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Chapter 16 Dark Matter, Dark Energy, and the Fate of the Universe

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Chapter 16 Dark Matter, Dark Energy, and the Fate of the Universe Chapter 16 Dark Matter, Dark Energy, and the Fate of the Universe Copyright © 2010 Pearson Education, Inc. Normal matter: The kind of matter we are familiar with, which is made of atoms and responds to the electromagnetic force and gravity. Dark matter: An undetected form of mass that emits little or no light but whose existence we infer from its gravitational influence Dark energy: An unknown form of energy that seems to be the source of a repulsive force causing the expansion of the universe to accelerate Copyright © 2010 Pearson Education, Inc. Contents of Universe • Normal matter: ~ 4.9% • Dark matter: ~ 26.8% • Dark energy: ~ 68.3% Copyright © 2010 Pearson Education, Inc. 1 Rotation curve A plot of orbital speed versus orbital radius Solar system’s rotation curve declines because Sun has almost all the mass. Rotation Curve of the Solar System Copyright © 2010 Pearson Education, Inc. The rotation curve of the Milky Way stays flat with distance. Mass must be more spread out than in the solar system. Rotation Curve of a Spiral Galaxy Copyright © 2010 Pearson Education, Inc. The mass in the Milky Way is spread out over a larger region than the stars. Most of the Milky Way’s mass seems to be dark matter! Encircled Mass as a Function of Distance for a Spiral Galaxy Copyright © 2010 Pearson Education, Inc. 2 The visible portion of a galaxy lies deep in the heart of a large halo of dark matter. Copyright © 2010 Pearson Education, Inc. We can measure orbital velocities in other spiral galaxies using the Doppler shift of the 21-cm line of atomic H. Copyright © 2010 Pearson Education, Inc. Spiral galaxies all tend to have orbital velocities that remain constant at large radii, indicating large amounts of dark matter. Copyright © 2010 Pearson Education, Inc. 3 We can measure the velocities of galaxies in a cluster from their Doppler shifts. Copyright © 2010 Pearson Education, Inc. The mass we find from galaxy motions in a cluster is about 50 times larger than the mass in stars! Copyright © 2010 Pearson Education, Inc. Clusters contain large amounts of X ray–emitting hot gas. The temperature of hot gas (particle motions) tells us cluster mass: 85% dark matter 13% hot gas 2% stars Copyright © 2010 Pearson Education, Inc. 4 Gravitational lensing, the bending of light rays by gravity, can also tell us a cluster’s mass. Copyright © 2010 Pearson Education, Inc. All three methods of measuring cluster mass indicate similar amounts of dark matter. Copyright © 2010 Pearson Education, Inc. Does dark matter really exist? Copyright © 2010 Pearson Education, Inc. 5 Our Options 1. Dark matter really exists, and we are observing the effects of its gravitational attraction. 2. Something is wrong with our understanding of gravity, causing us to mistakenly infer the existence of dark matter. Copyright © 2010 Pearson Education, Inc. Our Options 1. Dark matter really exists, and we are observing the effects of its gravitational attraction. 2. Something is wrong with our understanding of gravity, causing us to mistakenly infer the existence of dark matter. Because gravity is so well tested, most astronomers prefer option #1. Copyright © 2010 Pearson Education, Inc. What might dark matter be made of? Copyright © 2010 Pearson Education, Inc. 6 How dark is it? Copyright © 2010 Pearson Education, Inc. How dark is it? … not as bright as a star. Copyright © 2010 Pearson Education, Inc. Two Basic Options • Ordinary Matter (MACHOs) — Massive Compact Halo Objects: dead or failed stars in halos of galaxies • Exotic Particles (WIMPs) — Weakly Interacting Massive Particles: mysterious neutrino-like particles Copyright © 2010 Pearson Education, Inc. 7 Compact starlike objects occasionally make other stars appear brighter through lensing… Copyright © 2010 Pearson Education, Inc. Compact starlike objects occasionally make other stars appear brighter through lensing… … but there are not enough lensing events to explain all the dark matter. Copyright © 2010 Pearson Education, Inc. Why WIMPs? • There’s not enough ordinary matter. • WIMPs could be left over from the Big Bang. • Models involving WIMPs explain how galaxy formation works. Copyright © 2010 Pearson Education, Inc. 8 Will the universe continue expanding forever? Copyright © 2010 Pearson Education, Inc. Fate of universe depends on the amount of dark matter. Lots of Critical Not enough dark density of dark matter matter matter Copyright © 2010 Pearson Education, Inc. Amount of matter is ~25% of the critical density, suggesting fate is eternal expansion. Not enough dark matter Copyright © 2010 Pearson Education, Inc. 9 But expansion appears to be speeding up! Dark energy? Not enough dark matter Copyright © 2010 Pearson Education, Inc. old older oldest Estimated age depends on both dark matter and dark energy. Copyright © 2010 Pearson Education, Inc. The brightness of distant white dwarf supernovae tells us how much the universe has expanded since they exploded. Copyright © 2010 Pearson Education, Inc. 10 An accelerating universe is the best fit to supernova data. Copyright © 2010 Pearson Education, Inc. Chapter 17 The Beginning of Time Copyright © 2010 Pearson Education, Inc. The universe must have been much hotter and denser early in time. Estimating the Age of the Universe Copyright © 2010 Pearson Education, Inc. 11 The early universe must have been extremely hot and dense. Copyright © 2010 Pearson Education, Inc. Photons converted into particle–antiparticle pairs and vice versa. E = mc 2 The early universe was full of particles and radiation because of its high temperature. Copyright © 2010 Pearson Education, Inc. Defining Eras of the Universe • The earliest eras are defined by the kinds of forces present in the universe. • Later eras are defined by the kinds of particles present in the universe. Copyright © 2010 Pearson Education, Inc. 12 Four known forces in universe: Strong Force Electromagnetism Weak Force Gravity Copyright © 2010 Pearson Education, Inc. Copyright © 2010 Pearson Education, Inc. Copyright © 2010 Pearson Education, Inc. 13 Do forces unify at high temperatures? Four known forces in universe: Strong Force Electromagnetism Weak Force Gravity Copyright © 2010 Pearson Education, Inc. Do forces unify at high temperatures? Four known forces in universe: Strong Force Electromagnetism Weak Force Gravity Yes! (Electroweak) Copyright © 2010 Pearson Education, Inc. Do forces unify at high temperatures? Four known forces in universe: Strong Force Electromagnetism Weak Force Gravity Yes! Maybe (Electroweak) (GUT) Copyright © 2010 Pearson Education, Inc. 14 Do forces unify at high temperatures? Four known forces in universe: Strong Force Electromagnetism Weak Force Gravity Yes! Maybe Who knows? (Electroweak) (GUT) (String Theory) Copyright © 2010 Pearson Education, Inc. Copyright © 2010 Pearson Education, Inc. Planck Era Time: < 10 -43 s Temp: > 10 32 K No theory of quantum gravity All forces may have been unified Copyright © 2010 Pearson Education, Inc. 15 GUT Era Time: 10 -43 –10 -38 s Temp: 10 32 –10 29 K GUT era began when gravity became distinct from other forces. GUT era ended when strong force became distinct from electroweak force. This was accompanied by inflation. Copyright © 2010 Pearson Education, Inc. Electroweak Era Time: 10 -38 –10 -10 s Temp: 10 29 –10 15 K Strong force and electroweak force now separate Copyright © 2010 Pearson Education, Inc. Particle Era Time: 10 -10 –0.001 s Temp: 10 15 –10 12 K Amounts of matter and antimatter are nearly equal. (Roughly one extra proton for every 10 9 proton– antiproton pairs!) Copyright © 2010 Pearson Education, Inc. 16 Era of Nucleosynthesis Time: 0.001 s–5 min Temp: 10 12 –10 9 K Began when matter annihilates remaining antimatter at ~ 0.001 s. Nuclei began to fuse. Copyright © 2010 Pearson Education, Inc. Era of Nuclei Time: 5 min–380,000 yrs Temp: 10 9–3000 K Helium nuclei formed at age ~3 minutes. The universe became too cool to blast helium apart. Radiation could not travel far The universe was “opaque” Copyright © 2010 Pearson Education, Inc. Era of Atoms Time: 380,000 years– 1Gy Temp: 3000–20 K Atoms formed at age ~380,000 years. The universe became “transparent”. Cosmic background radiation is released. Copyright © 2010 Pearson Education, Inc. 17 Era of Galaxies Time: ~1Gy–present Temp: 20–3 K The first stars and galaxies formed by ~1 billion years after the Big Bang. Copyright © 2010 Pearson Education, Inc. Primary Evidence for the Big Bang 1. We have detected the leftover radiation from the Big Bang. 2. The Big Bang theory correctly predicts the abundance of helium and other light elements in the universe. Copyright © 2010 Pearson Education, Inc. The cosmic microwave background — the radiation left over from the Big Bang— was detected by Penzias and Wilson in 1965. Copyright © 2010 Pearson Education, Inc. 18 Background radiation from the Big Bang has been freely streaming across the universe since atoms formed at temperature ~3000 K: visible/IR. Copyright © 2010 Pearson Education, Inc. Background has perfect thermal radiation spectrum at temperature 2.73 K. Expansion of the universe has redshifted thermal radiation from that time to ~1000 times longer wavelength: microwaves. Copyright © 2010 Pearson Education, Inc. Protons and neutrons combined to make long-lasting helium nuclei when the universe was ~5 minutes old. Copyright © 2010 Pearson Education, Inc. 19 Big Bang theory prediction: 75% H, 25% He (by mass) Matches observations of nearly primordial gases Copyright © 2010 Pearson Education, Inc. 20.
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