The Fate of the Universe •  A universe has a certain number of dimensions, or direc8ons of movement •  A universe has a certain shape or geometry •  A universe can be finite or infinite, depending on the shape •  A universe has no edge or boundary, even if it is finite •  A universe has no point within it that is the center Milestones in the Early Universe At the moment of the Big Bang, the universe contained only energy in the form of radia8on, and only one force existed, a “superforce”. The universe then changed very quickly: •  10-­‐43 seconds: the superforce splits into gravity and the grand unified theory (GUT) force •  10-­‐35 seconds: the GUT force splits into the electroweak and strong nuclear forces; infla8on begins; radia8on converts to quarks •  10-­‐32 seconds: infla8on ends •  10-­‐10 seconds: electroweak splits into electromagne8c and weak nuclear forces •  0.00001 seconds: quarks form protons and neutrons •  0.01 sec to 3 minutes: fusion of hydrogen to helium •  400,000 yrs: microwave background seen today is produced Milestones in the Early Universe The Fate of the Universe Will the Universe expand forever? Or will it stop expanding and collapse (a Big Crunch)? The Fate of the Universe The gravity from the maUer in the universe should slow the expansion of the universe over 8me. There are 2 possible fates for the expansion: Big Bang expansion gravity The Decelera8on of the Universe For either of the possible fates (bound or unbound), we expect that the expansion was faster in the past and slower now. As a result, the Hubble rela8on should bend upward for the most distant galaxies. now past 1.5 3 (billion light years) In 1998, Type Ia supernovae were used to measure distances for some of the most distant galaxies. When redshi\s were measured for these galaxies, it was found that the universe was expanding more slowly in the distant past than now. The expansion of the universe is accelera?ng! Accelera8ng Expansion In 1998, Type Ia supernovae were used to measure distances for some of the most distant galaxies. When redshi\s were measured for these galaxies, it was found that the universe was expanding more slowly in the distant past than now. The expansion of the universe is accelera?ng! now past 1.5 3 (billion light years) The Age of the Universe In the previous lecture, we found that the slope of the Hubble Law implies an age of <13 billion years for the universe. But this age doesn’t make sense since some globular clusters appear to have ages near 13 billion years. Our previous es8mate of the age was based on the current expansion rate of the universe and an assump8on that the universe expanded more rapidly in the past (i.e., decelera8on). But the universe is actually accelera8ng, and was expanding more slowly in the past, which produces an older age >13 billion years (more precisely, 13.8 billion years) that is consistent with the globular cluster ages. The Cosmological Constant Einstein postulated the existence of a Cosmological Constant, an an8-­‐gravity force to counteract the collapse of the universe. He went to his deathbed thinking that he was wrong, and that the Cosmological Constant was unnecessary because the observed Hubble expansion of the universe would keep it from collapsing. But he was right a\er all! Dark Energy Today, Dark Energy is the name given to the force behind the Cosmological Constant. Dark energy is now the dominant component of the universe, dwarfing dark maUer. Regular maUer (stars, planets, etc.) makes up a 8ny frac8on of the universe. Dark Energy The universe was dominated by radia8on shortly a\er the Big Bang, and then maUer. The influence of dark energy is related to the amount of space in the universe, so as the universe to expands, dark energy becomes increasingly dominant in the universe. But since we don’t know what dark energy is, we don’t know if the universe will expand forever. Big Bang now From Here to Eternity •  A\er 100 trillion years, there won’t be enough remaining gas for making new stars, and the last stars will have died. •  The remaining maUer consists of brown dwarfs, white dwarfs, black holes, and a liUle gas and dust. •  A few new stars are made by the collision and merger of brown dwarfs. At any given 8me, there will be 2-­‐3 stars in the en8re Milky Way made in this way. •  Black holes con8nue to grow by accre8ng any maUer that comes too close. •  The Milky Way disintegrates as its stars interact with each other and are kicked into intergalac8c space. From Here to Eternity •  A\er 1040 years, all protons and neutrons have decayed into smaller elementary par8cles. The only remaining maUer consists of these par8cles and black holes. •  A\er 10100 years, all black holes have evaporated and disappeared. The universe now contains only photons, electrons, neutrinos, and other elementary par8cles. 1040=10000000000000000000000000
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