Copyright © 2010 Pearson Education, Inc. Chapter 17 Cosmology

Copyright © 2010 Pearson Education, Inc. Chapter 17 Cosmology Ultra Deep Space Field HST

Copyright © 2010 Pearson Education, Inc. Units of Chapter 17 The Universe on the Largest Scales The Expanding Universe Cosmic Dynamics and the Geometry of Space The Fate of the Cosmos The Early Universe The Formation of Nuclei and Atoms Cosmic Inflation The Formation of Large-Scale Structure in the Universe

Copyright © 2010 Pearson Education, Inc. a) the Milky Way must be located at the edge of the universe. b) the Milky Way is at the center of the universe. c) the universe is expanding. d) the sky is dark at night. e) the universe has not changed significantly. Question 1 Because distant galaxies in every direction are moving away from us,

Copyright © 2010 Pearson Education, Inc. a) the Milky Way must be located at the edge of the universe. b) the Milky Way is at the center of the universe. c) the universe is expanding. d) the sky is dark at night. e) the universe has not changed significantly. Question 1 Because distant galaxies in every direction are moving away from us,

Copyright © 2010 Pearson Education, Inc. This galaxy map shows the largest structure known in the universe, the Sloan Great Wall. No structure larger than 300 Mpc is seen. The Universe on the Largest Scales

Copyright © 2010 Pearson Education, Inc. Therefore, the Universe is homogenous (any 300-Mpc-square block appears much like any other) on scales greater than about 300 Mpc. The universe also appears to be isotropic – the same in all directions. The cosmological principle includes the assumptions of isotropy and homogeneity. The Universe on the Largest Scales

Copyright © 2010 Pearson Education, Inc. Olbers’s Paradox: If the universe is homogeneous, isotropic, infinite, and unchanging, the entire sky should be as bright as the surface of the Sun. The Expanding Universe

Copyright © 2010 Pearson Education, Inc. So, why is it dark at night? The universe is homogeneous and isotropic – It must not be infinite and/or unchanging. We have already found that galaxies are moving away from us faster the farther away they are: recessional velocity = H 0  distance The Expanding Universe

Copyright © 2010 Pearson Education, Inc. So, how long did it take the galaxies to get there? The Expanding Universe

Copyright © 2010 Pearson Education, Inc. Using H 0 = 70 km/s/Mpc, we find that time is about 14 billion years. Note that Hubble’s law is the same no matter who is making the measurements. The Expanding Universe 1 Mpc = 3.3 x km 1/h 0 = 3.1 x km / 70 km/s = 4.4 x s 4.4 x s = 14 billion years

Copyright © 2010 Pearson Education, Inc. a) the size of the universe. b) the age of the universe. c) the shape of the universe. d) the temperature of the universe. e) the distance the universe has expanded. Question 2 Hubble’s constant, H 0, can be related to

Copyright © 2010 Pearson Education, Inc. a) the size of the universe. b) the age of the universe. c) the shape of the universe. d) the temperature of the universe. e) the distance the universe has expanded. Question 2 Hubble’s constant, H 0, can be related to H 0 is currently estimated to be about 70 km/sec/Mpc. This translates to an age for the universe of about 14 billion years.

Copyright © 2010 Pearson Education, Inc. If this expansion is extrapolated backward in time, all galaxies are seen to originate from a single point in an event called the Big Bang. So, where was the Big Bang?Big Bang It was everywhere! No matter where in the universe we are, we will measure the same relation between recessional velocity and distance, with the same Hubble constant. The Expanding Universe

Copyright © 2010 Pearson Education, Inc. This can be demonstrated in two dimensions. Imagine a balloon with coins stuck to it. As we blow up the balloon, the coins all move farther and farther apart. There is, on the surface of the balloon, no “center” of expansion. The Expanding Universe

Copyright © 2010 Pearson Education, Inc. The same analogy can be used to explain the cosmological redshift. The Expanding Universe

Copyright © 2010 Pearson Education, Inc. a) a Doppler shift of the random motions of galaxies. b) an aging of light as the universe ages. c) space itself expanding with time, stretching light. d) the result of the Milky Way’s position at the center. e) due to the temperature differences in the early and late universe. Question 3 The redshift of galaxies is explained best as

Copyright © 2010 Pearson Education, Inc. a) a Doppler shift of the random motions of galaxies. b) an aging of light as the universe ages. c) space itself expanding with time, stretching light. d) the result of the Milky Way’s position at the center. e) due to the temperature differences in the early and late universe. Question 3 The redshift of galaxies is explained best as As the universe expands, photons of radiation are stretched in wavelength as they move through space.

Copyright © 2010 Pearson Education, Inc. These concepts are hard to comprehend, and not at all intuitive. A full description requires the very high-level mathematics of general relativity. However, there are aspects that can be understood using relatively simple Newtonian physics – we just need the full theory to tell us which ones! The Expanding Universe

Copyright © 2010 Pearson Education, Inc. a) is infinitely old and getting larger. b) began expanding long ago, and has a finite age. c) will slow down because of dark matter. d) has repeatedly expanded and contracted. e) will eventually stop and recollapse. Question 4 Hubble’s law implies that the universe

Copyright © 2010 Pearson Education, Inc. a) is infinitely old and getting larger. b) began expanding long ago, and has a finite age. c) will slow down because of dark matter. d) has repeatedly expanded and contracted. e) will eventually stop and recollapse. Question 4 Hubble’s law implies that the universe Using the Hubble constant H 0, astronomers can estimate that the universe was born about 14 billion years ago.

Copyright © 2010 Pearson Education, Inc. There are two possibilities for the universe in the far future: 1. It could keep expanding forever. 2. It could collapse. Assuming that the only relevant force is gravity, which way the universe goes depends on its density. Cosmic Dynamics and the Geometry of Space

Copyright © 2010 Pearson Education, Inc. If the density is low, the universe will expand forever. If it is high, the universe will ultimately collapse. Cosmic Dynamics and the Geometry of Space

Copyright © 2010 Pearson Education, Inc. If space is homogenous, there are three possibilities for its overall structure: 1. Closed – this is the geometry that leads to ultimate collapse 2. Flat – this corresponds to the critical density 3. Open – expands forever Cosmic Dynamics and the Geometry of Space

Copyright © 2010 Pearson Education, Inc. These three possibilities are illustrated here. The closed geometry is like the surface of a sphere; the flat one is flat; and the open geometry is like a saddle. Cosmic Dynamics and the Geometry of Space

Copyright © 2010 Pearson Education, Inc. In a closed universe, you can travel in a straight line and end up back where you started. Cosmic Dynamics and the Geometry of Space

Copyright © 2010 Pearson Education, Inc. a) the universe will eventually stop expanding and recollapse. b) dark matter will dominate over dark energy. c) the universe will stop expanding and remain stationary. d) dark energy will dominate over dark matter. e) the universe will not stop expanding. Question 7 In a closed universe

Copyright © 2010 Pearson Education, Inc. a) the universe will eventually stop expanding and recollapse. b) dark matter will dominate over dark energy. c) the universe will stop expanding and remain stationary. d) dark energy will dominate over dark matter. e) the universe will not stop expanding. Question 7 In a closed universe Greater density means more matter in a smaller volume, and gravity will be strong enough to stop the expansion and cause a “Big Crunch.”

Copyright © 2010 Pearson Education, Inc. The answer to this question lies in the actual density of the universe. Measurements of luminous matter suggest that the actual density is only a few percent of the critical density. But – we know there must be large amounts of dark matter. The Fate of the Cosmos

Copyright © 2010 Pearson Education, Inc. However, the best estimates for the amount of dark matter needed to bind galaxies in clusters, and to explain gravitational lensing, still only bring the observed density up to about 0.3 times the critical density, and it seems very unlikely that there could be enough dark matter to make the density critical. The Fate of the Cosmos

Copyright © 2010 Pearson Education, Inc. Type I supernovae can be used to measure the behavior of distant galaxies. If the expansion of the universe is decelerating, as it would if gravity were the only force acting, the farthest galaxies had a more rapid recessional speed in the past, and will appear as though they were receding faster than Hubble’s law would predict. The Fate of the Cosmos

Copyright © 2010 Pearson Education, Inc. However, when we look at the data, we see that it corresponds not to a decelerating universe, but to an accelerating one. The Fate of the Cosmos

Copyright © 2010 Pearson Education, Inc. Possible explanation for the acceleration: Vacuum pressure (cosmological constant), also called dark energy. The Fate of the Cosmos

Copyright © 2010 Pearson Education, Inc. The cosmic microwave background was discovered fortuitously in 1964, as two researchers tried to get rid of the last bit of “noise” in their radio antenna. Instead they found that the “noise” came from all directions and at all times, and was always the same. They were detecting photons left over from the Big Bang. The Early Universe Robert Wilson & Arno Penzias

Copyright © 2010 Pearson Education, Inc. When these photons were created, it was only one second after the Big Bang, and they were very highly energetic. The expansion of the universe has redshifted their wavelengths so that now they are in the radio spectrum, with a blackbody curve corresponding to about 3 K. The Early Universe

Copyright © 2010 Pearson Education, Inc. Since then, the cosmic background spectrum has been measured with great accuracy. The Early Universe

Copyright © 2010 Pearson Education, Inc. Cosmic Microwave Background COBE 1990sWMAP 2000s Video

Copyright © 2010 Pearson Education, Inc. Question 5 The cosmic microwave background radiation is a) evidence supporting the Big Bang. b) proof that the universe is getting warmer. c) a result of the hot intergalactic gas between clusters. d) the observable form of dark energy. e) released from the first generation of stars in the universe.

Copyright © 2010 Pearson Education, Inc. a) evidence supporting the Big Bang. b) proof that the universe is getting warmer. c) a result of the hot intergalactic gas between clusters. d) the observable form of dark energy. e) released from the first generation of stars in the universe. Question 5 The cosmic microwave background radiation is The radiation observed is the “fossil remnant” of the primeval fireball that existed at the very beginning of the universe.

Copyright © 2010 Pearson Education, Inc. The total energy of the universe consists of both radiation and matter. As the universe cooled, it went from being radiation- dominated to being matter-dominated. Dark energy becomes more important as the universe expands. The Early Universe

Copyright © 2010 Pearson Education, Inc. Question 9 In the first few minutes after the Big Bang a) the universe cooled and formed neutral matter. b) the cosmic microwave background radiation was released. c) electrons recombined with protons. d) hydrogen fused into deuterium and then helium. e) the universe was governed by one unified super-force.

Copyright © 2010 Pearson Education, Inc. a) the universe cooled and formed neutral matter. b) the cosmic microwave background radiation was released. c) electrons recombined with protons. d) hydrogen fused into deuterium and then helium. e) the universe was governed by one unified super-force. Question 9 In the first few minutes after the Big Bang The production of elements heavier than hydrogen by nuclear fusion is “primordial nucleosynthesis.” The amount of deuterium we see today is an important clue to the density of this early universe.

Copyright © 2010 Pearson Education, Inc. Hydrogen will be the first atomic nucleus to be formed, as it is just a proton and an electron. Beyond that, helium can form through fusion. The Formation of Nuclei and Atoms

Copyright © 2010 Pearson Education, Inc. Most deuterium fused into helium as soon as it was formed, but some did not. Deuterium is not formed in stars, so any deuterium we see today must be primordial. The Formation of Nuclei and Atoms

Copyright © 2010 Pearson Education, Inc. The time during which nuclei and electrons combined to form atoms is referred to as the decoupling epoch. This is when the cosmic background radiation originated. The Formation of Nuclei and Atoms

Copyright © 2010 Pearson Education, Inc. The horizon problem: Cosmic background radiation appears the same in diametrically opposite directions from Earth, even though there hasn’t been enough time since the Big Bang for these regions to be in thermal contact. Cosmic Inflation

Copyright © 2010 Pearson Education, Inc. The flatness problem: In order for the universe to have survived this long, its density in the early stages must have differed from the critical density by no more than 1 part in Cosmic Inflation

Copyright © 2010 Pearson Education, Inc. Geometry What shape does the universe have? Geometry

Copyright © 2010 Pearson Education, Inc. Between s and s after the Big Bang, some parts of the universe may have found themselves in an extreme period of inflation, as shown on the graph. Between s and s, the size of this part of the universe expanded by a factor of ! Cosmic Inflation

Copyright © 2010 Pearson Education, Inc. Inflation would solve both the horizon and the flatness problems. This diagram shows how the flatness problem is solved – after the inflation the need to be very close to the critical density is much more easily met. Cosmic Inflation

Copyright © 2010 Pearson Education, Inc. a) there is more matter than energy. b) the universe is closed, and will recollapse. c) the universe is open, and will keep expanding. d) dark matter will dominate, and galaxies will stop expanding. e) there was more helium than hydrogen created in the Big Bang. Question 8 If the density of the universe is greater than “critical”,

Copyright © 2010 Pearson Education, Inc. a) there is more matter than energy. b) the universe is closed, and will recollapse. c) the universe is open, and will keep expanding. d) dark matter will dominate, and galaxies will stop expanding. e) there was more helium than hydrogen created in the Big Bang. Question 8 If the density of the universe is greater than “critical”,

Copyright © 2010 Pearson Education, Inc. Cosmologists realized that galaxies could not have formed just from instabilities in normal matter: Before decoupling, background radiation kept clumps from forming. Variations in the density of matter before decoupling would have led to variations in the cosmic microwave background. The Formation of Large-Scale Structure in the Universe Ripples

Copyright © 2010 Pearson Education, Inc. Because of the overall expansion of the universe, any clumps formed by normal matter could only have had 50–100 times the density of their surroundings. Dark matter, being unaffected by radiation, would have started clumping long before decoupling. The Formation of Large-Scale Structure in the Universe

Copyright © 2010 Pearson Education, Inc. Galaxies could then form around the dark-matter clumps, resulting in the universe we see. The Formation of Large-Scale Structure in the Universe

Copyright © 2010 Pearson Education, Inc. This figure is the result of simulations of a cold dark matter universe with critical density. The Formation of Large-Scale Structure in the Universe

Copyright © 2010 Pearson Education, Inc. Although dark matter does not interact directly with radiation, it will interact through the gravitational force, leading to tiny “ripples” in the cosmic background radiation. These ripples have now been observed. The Formation of Large-Scale Structure in the Universe

Copyright © 2010 Pearson Education, Inc. This is a much higher-precision map of the cosmic background radiation. The Formation of Large-Scale Structure in the Universe

Copyright © 2010 Pearson Education, Inc. a) the Steady State Theory. b) the Grand Unified Theories. c) the Inflationary epoch. d) dark matter. e) decoupling of matter from radiation. Question 10 The universe appears flat; this is explained by

Copyright © 2010 Pearson Education, Inc. a) the Steady State Theory. b) the Grand Unified Theories. c) the Inflationary epoch. d) dark matter. e) decoupling of matter from radiation. Question 10 The universe appears flat; this is explained by

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Copyright © 2010 Pearson Education, Inc. Final Exam Study Guide Density is defined as… A.Weight per square inch B.Mass times weight C.Size divided by weight D.Weight divided by a planet’s radius E.Mass per unit volume

Copyright © 2010 Pearson Education, Inc. E. Mass per unit volume kg / m 3

Copyright © 2010 Pearson Education, Inc. Comets The tail of a comet always points… A.toward Earth B.toward the Sun C.In the direction of the comet’s motion D.Away from the sun E.Toward the galactic center

Copyright © 2010 Pearson Education, Inc. D. Away from the sun

Copyright © 2010 Pearson Education, Inc. Techniques When we detect Doppler shifting of a star we can assume that… A.The star is about to go supernova B.The star is a class OB C.There is intelligent life near by D.There might be an orbiting planet

Copyright © 2010 Pearson Education, Inc. D. There might be an orbiting planet

Copyright © 2010 Pearson Education, Inc. Water Water in the form of permafrost has been found on which planet? A.Europa B.Venus C.Jupiter D.Mercury E.Mars

Copyright © 2010 Pearson Education, Inc. E. Mars

Copyright © 2010 Pearson Education, Inc. Saturn Saturn is the only known planet that has rings. A.True B.False

Copyright © 2010 Pearson Education, Inc. B. False Rings of Uranus

Copyright © 2010 Pearson Education, Inc. Jupiter The light and dark bands of Jupiter are caused by… A.Zones of rising and sinking gas B.Dirty sooty rings C.Huge magnetic fields D.Differential rotation E.Ice crystals

Copyright © 2010 Pearson Education, Inc. A. Zones of rising and sinking gas

Copyright © 2010 Pearson Education, Inc. Venus What greenhouse gas does Earth and Venus have in common? A.Oxygen B.Argon C.Hydrogen D.Carbon dioxide E.Nitrogen

Copyright © 2010 Pearson Education, Inc. D. Carbon dioxide

Copyright © 2010 Pearson Education, Inc. Bright! What is the third brightest object in the sky? A.Jupiter B.Moon C.Sirius D.Venus E.Mars

Copyright © 2010 Pearson Education, Inc. D. Venus

Copyright © 2010 Pearson Education, Inc. Moon It takes a full year for us to see the entire surface of the moon. A.No, only one month B.No, only one day C.No, we see only one half of its surface D.Yes, but it is a sidereal year E.Yes, but it is a tropical year

Copyright © 2010 Pearson Education, Inc. C. No, we see only one half of its surface

Copyright © 2010 Pearson Education, Inc. Moon The reason we see only one side of the moon is because of something called… A.Tidal lock B.Lunar gravity C.Solar equalization D.Earth sync E.Kepler’s Law

Copyright © 2010 Pearson Education, Inc. A. Tidal lock

Copyright © 2010 Pearson Education, Inc. Kepler’ Laws Kepler’s Laws allow us to determine what if we know the period and distance of one object orbiting another? A.Length of year B.Distance to that object C.Composition of the object D.Total mass of both objects

Copyright © 2010 Pearson Education, Inc. D. Total mass of both objects

Copyright © 2010 Pearson Education, Inc. On scales larger than a few hundred megaparsecs, the universe is homogeneous and isotropic. The universe began about 14 million years ago, in a Big Bang. Future of the universe: it will either expand forever, or collapse. Density between expansion and collapse is critical density. Summary of Chapter 17

Copyright © 2010 Pearson Education, Inc. A high-density universe has a closed geometry; a critical universe is flat; and a low- density universe is open. Acceleration of the universe appears to be speeding up, due to some form of dark energy. The universe is about 14 billion years old. Cosmic microwave background is photons left over from Big Bang. Summary of Chapter 17, cont.

Copyright © 2010 Pearson Education, Inc. At present the universe is matter-dominated; at its creation it was radiation-dominated. When the temperature became low enough for atoms to form, radiation and matter decoupled. The cosmic background radiation we see dates from that time. Horizon and flatness problems can be solved by inflation. Summary of Chapter 17, cont.

Copyright © 2010 Pearson Education, Inc. The density of the universe appears to be the critical density; 2/3 of the density comes from dark energy, and dark matter makes up most of the rest. Structure of universe today could not have come from fluctuations in ordinary matter. Fluctuations in dark matter can account for what we see now. Summary of Chapter 17, cont.