawkingBlack Holes decay 7 Far futureThe Big Fizzlean everexpanding gas of electrons protons neutrons photonsFuture of a closed or oscillating universe 1 The expansion phasestatements 1 Through 5 above 2 At some unknown future time recontraction begins the density of the Universe increases the number of Black Holes increases They feast on everything within reach 3 Black Holes will meet and coalesce merge into one more massive black holethe Universethe new singularityThe Big CrunchHow to decide among these futures y Is there sufficient matter to exert a strong gravitational force y More or equal to 5 protons per m3 of space o Gravity will dominate and recontract the Universe o A closed or oscillating universe y Less than 5 protons per m3 of space
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