The Fate of the Universe

1. The Fate of the Universe

2. The Fate of the Universe Will the Universe expand forever? Or will it stop expanding and collapse (a Big Crunch)?

3. The Shape of the Universe Whether the universe expands forever or eventually collapses depends on the amount of gravity in the universe, which is determined by the average density of matter. The density of matter (and energy) () in the universe also determines the shape of the universe since they curve and distort space. There are 3 possible shapes for a 3-D universe. We can’t visualize them, but we can draw 2-D versions of these shapes.  >1: space curved so much that it wraps back on itself; universe has positive curvature and is finite  <1: negative curvature; infinite  =1: no curvature (flat); infinite

4. The Shape of the Universe In a closed universe, moving in a straight line would eventually bring us back to our original position!

5. The Shape of the Universe As a result, the apparent size of an object from our point of view is affected by the shape of the universe. We can determine this shape by measuring the sizes of clumps in the microwave background. By doing so, we find that the universe is flat. As light travels through the universe, its path will follow the curvature of space.

7. The Fate of the Universe Big Bang expansion The gravity from the matter in the universe should slow the expansion of the universe over time. There are 2 possible fates for the expansion: gravity

8. The Deceleration 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 relation should bend upward for the most distant galaxies. now past constant expansion 1.5 3 (billion light years)

9. In 1998, Type Ia supernovae were used to measure distances for some of the most distant galaxies. When redshifts 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 accelerating!

10. Accelerating Expansion In 1998, Type Ia supernovae were used to measure distances for some of the most distant galaxies. When redshifts 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 accelerating! now past constant expansion 1.5 3 (billion light years)

12. 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 estimate of the age was based on the current expansion rate of the universe and an assumption that the universe expanded more rapidly in the past (i.e., deceleration). But the universe is actually accelerating, and was expanding more slowly in the past, which produces an olderage >13 billion years (more precisely, 13.8 billion years) that is consistent with the globular cluster ages.

13. The Cosmological Constant Einstein postulated the existence of a Cosmological Constant, an anti-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 after all!

14. 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 matter. Regular matter (stars, planets, etc.) makes up a tiny fraction of the universe.

15. Dark Energy The universe was dominated by radiation shortly after the Big Bang, and then matter. 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. now Big Bang

16. From Here to Eternity • After 100 trillion years, there won’t be enough remaining gas for making new stars, and the last stars will have died. • The remaining matter consists of brown dwarfs, white dwarfs, black holes, and a little gas and dust. • A few new stars are made by the collision and merger of brown dwarfs. At any given time, there will be 2-3 stars in the entire Milky Way made in this way. • Black holes continue to grow by accreting any matter that comes too close. • The Milky Way disintegrates as its stars interact with each other and are kicked into intergalactic space.

17. From Here to Eternity • After 1040 years, all protons and neutrons have decayed into smaller elementary particles. The only remaining matter consists of these particles and black holes. • After 10100 years, all black holes have evaporated and disappeared. The universe now contains only photons, electrons, neutrinos, and other elementary particles.