Popular in Visions of The Universe
Popular in Physics 2
This 72 page Study Guide was uploaded by AlissaRabat on Sunday February 7, 2016. The Study Guide belongs to ISP 205 at Michigan State University taught by C. Wrede in Fall 2015. Since its upload, it has received 81 views. For similar materials see Visions of The Universe in Physics 2 at Michigan State University.
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Chapter 15: Our Galaxy 12/17/15 5:29 PM 15.1 The Milky Way Revealed • What does our galaxy look like? o o Another Galaxy Far Far Away o The Milky Way Galaxy appears in our sky as a faint band of light. o Dusty gas clouds obscure our view because they absorb visible light. o This is the interstellar medium that makes new star systems. o Artist’s impression of our galaxy edge-on. o Primary features: disk, bulge, halo, globular clusters o o If we could view the Milky Way from above the disk, we would see its spiral arms. o • Milky Way size o The Milky Way is big! o 100 000 light-year diameter o Contains over 100 billion stars o Along with Andromeda, Milky Way is one of the two largest galaxies in our local group of ~70 galaxies • Satellite Galaxies o Eg. Large and Small Magellanic Clouds are small galaxies orbiting the Milky Way at distances of about 150 000 light years. They contain a few billion stars. o • How do stars orbit in our galaxy? o o Stars in the disk all orbit in the same direction with a minor up-and-down motion (yellow). o Orbits of stars in the halo (green) have random orientations. o Orbits of stars in the bulge (red) have both random and collective orientations. o The Sun is moving with an orbital velocity of 220 km/s and orbits the Galaxy once every 230 million years o Sun’s orbital motion (radius and velocity) tells us mass within Sun’s orbit: o 1.0 × 1MSun o • Orbital Velocity Law 2 r×v M = r o G o The orbital speed (v) and radius (r) of an object on a circular orbit around the galaxy trll us the mass (M ) within that orbit. G is the gravitational constant. o Sun’s orbital motion (radius and velocity) tells us mass within Sun’s orbit: o 1.0 × 1M Sun o Using this formula for other stars suggests that most of the mass of our galaxy is dark! 15.2 Galactic Recycling • How is gas recycled in our galaxy? o o Star–gas–star cycle § Recycles gas from old stars into new star systems. o High-mass stars have strong stellar winds that blow bubbles of hot gas. o Lower-mass stars return gas to interstellar space through weak stellar winds and planetary nebulae. o X rays from hot gas in young supernova remnants, which shock and heat interstellar medium. o A supernova remnant cools and begins to emit visible light as it expands. o New elements made by supernova mix into interstellar medium. § o Multiple supernovae create huge hot bubbles that can blow out of disk. o Gas clouds cooling in the halo can rain back down on disk. o o Ionized hydrogen (H+) gas heated to 1 000 000 K by stellar winds or supernovae o Atomic hydrogen (H) gas forms as hot gas cools to 100- 10 000 K, allowing electrons to join with protons. § -we can see H gas through a 21-cm (radio) wavelength emission line o Molecular clouds (mostly H ) for2 next, after gas cools below 30 K to allow atoms to combine into molecules. § -we can’t see H 2irectly because no emission line but can see other molecules like CO o Molecular clouds in Orion: colors indicate gas motion from Doppler shift of CO line o Composition: § Mostly H 2 § About 28% He § About 1% CO § Many other molecules o Gravity forms stars out of the gas in molecular clouds, completing the star–gas– star cycle. o Radiation from newly formed stars is eroding these star- forming clouds. • Summary of Galactic Recycling o Stars make new elements by fusion. o Dying stars expel gas and new elements, producing hot bubbles (~10 K). o Hot gas cools, allowing atomic hydrogen clouds to form (~100–10,000 K). o Further cooling permits molecules to form, making molecular clouds (~30 K). o Gravity forms new stars (and planets) in molecular clouds. • We observe the star–gas–star cycle operating in Milky Way’s disk using many different wavelengths of light. • Infrared light reveals stars whose visible light is blocked by gas clouds. o § Top: Infrared § Bottom: Visible • X rays are observed from hot gas above and below the Milky Way’s disk (and x-ray binaries). o • 21-cm radio waves emitted by atomic hydrogen show where gas has cooled and settled into disk. o • Radio waves from carbon monoxide (CO) show locations of molecular clouds. o • Long-wavelength infrared emission shows where young stars are heating dust grains. o • Gamma rays show where cosmic rays from supernovae collide with atomic nuclei in gas clouds. o • Where do stars tend to form in our galaxy? o Electrons in atoms move to higher energy orbits when they absorb ultraviolet photons, then emit light when electrons fall back down • Ionization nebulae are reddish and found around short-lived high-mass stars, signifying active star formation. o • Reflection nebulae contain dust that scatters the light from stars. o Why do reflection nebulae look bluer than the nearby stars? § For the same reason that our sky is blue! § (blue light from hot stars is scattered more than red light) • What features of the Interstellar medium do you see in the picture ? o Ionization nebulae o Reflection nebulae o Molecular clouds o o • Much of star formation in disk happens in spiral arms. o -Ionization nebulae are red o -Blue stars and reflection nebulae are blue o -Molecular gas and dust clouds are dark • Spiral arms are waves of star formation: o Gas clouds get squeezed as they move into spiral arms. o The squeezing of clouds triggers star formation. o Young stars flow out of spiral arms. § 15.3 The History of the Milky Way • What do halo stars tell us about our galaxy’s history? o § Halo stars formed first, then stopped. § Disk stars formed later, and kept forming. o • How did our galaxy form? o Our galaxy probably formed from a giant protogalactic gas cloud. o Halo stars formed first as gravity caused the protogalactic cloud to contract. o The remaining gas settled into a spinning disk. o Stars continuously form in the disk as the galaxy grows older. o Warning: This model is oversimplified. § o Detailed studies: Halo stars formed in clumps that later merged. § 15.4 The Galactic Center • What is the evidence for a black hole at our galaxy’s center? o Infrared light from center of Milky Way § o Radio emission from center § o Swirling gas near center (radio) § o Orbiting stars near center (infrared) § o Stars appear to be orbiting something massive but invisible a black hole? § § Orbits of stars indicate a mass of about 4 million Msun. o Orbits of stars around Sagittarius-A* § o X-ray flares from galactic center suggest that tidal forces of suspected black hole occasionally tear apart chunks of matter about to fall in. § Reading Review Chapter 15 12/17/15 5:29 PM Reading Review Ch. 15a 1. How does the diameter of the disk of the Milky Way Galaxy compare to its thickness? a. The diameter is about 100 times as great as the thickness. 2. The Sun's location in the Milky Way Galaxy is _________. a. in the galactic disk, roughly halfway between the center and the outer edge of the disk 3. How do disk stars orbit the center of the galaxy? a. They all orbit in roughly the same plane and in the same direction. 4. How do we know the total mass of the Milky Way Galaxy that is contained within the Sun's orbital path? a. by applying Newton's version of Kepler's third law (or the equivalent orbital velocity formula) to the Sun's orbit around the center of the Galaxy 5. What do we mean by the star-gas-star cycle? a. It is the continuous recycling of gas in the galactic disk between stars and the interstellar medium 6. Which of the following models best explains why our galaxy has spiral arms? a. The spiral arms are a wave of star formation caused by wave of density propagating outward through the disk of the galaxy. 7. Where does most star formation occur in the Milky Way Galaxy? a. In the spiral arms R.R Chapter 15b 1. What do halo stars do differently from disk stars? a. They orbit the galactic center with many different inclinations, while disk stars all orbit in nearly the same plane. 2. Based on observations, which of the following statements about stars in the Milky Way is generally true? a. The older the star, the lower its abundance of heavy elements. R.R Chapter 15c 1. What kind of object do we think lies in the center of the Milky Way Galaxy? a. A black hole of about 4 million solar masses Chapter 16: A Universe of Galaxies 12/17/15 5:29 PM Hubble Deep Field • Galaxies and Cosmology o A galaxy’s age, its distance, and the age of the universe are all closely related. o The study of galaxies is thus intimately connected with cosmology—the study of the structure and evolution of the universe. o 16.1 Islands of Stars • What are the three major types of galaxies? o o Spiral Galaxy § § § § Barred Spiral Galaxy: Has a bar of stars across the bulge. ú o Elliptical Galaxy: § All spheroidal component; virtually no disk component § Red-yellow color indicates older star population. § o Lenticular Galaxy: § Has a disk like a spiral galaxy but much less dusty gas (intermediate between spiral and elliptical). § o Hubble’s Galaxy Classes § o Irregular Galaxy: § Neither spiral nor elliptical. Blue-white color indicates ongoing star formation. § • How are galaxies grouped together? o Spiral galaxies are often found in groups of galaxies (up to a few dozen galaxies per group). § o Elliptical galaxies are much more common in huge clusters of galaxies (hundreds to thousands of galaxies). § 16.2 Distances of Galaxies • How do we measure the distances to galaxies? o Brightness alone does not provide enough information to measure distance. • Step 1 o Determine size of solar system using radar. • Step 2 o Determine distances of stars out to several hundred light-years using parallax. o Eg. distance to nearest star clusters o o Luminosity passing through each sphere is the same. o Area of sphere: § 4π (radius)2 § Divide luminosity by area to get brightness. o The relationship between apparent brightness and luminosity depends on distance. § § We can determine a star’s distance if we know its luminosity and can measure its apparent brightness. ú Distance = Square root of (Luminosity / 4pi * Brightness o A standard candle is an object whose luminosity we can determine without measuring its distance. • Step 3 o Apparent brightness of star cluster’s main sequence tells us its distance. o This method works to measure distances within our galaxy o Knowing a star cluster’s distance, we can also determine the luminosity of each type of star within it. • Cepheid Variable Stars o Cepheid variable stars are very luminous. o Therefore, they could be very good standard candles, if we calibrate their luminosity somehow… o The light curve of this Cepheid variable star shows that its brightness alternately rises and falls over a 50-day period. o o Cepheid variable stars with longer periods have greater luminosities. o • Step 4 o Because the period of a Cepheid variable star tells us its luminosity, we can use these stars as standard candles. o This works to measure distances to nearby galaxies (up to 100 million light years away) o White dwarf supernovae can also be used as standard candles. o The 1.4 M Sunwhite dwarf mass limit makes them all similar. • Step 5 o Apparent brightness of a white dwarf supernova tells us the distance to its galaxy (up to 10 billion light-years!). o • What is Hubble’s law? o • The Puzzle of “Spiral Nebulae” o Before Hubble, some scientists (eg. Curtis) argued that “spiral nebulae” were entire galaxies like our Milky Way, whereas other scientists (eg. Shapley) maintained they were smaller collections of stars within the Milky Way. o The debate remained unsettled until someone finally measured the distances of spiral nebulae… o Hubble settled the debate in 1924 by measuring the distance to the Andromeda Galaxy using Cepheid variables as standard candles. o Hubble also knew that the spectral features of virtually all galaxies are redshifted ⇒ they’re all moving away from us. § o By measuring distances to nearby galaxies, Hubble found that redshift and distance are related in a special way. o Hubble’s law: velocity = H0× distance § o Redshift (velocity) of a very distant galaxy tells us its distance through Hubble’s law: § distance = (velocity)/ H 0 o Distances of the farthest galaxies are measured using redshifts. o Difficulties with determining distances using Hubble’s law § Galaxies may have some motion besides that from Hubble’s law (problem for short distances where cosmological red-shift effect is small) § Distances are only as good as our value for Hubble’s constant, H 0 • Summary: We measure galaxy distances using a chain of interdependent techniques. o • How do distance measurements tell us the age of the universe? o Hubble's constant tells us the age of the universe because it relates velocities and distances of all galaxies. § Age = (Distance)/(Velocity) about 1 / H 0 o The expansion rate appears to be the same everywhere in space. o Can think of expansion two ways: § Expansion of space itself, carrying galaxies § Galaxies moving apart in static space o A two-dimensional example of something that expands, but has no center or edge, is the surface of a balloon. o • Cosmological Principle o The universe looks about the same no matter where you are within it. o Matter is evenly distributed on very large scales in the universe. o The universe has no center and no edges o Our Galaxy is not the center of expansion o Not proved but consistent with all observations to date o Distances between faraway galaxies change while light travels. o Astronomers think in terms of lookback time rather than distance distance? § o Expansion stretches photon wavelengths, causing a cosmological redshift directly related to lookback time. § 16.3 Galaxy Evolution • How do we study galaxy evolution? o o Deep observations show us very distant galaxies as they were much earlier in time (old light from young galaxies). o Our best models for galaxy formation assume that: § Matter originally filled all of space almost uniformly, with slight density fluctuations. § Gravity of denser regions pulled in surrounding matter. § o Denser regions contracted, forming protogalactic clouds. o H and He gases in these clouds formed the first stars, which were massive and died quickly. § o Supernova explosions from the first stars kept much of the gas from forming stars. o Leftover gas settled into a spinning disk. o Conservation of angular momentum § • But why do some galaxies end up looking so different? o Conditions in Protogalactic Cloud? § Spin: Initial angular momentum of protogalactic cloud could determine the size of the resulting disk. ú § Density: Elliptical galaxies could come from dense protogalactic clouds that were able to cool and form stars before gas settled into a disk. ú § Distant Red Ellipticals ú Observations of some distant red elliptical galaxies support the idea that most of their stars formed very early in the history of the universe. ú o Galaxies are not very far apart compared to their sizes, so we must also consider the effects of collisions. o Collisions were much more likely early in time, because galaxies were closer together due to expansion of universe. § o Many of the galaxies we see at great distances (and early times) do indeed look violently disturbed. o The collisions we observe nearby trigger bursts of star formation. § o Modeling such collisions on a computer shows that two spiral galaxies can merge to make an elliptical. § o Collisions may explain why elliptical galaxies tend to be found where galaxies are closer together. o Giant elliptical galaxies at the centers of clusters seem to have consumed a number of smaller galaxies. § o Starburst galaxies are forming stars so quickly that they will use up all their gas in less than a billion years. o The intensity of supernova explosions in starburst galaxies can drive galactic winds, removing the gas that could form more stars. o The intensity of supernova explosions in starburst galaxies can drive galactic winds. § § X-Ray Image 16.4 Quasars and Other Active Galactic Nuclei • What is the evidence for supermassive black holes at the centers of galaxies? o If the center of a galaxy is unusually bright, we call it an active galactic nucleus. o Quasars are the most luminous examples. § o The highly redshifted spectra of quasars indicate large distances. o From brightness and distance, we find that luminosities of some quasars are >10 L 12 ! Sun o Variability over timescales of hours shows that all this energy comes from a region smaller than the solar system. § o Quasars powerfully radiate energy over a very wide range of wavelengths, indicating that they contain matter with a wide range of temperatures. § o Radio galaxies contain active nuclei shooting out vast jets of plasma, which emit radio waves coming from electrons moving at near light speed. § o An active galactic nucleus can shoot out blobs of plasma moving at nearly the speed of light. § o The lobes of radio galaxies can extend up to a million light- years. § o Radio galaxies don’t appear as quasars because dusty gas clouds block our view of their accretion disks. § • Characteristics of Active Galaxies o Luminosity can be enormous (>10 L 12 Sun). o Luminosity can vary rapidly (comes from a space smaller than solar system). o They emit energy over a wide range of wavelengths (contain matter with wide temperature range). o Some drive jets of plasma at near light speed. • What is the power source for quasars and other active galactic nuclei? o The accretion of gas onto a supermassive black hole appears to be the only way to explain all the properties of quasars. o • Energy from a Black Hole o The gravitational potential energy of matter falling into a black hole turns into kinetic energy. o Friction in the accretion disk turns kinetic energy into thermal energy (heat). o Heat produces thermal radiation (photons). o This process can convert 10–40% of E = mc into 2 radiation. o Jets may come from the twisting of the magnetic field lines in the inner part of the accretion disk. § • Why do we think the growth of central black holes is related to galaxy evolution? o Orbits of stars at center of Milky Way indicate a black hole with mass of 4 million M Sun. § o Orbital speed (800 km/s) and distance (60 light years) of gas orbiting center of M87 indicate a black hole with mass of at least 2-3 billion Sun. § • Black Holes in Galaxies o Many nearby galaxies—perhaps all of them—have supermassive black holes at their centers. o These black holes seem to be dormant active galactic nuclei. o All galaxies may have passed through a quasar-like stage earlier in time. • Galaxies and Black Holes • The mass of a galaxy’s central black hole is closely related to the mass of its spheroidal bulge of stars. o • The development of a central black hole must somehow be related to galaxy evolution. Reading Review Chapter 16 12/17/15 5:29 PM R.R Chapter 16a 1. A standard candle is _________. a. a light source of known luminosity 2. What two observable properties of a Cepheid variable are directly related to one another? a. the period between its peaks of brightness and its luminosity 3. What does Hubble's law tell us? a. The more distant a galaxy, the faster it is moving away from us. 4. If we say that a galaxy has a lookback time of 1 billion years, we mean that _________. a. its light traveled through space for 1 billion years to reach us 5. Current estimates place the age of the universe at about _________. a. 14 billion years R. Review Ch. 16 1. Which of the following is NOT one of the three major categories of galaxies? a. globular galaxies 2. Telescopes designed to study the earliest stages in galactic lives should be optimized for observations in ______. a. infrared light 3. Current understanding holds that a galaxy's type (spiral, elliptical, or irregular) ______. a. may either be the result of conditions in the protogalactic cloud that formed it or the result of later interactions with other galaxies 4. Why should galaxy collisions have been more common in the past than they are today? a. Galaxies were closer together in the past because the universe was smaller. R. Review Ch. 16c 1. Which of the following phenomena is probably NOT related to the presence of a supermassive black hole? a. the presence of globular clusters in the halos of galaxies 2. According to the theory that active galactic nuclei are powered by supermassive black holes, the high luminosity of an active galactic nucleus primarily consists of ______. a. Light emitted by a hot gas in an accretion disk that swirls around a black hole Chapter 17: The Birth of the Universe 12/17/15 5:29 PM 17.1 The Big Bang • What were conditions like in the early universe? o The early universe must have been extremely hot and dense, and it cooled as it expanded over time. § o The early universe was full of particles and radiation at high temperature. o Photons converted into particle–antiparticle pairs and vice versa. 2 § E = mc § o We can understand which particles are produced by doing experiments at high temperatures. § Eg. Proton-proton collisions at the LHC at CERN in France/Switzerland. o o Four known forces in universe: § Strong Force § Electromagnetism § Weak Force § Gravity o • How did the early universe change with time? o 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. o Planck Era -43 § Time: < 10 s § Temp: > 10 32K § We have no theory of quantum gravity to describe this era § All four forces may have been unified as one o GUT Era § Time: 10 -4–10 -38s § Temp: 10 –10 29K § GUT era began when gravity became distinct from other forces. § GUT era ended when strong force became distinct from electroweak force. § Note: GUT = grand unified theory o Electroweak Era § Time: 10 -3–10 -10s 29 15 § Temp: 10 –10 K § Began when strong force became distinct from electroweak force. § Ended when electroweak force separated into the weak force and the electromagnetic force o Particle Era § Time: 10 -1–0.001 s 15 12 § Temp: 10 –10 K § Amounts of matter and antimatter were nearly equal. 9 § Roughly one extra proton for every 10 proton– antiproton pairs! o Era of Nucleosynthesis § Time: 0.001 s–5 min § Temp: 10 –10 K 9 § Began when matter annihilated remaining antimatter, leaving small excess of matter § Ended when Helium finished fusing from protons and neutrons o Era of Nuclei § Time: 5 min–380,000 yrs § Temp: 10 –3000 K § Plasma of nuclei, electrons, and photons for next 380,000 years. § Ended when electrons combined with nuclei o Era of Atoms § Time: 380,000 years– 1 billion years § Temp: 3000–20 K § Began when atoms formed (electrons became bound to nuclei) and universe became transparent to photons. o Era of Galaxies § Time: ~1 billion years–present § Temp: 20–3 K § The first stars and galaxies formed by ~1 billion years after the Big Bang. 17.2 Evidence for the Big Bang • Primary Evidence for the Big Bang o We have detected the leftover radiation from the Big Bang. o The Big Bang theory correctly predicts the abundance of helium and other light elements in the universe. • How do observations of the cosmic microwave background support the Big Bang theory? o The cosmic microwave background— the radiation left over from the Big Bang— was detected by Penzias and Wilson in 1965. o Background radiation from the Big Bang has been freely streaming across the universe since atoms formed at temperature ~3000 K: visible/IR. § o Expansion of the universe has redshifted thermal radiation from that time to ~1000 times longer wavelength: microwaves. § o But, temperature is slightly (one part in 100,000) different depending on which direction we look. o This tells us that there were regions with slightly higher density in the early universe • How do the abundances of elements support the Big Bang theory? o o Protons and neutrons combined to make long-lasting helium nuclei by the time the universe was ~5 minutes old. o o Big Bang theory prediction: 75% H, 25% He (by mass) o Matches observations! o 17.3 The Big Bang and Inflation • Mysteries Needing Explanation o Where does structure come from? § Inflation can make structure by stretching tiny quantum ripples to enormous sizes. § These ripples in density then become the seeds for all structure in the universe. o Why is the overall distribution of matter so uniform? § How can microwave temperature be nearly identical on opposite sides of the sky if those regions have never been in contact? § § Regions now on opposite sides of the sky were close together before inflation pushed them far apart. § o Why is the density of the universe so close to the critical density? § The overall geometry of the universe is closely related to total density of matter and energy. § ú Density = Critical ú Density > Critical ú Density < Critical § The inflation of the universe flattens the overall geometry like the inflation of a balloon, causing overall density of matter plus energy to be very close to critical density. ú • MIT Professor o Where does structure come from? o Why is the overall distribution of matter so uniform? o Why is the density of the universe so close to the critical density? o An early episode of rapid inflation at the end of the GUT era (10 -38s into the Big Bang) can solve all three mysteries! • Did inflation really occur? o Patterns of structure observed by Planck show us the “seeds” of the universe. o Observed patterns of structure in the cosmic microwave background agree (so far) with the “seeds” that inflation would produce. o 17.4 Observing the Big Bang for Yourself • Why is the darkness of the night sky evidence for the Big Bang? o Olbers’ Paradox § If the Universe were: ú infinite ú unchanging ú everywhere the same § then stars would cover the night sky… § but they don’t o The night sky is dark because the universe changes with time. o As we look out in space, we can look back to a time when there were no stars. o The night sky is dark because the universe changes with time. o As we look out in space, we can look back to a time when there were no stars. Reading Review Chapter 17 12/17/15 5:29 PM R. Review Ch. 17a 1. What happens when a particle of matter meets its corresponding antiparticle of antimatter? a. The combined mass of the two particles is completely transformed into energy (photons). 2. What happens to gas as it freely expands? a. It gets less dense and cools 3. A "GUT" (grand unified theory) refers to theories that _________. a. unify the strong force with the electromagnetic and weak forces 4. What do we mean by inflation? a. a sudden and extremely rapid expansion of the universe that occurred in a tiny fraction of a second during the universe's first second of existence 5. Which statement about the cosmic microwave background is NOT true? a. It is the result of a mixture of radiation from many independent sources, such as stars and galaxies. 6. In the past, the temperature of the universe was _____. a. Hotter than it is today 7. The Big Bang theory is supported by two major lines of evidence that alternative models have not successfully explained. What are they? a. (1) the existence and specific characteristics of the observed cosmic microwave background; (2) the observed overall chemical composition of the universe. R. Review Ch. 17b 1. Which of the following observations cannot be explained by the Big Bang theory unless we assume that an episode of inflation occurred? a. the fact that the temperature of the cosmic microwave background is almost the same everywhere 2. Which of the following statements can NOT be tested by science today? a. Prior to the Planck time, our universe sprouted from another universe. 3. Which of the following statements explains why the night sky is dark? a. The universe has a finite age Dark Matter, Dark Energy, and the Fate of the Universe 12/17/15 5:29 PM 18.1 Unseen Influences in the Cosmos • What do we mean by dark matter and dark energy? o Unseen Influences § Dark matter: An undetected form of mass that emits little or no light but whose existence we infer from its gravitational influence on stars and gas § 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 • Contents of Universe o Normal matter: ~ 5% § Normal matter inside stars: ~ 0.5% § Normal matter outside stars: ~ 4.5% o Dark matter: ~ 27% o Dark energy: ~ 68% o 18.2 Evidence for Dark Matter • What is the evidence for dark matter in galaxies? o Introduction to rotation curves § § Rotation curve: A plot of orbital speed versus orbital radius o Rotation curve for our Solar System § We measure the mass of the solar system using the orbits of planets. § Orbital speed depends on force of gravity. § Gravity depends on distance and mass. § Solar system’s rotation curve declines with distance because Sun has almost all the mass. § Mass encircled by orbits does not change with distance from Sun. § o Rotation curve for the Milky Way § The rotation curve of the Milky Way stays flat with distance. § Mass must be more spread out than in the solar system. § 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! § o Mass within Sun’s orbit: 1.0 × 10 M 11 Sun 12 § Total mass: ~10 M Sun § o The visible portion of the Milky Way lies deep in the heart of a large halo of dark matter. § o We can measure orbital velocities in other spiral galaxies using the Doppler shift of the 21-cm line of atomic H. § o Spiral galaxies all tend to have orbital velocities that remain constant at large radii, indicating large amounts of dark matter. § o The Doppler broadening of spectral lines in elliptical galaxies tells us how fast the stars are orbiting, indicating more mass than we can see. § These galaxies also have large amounts of dark matter. § • What is the evidence for dark matter in clusters of galaxies? o We can measure the velocities of galaxies in a cluster from their Doppler shifts. § The mass we find from galaxy motions in a cluster is about 40 times larger than the mass in stars! o Galaxy clusters contain large amounts of X ray–emitting hot gas in gravitational equilibrium. § The temperature of hot gas (particle motions) tells us cluster mass: ú 85% dark matter ú 13% hot gas ú 2% stars o Gravitational lensing, the bending of light rays by gravity, can also tell us a cluster’s mass. § A gravitational lens distorts our view of things behind it. o All three methods of measuring cluster mass indicate similar amounts of dark matter. • Does dark matter really exist? • Our Options o Dark matter really exists, and we are observing the effects of its gravitational attraction. o Something is wrong with our understanding of gravity, causing us to mistakenly infer the existence of dark matter. o Because gravity is so well tested, most astronomers prefer option #1. • The Bullet Cluster, the collision of two smaller clusters, provides strong evidence for the existence of dark matter. Here the blue represents the bulk of the cluster mass, while the pink represents the gas (visible matter.) o • What might dark matter be made of? o Generally, dark matter is matter that we know exists from its gravitational influence, but is not as bright as a star, so we can’t see it o So, what could it actually be made of? o Two Basic Options § Ordinary “baryonic” matter ú Massive Compact Halo Objects (MACHOs) : dead or failed stars in halos of galaxies § Exotic “non-baryonic” particles ú Weakly Interacting Massive Particles (WIMPs) : mysterious neutrino-like particles that only interact through gravity and the weak force ú Best Bet o Compact starlike objects occasionally make other stars appear brighter through lensing… § o Compact starlike objects (MACHOs) occasionally make other stars appear brighter through lensing…but there are not enough lensing events from MACHOs to explain all the dark matter. o Also, the observed abundance of deuterium left over from the Big Bang is consistent with a density of ordinary matter that is 5% of the critical density § o 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. o How might we detect WIMPs? § 1) Deep underground detectors look for WIMPs bouncing off their atoms as the Sun orbits through the Galaxy § 2) Colliding protons at very high energies may produce WIMPs whose presence can be inferred 18.3 Structure Formation • What is the role of dark matter in galaxy formation? o Gravity of dark matter is what caused protogalactic clouds to form and contract early in time. o WIMPs can’t contract to the center because they don’t radiate away their orbital energy. o Dark matter is still pulling things together. o After correcting for Hubble’s law, we can see that galaxies are flowing toward the densest regions of space. • What are the largest structures in the universe? o Maps of galaxy positions reveal extremely large structures: superclusters and voids. § o Models show that the gravity of dark matter pulls mass into denser regions—the universe grows lumpier with time. § o Structures in galaxy maps look very similar to the ones found in models in which dark matter is WIMPs. 18.4 The Fate of the Universe • Einstein’s biggest blunder? o Einstein created the “cosmological constant” a repulsive force to prevent the Universe from collapsing on itself. o After Hubble showed the Universe is expanding, Einstein called it “the greatest blunder” of his career. o Was he right all along? • What is the evidence for an accelerating expansion? o Does the universe have enough kinetic energy to escape its own gravitational pull? o Fate of universe depends on the amount of dark matter. o Critical density is the density of matter needed to halt expansion via gravity in the absence of a repulsive force o Amount of matter is ~30% of the critical density, suggesting fate is eternal expansion. § § Not Enough Dark Matter o But expansion appears to be speeding up! § § Dark Energy? o Estimated age of universe depends on both dark matter and dark energy. § § Left to Right: Old to Oldest o The brightness of distant white dwarf supernovae combined with the cosmological redshift of their host galaxies tells us how much the universe has expanded since they exploded. o An accelerating universe is the best fit to supernova and redshift data. § • Why is flat geometry evidence for dark energy? o Observations of the cosmic microwave background tell us that the density of the universe is equal to the critical density, but we can only account for 30% of that with matter! o Age of the universe § Models to explain temperature variations in cosmic microwave background predict age of Universe to be 13.8 billion years § Agrees with simple estimates using inverse of Hubble’s constant § Agrees with the fact that the oldest stars we in the Universe are 13 billion years old • What is the fate of the universe? o We expect that the universe will continue expanding forever, as dark energy continues to accelerate the expansion! Reading Review Chapter 18 12/17/15 5:29 PM R. Review Ch. 18a 1. Which of the following best summarizes what we mean by dark matter? a. matter that we have identified from its gravitational effects but that we cannot see in any wavelength of light 2. What is a rotation curve? a. a graph showing how orbital velocity depends on distance from the center for a spiral galaxy 3. What do we mean when we say that the rotation curve for a spiral galaxy is "flat"? a. Gas clouds orbiting far from the galactic center have approximately the same orbital speed as gas clouds located further inward. 4. Which of the following is NOT one of the three main strategies used to measure the mass of galaxy clusters? a. measuring the temperatures of stars in the halos of the galaxies 5. Which of the following statements best summarizes current evidence concerning dark matter in individual galaxies and in clusters of galaxies? a. It bends or distorts the light coming from galaxies located behind it. b. Dark Matter is the dominant form of mass in both clusters and in individual galaxies 6. What do we mean when we say that particles such as neutrinos or WIMPs are weakly interacting? a. They respond to the weak force but not to the electromagnetic force, which means they cannot emit light. 7. Measuring the amount of deuterium in the universe allows us to set a limit on _________. a. the density of ordinary (baryonic) matter the universe 8. Which of the following best sums up current scientific thinking about the nature of dark matter? • Most dark matter probably consists of weakly interacting particles of a type that we have not yet identified. 9. The critical density of the universe is the _______. a. average density the universe would need for gravity to someday halt the current expansion if dark energy did not exist R. Review Ch. 18b What is the primary form of evidence that has led astronomers to conclude that the expansion of the universe is accelerating? • observations of white dwarf supernovae Which of the following best sums up current scientific thinking about the nature of dark energy? • It is a name given to whatever is causing the expansion of the universe to accelerate with time. • Dark energy probably exists, but we have little (if any) idea what it is.
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