EARTH SCIENCE geology, the environment, and the universe 2008 (31)

Spiral galaxy BIG Idea Observations of galaxy expansion, cosmic background radiation, and the Big Bang theory describe an expanding universe that is 13.7 billion years old 30.1 The Milky Way Galaxy MAIN Idea Stars with varying light output allowed astronomers to map the Milky Way, which has a halo, spiral arms, and a massive galactic black hole at its center Merging galaxies 30.2 Other Galaxies in the Universe MAIN Idea Finding galaxies with different shapes reveals the past, present, and future of the universe 30.3 Cosmology MAIN Idea The Big Bang theory was formulated by comparing evidence and models to describe the beginning of the universe GeoFacts • Distance measurements in the universe are usually expressed in light-years, measured by how far light travels in one year The nearest galaxy like ours is nearly million light-years away • Each galaxy contains billions of stars, and there are billions of galaxies • Galaxies come in a variety of shapes, and most are either spiral or elliptical 860 Elliptical galaxy (t)NASA/ESA/S Beckwith (STScI)/The Hubble Heritage Team (STScI/AURA), (c)NASA/Holland Ford (JHU)/ACS Science Team/ESA, (b)NASA/ESA/The Hubble Heritage Team (STScI/AURA), (bkgd)NOAO/AURA/NSF/Photo Researchers Galaxies and the Universe Start-Up Activities Types of Galaxies Make this Foldable to show how galaxies are classified LAUNCH Lab How big is the Milky Way? Our solar system seems large when compared to the size of Earth However, the Milky Way dwarfs the size of our solar system Procedure Read and complete the lab safety form The Milky Way has a diameter of approximately 8.25 × 109 AU What is the diameter of the Milky Way in light-years? (206,265 AU = 3.26 ly) Given that the Kuiper belt has a diameter of 50 AU, what is the diameter of the Kuiper belt in ly? If you were to apply the scale mm = ly, how large would the Milky Way be? The Sun is located 28,000 ly from the center of the Milky Way Based on the scale that you used in Question 4, what would be the distance, in millimeters, from the center of the Milky Way to the Sun? If you included the Kuiper belt in your model, how many millimeters across would its orbit be? Analysis Observe In your science journal, describe what your model of the Milky Way would look like if you actually built it Explain why it would be a problem to show the size of our solar system in comparison to the Milky Way Explain how you would change your model to include the size of Earth Fold the top of a horizontal sheet of paper down about cm STEP STEP Fold the sheet into thirds Unfold and draw lines along all fold lines STEP Label the columns Spiral, Elliptical, and Irregular Spiral Elliptical Irregular FOLDABLES Use this Foldable with Section 30.2 As you read this section, record information about each type of galaxy, including sketches when appropriate Visit glencoe.com to study entire chapters online; explore • Interactive Time Lines • Interactive Figures • Interactive Tables animations: access Web Links for more information, projects, and activities; review content with the Interactive Tutor and take Self-Check Quizzes Chapter Section 301••Galaxies XXXXXXXXXXXXXXXXXX and the Universe 861 Section 0.1 Objectives ◗ Determine the size and shape of our galaxy ◗ Distinguish the different kinds of variable stars ◗ Identify the different kinds of stars in a galaxy and their locations Review Vocabulary galaxy: any of the very large groups of stars and associated matter found throughout the universe New Vocabulary variable star RR Lyrae variable Cepheid variable halo Population I star Population II star spiral density wave MAIN Idea Stars with varying light output allowed astronomers to map the Milky Way, which has a halo, spiral arms, and a massive black hole at its center Real-World Reading Link From inside your home, you have only a few ways to find out what is going on outside You can look out a window or door, use a telephone or a computer, or bring in news and entertainment on a radio or TV Similarly, scientists also have a few ways to learn about the stars in the galaxy around us Discovering the Milky Way When looking at the Milky Way galaxy, it is difficult to see its size and shape because not only is the observer too close, but he or she is also inside the galaxy Observing the band of stars stretching across the sky, you are looking at the edge of a disk from the inside of the disk However, it is difficult to tell how big the galaxy is, where its center is, or what Earth’s location is within this vast expanse of stars Though astronomers have answers to these questions, they are still refining their measurements ■ Figure 30.1 The diameters of variable stars change over a period of to 100 days, causing them to brighten and dim Variable star dim 862 Chapter 30 • Galaxies and the Universe (l)NASA, (r)NASA The Milky Way Galaxy Variable stars In the 1920s, astronomers focused their attention on mapping out the locations of globular clusters of stars These huge, spherical star clusters are located above or below the plane of the galactic disk Astronomers estimated the distances to the clusters by identifying variable stars in them Variable stars are located in the giant branch of the Hertzsprung-Russell diagram, discussed in Chapter 29, and pulsate in brightness because of the expansion and contraction of their outer layers Variable stars are brightest at their largest diameters and and dimmest at their smallest diameters Figure 30.1 shows the dim and bright extremes of a variable star Variable star bright Types of variables For certain types of variable stars, there is a relationship between a star’s luminosity and its pulsation period, which is the time between its brightest pulses The longer the period of pulsation takes, the greater the luminosity of the star RR Lyrae variables are stars that have periods of pulsation between 1.5 hours and day, and on average, they have the same luminosity Cepheid variables, however, have pulsation periods between and 100 days, and the luminosity as much as doubles from dimmest to brightest By measuring the star’s period of pulsation, astronomers can determine the star’s absolute luminosity This, in turn, allows them to compare the star’s luminosity (energy) to its apparent magnitude (brightness) and calculate how far away the star must be to appear this dim or bright ■ Figure 30.2 The top two images are views of the Milky Way—one toward the outer galaxy and one close to the center The third figure is an artist’s concept of what the Milky Way galaxy looks like from space Along the disk toward space The galactic center After reasoning there were globular clusters orbiting the center of the Milky Way, astronomers then used RR Lyrae variables to determine the distances to them They discovered that these clusters are located far from our solar system, and that their distribution in space is centered on a distant point 28,000 light-years (ly) away The galactic center is a region of high star density, shown in Figure 30.2, much of which is obscured by interstellar gas and dust The direction of the galactic center is toward the constellation Sagittarius The other view of the Milky Way that is shown is along the disk into space View toward the galactic center Reading Check Describe how astronomers located the galactic center of the Milky Way 100,000 ly The Shape of the Milky Way Only by mapping the galaxy with radio waves have astronomers been able to determine its shape This is because radio waves are long enough that they can penetrate the interstellar gas and dust without being scattered or absorbed By measuring radio waves as well as infrared radiation, astronomers have discovered that the galactic center is surrounded by a nuclear bulge, which sticks out of the galactic disk much like the yolk in a fried egg Around the nuclear bulge and disk is the halo, a spherical region where globular clusters are located, as illustrated in Figure 30.2 28,000 ly Nuclear bulge Disk Sun Globular clusters Halo The Milky Way galaxy Section • The Milky Way Galaxy 863 (t)Jerry Schad/Photo Researchers, (b)Ronald Royer/Science Photo Library/Photo Researchers Centaurus Sagittarius Rotation Orion Cygnus Sun Perseus Figure 30.3 The Sun is located on the minor Orion spiral arm and follows an orbital path around the nuclear center as shown (Note: Drawing is not to scale.) Infer how the arms were named ■ Spiral arms Knowing that the Milky Way galaxy has a disklike shape with a central bulge, astronomers speculated that it might also have spiral arms, as many other galaxies This was difficult to prove Because of the distance, astronomers have no way to get outside of the galaxy and look down on the disk Astronomers decided to use hydrogen atoms to look for the spiral arms To locate the spiral arms, hydrogen emission spectra are helpful for three reasons First, hydrogen is the most abundant element in space; second, the interstellar gas, composed mostly of hydrogen, is concentrated in the spiral arms; and third, the 21-cm wavelength of hydrogen emission can penetrate the interstellar gas and dust and be detected all the way across the galactic disk Using the hydrogen emission as a guide, astronomers have identified four major spiral arms and numerous minor arms in the Milky Way Using these data, scientists discovered that the Sun is located in the minor Orion arm at a distance of about 28,000 ly from the galactic center The Sun’s orbital speed is about 220 km/s, and thus its orbital period is about 240 million years In its 5-billion-year life, the Sun has orbited the galaxy approximately 20 times Figure 30.3 shows the orbit that the Sun follows in a spinning galaxy Reading Check Explain how astronomers used the Milky Way’s hydrogen emission spectrum to locate the arms ■ Figure 30.4 A barred galaxy has an elongated central bulge 864 Chapter 30 • Galaxies and the Universe NOAO/Photo Researchers Nuclear bulge or bar? Many spiral galaxies have a barlike shape rather than having a round disk to which the arms are attached Recent radio observation of interstellar gas indicates that the Milky Way has a slightly elongated shape Astronomers theorize that the gas density in the halo determines whether a bar will form Figure 30.4 shows a barred galaxy Using a variety of wavelengths, astronomers are discovering what the center of the Milky Way looks like The nuclear bulge of a galaxy is typically made up of older, red stars The bar in a galaxy center, however, is associated with younger stars and a disk that forms from neutral hydrogen gas Star formation does continue to occur in the bulge, and most stars are about 1000 AU apart compared to 207,000 AU separation in the locale of the Sun Recent measurements of 30 million stars in the Milky Way indicate a bar about 27,000 ly in length Mass of the Milky Way The mass located within the circle of the Sun’s orbit through the galaxy, outlined in Figure 30.3, is about 100 billion times the mass of the Sun Using this figure, astronomers have concluded that the galaxy contains about 100 billion stars within its disk Mass of the halo Evidence of the movement of outer disk stars and gas suggests that as much as 90 percent of the galaxy’s mass is contained in the halo Some of this unseen matter is probably in the form of dim stellar remnants such as white dwarfs, neutron stars, or black holes, but the nature of the remainder of this mass is unknown As you will read in Section 30.2, the nature of unseen matter extends to other galaxies and to the universe as a whole Figure 30.5 shows the halo of the Sombrero galaxy A galactic black hole Weighing in at a few million to a few billion times the mass of the Sun, supermassive black holes occupy the centers of most galaxies When the center of the galaxy is observed at infrared and radio wavelengths, several dense star clusters and supernova remnants stand out Among them is a complex source called Sagittarius A (Sgr A), with sub-source called Sgr* (Sagittarius star), which appears to be an actual point around which the whole galaxy rotates Careful studies of the motions of the stars that orbit close to Sagittarius A* (pronounced A star) indicate that this region has about 2.6 million times the mass of the Sun but is smaller than our solar system Data gathered by the Chandra X-Ray Observatory reveal intense X-ray emissions Astronomers think that Sagittarius A* is a supermassive black hole that glows brightly because of the hot gas surrounding it and spiraling into it This black hole probably formed early in the history of the galaxy, at the time when the galaxy’s disk was forming Gas clouds and stars within the disk probably collided and merged to form a single, massive object that collapsed to form a black hole Figure 30.6 illustrates how a supermassive black hole develops This kind of black hole should not be confused with the much smaller, stellar black hole, which is usually made from the collapsing core of a massive star ■ Figure 30.5 The galaxy halo is populated by older, dimmer stars, while the central bulge is populated by newer, brighter stars, as shown in this view of the Sombrero galaxy ■ Figure 30.6 The formation of a supermassive black hole begins with the collapse of a dense gas cloud The accumulation of mass releases photons of many wavelengths, and perhaps even a jet of matter, as shown here Section • The Milky Way Galaxy 865 NOAO/SPL/Photo Researchers Figure 30.7 Globular clusters and the nuclear bulge contain old stars poor in heavy elements The disk contains young stars that have a higher heavy element content (Note: Drawing is not to scale.) ■ Nuclear bulge (Population II) Disk (Population I) Halo (Population II) Globular clusters (Population II) Stellar populations in the Milky Way Even though the basic compositions of all stars are the same, there are several distinct differences in detail The differences among stars include differences in location, motion, and age, leading to the notion of stellar populations The population of a star provides information about its galactic history In fact, the galaxy could be divided into two components: the round part made up of the halo and bulge noted in Figure 30.7, where the stars are old and contain only traces of heavy elements; and the disk, especially the spiral arms To astronomers, heavy elements are any elements with a mass larger than helium Astronomers divide stars in these two regions into two classes Population I stars are in the disk and arms and have small amounts of heavy elements Population II stars are found in the halo and bulge and contain even smaller traces of heavy elements Refer to Table 30.1 for more details Population I Most of the young stars in the galaxy are located in the spiral arms of the disk, where the interstellar gas and dust are concentrated Most star formation takes place in the arms Population I stars tend to follow circular orbits with low (flat) eccentricity, and their orbits lie close to the plane of the disk Finally, Population I stars have normal compositions, meaning that approximately percent of their mass is made up of elements heavier than helium The Sun is a Population I star Table 30.1 Population I stars Population II stars 866 Population I and II Stars of the Milky Way Interactive Table To explore more about Population I and II stars, visit glencoe.com Location in Galaxy Percent of H & He Percent Heavy Elements Age (years) disk arms and open clusters 98 2.0 15 billion old mainsequence stars (type K and M) elliptical and spiral halos and bulges HD 92531 and most white dwarfs Chapter 30 • Galaxies and the Universe Type of Star Type of Galaxy Example Population II There are few stars and little interstellar material currently forming in the halo or the nuclear bulge of the galaxy, and this is one of the distinguishing features of Population II stars Age is another The halo of the Milky Way contains the oldest known objects in the galaxy—globular clusters These clusters are estimated to be 12 to 14 billion years old Stars in the globular clusters have extremely small amounts of elements that are heavier than hydrogen and helium All stars contain small amounts of these heavy elements, but in globular clusters, the amounts are mere traces Stars like the Sun are composed of about 98 percent hydrogen and helium, whereas in globular cluster stars, this composition can be as high as 99.9 percent This indicates their extreme age The nuclear bulge of the galaxy also contains stars with compositions like those of globular cluster stars Table 30.1 points out some other comparisons of Population I and II stars Formation and Evolution of the Milky Way The fact that the halo and nuclear bulge are made exclusively of old stars suggests that these parts of the galaxy formed first, before the disk that contains only younger stars Astronomers therefore hypothesize that the galaxy began as a spherical cloud in space The first stars formed while this cloud was round This explains why the halo, which contains the oldest stars, is spherical The nuclear bulge, which is also round, represents the inner portion of the original cloud The cloud eventually collapsed under the force of its own gravity, and rotation forced it into a disklike shape Stars that formed after this time have orbits lying in the plane of the disk They also contain greater quantities of heavy elements because they formed from gas that had been enriched by previous generations of massive stars In Figure 30.8, the nuclear bulge makes up the hat of the Sombrero galaxy ■ Figure 30.8 Easily seen through small telescopes, the Sombrero galaxy gets its name from the bright glow of the nuclear bulge and the dust and gas lanes along the outer edge of its disk Predict which type of stars would be found in the nuclear bulge Section • The Milky Way Galaxy 867 European Southern Observatory/Photo Researchers Spiral Arms Knot of traffic Figure 30.9 A slow truck on a highway causing a build up of cars around it illustrates one theory as to how spiral density waves maintain spiral arms in a galaxy ■ Section 0.1 Most of the main features of the galaxy are understood by astronomers, except for the way in which the spiral arms are retained The Milky Way is subject to gravitational tugs by neighboring galaxies and is periodically disturbed by supernova explosions from within, both of which can create or affect spiral arms There are several hypotheses about why galaxies keep this spiral shape One hypothesis is that a kind of wave called a spiral density wave is responsible A spiral density wave has spiral regions of alternating density, which rotate as a rigid pattern As the wave moves through gas and dust, it causes a temporary buildup of material, like a slow truck on the highway causes a buildup of cars, shown in Figure 30.9 A second hypothesis is that the spiral arms are not permanent structures but instead are continually forming as a result of disturbances such as supernova explosions The Milky Way has a broken spiral-arm pattern, which most astronomers think fits this second model best However, some galaxies have a prominent two-armed pattern, that was more likely created by density waves A third possibility is considered for faraway galaxies It suggests that the arms are only visible because they contain hot, blue stars that stand out more brightly than dimmer, redder stars When viewed in UV wavelengths, the arms stand out, but when viewed in infrared wavelengths, they seem to disappear Assessment Section Summary Understand Main Ideas ◗ The discovery of variable stars aided in determining the shape of the Milky Way ◗ RR Lyrae and Cepheid are two types of variable stars used to measure distances Analyze How are Population I stars and Population II stars different? MAIN Idea Explain How did astronomers determine where Earth is located within the Milky Way? Determine What measurements of the mass of the Milky Way indicate? Summarize How can variable stars be used to determine the distance to globular clusters? ◗ The nuclear bulge and halo of the Milky Way is a globular cluster of old stars Think Critically ◗ The spiral arms of the Milky Way are made of younger stars and gaseous nebulae Hypothesize What would happen to the stellar orbits near the center of the Milky Way galaxy if there were no black hole? ◗ Population I stars are found in the spiral arms, while Population II stars are in the central bulge and halo 868 Chapter 30 • Galaxies and the Universe Explain If our solar system were slightly above the disk of the Milky Way, why would astronomers still have difficulty determining the shape of the galaxy? Earth Science Write a description of riding a spaceship from above the Milky Way galaxy into its center Point out all of the galaxy’s parts and star types Self-Check Quiz glencoe.com Section 0.2 Objectives ◗ Describe how astronomers classify galaxies ◗ Identify how galaxies are organized into clusters and superclusters ◗ Describe the expansion of the universe Other Galaxies in the Universe MAIN Idea Finding galaxies with different shapes reveals the past, present, and future of the universe Real-World Reading Link Have you ever read an old newspaper to find out Review Vocabulary what life was like in the past? Astronomers observe distant, older galaxies to get an idea of what the universe was like long ago elliptical: relating to or shaped like an ellipse or oval Discovering Other Galaxies Long before they knew what galaxies were, astronomers observed many objects scattered throughout the sky Some astronomers hypothesized that these objects were nebulae or star clusters within the Milky Way Others hypothesized that they were distant galaxies that were as large as the Milky Way The question of what these objects were was answered by Edwin Hubble in 1924, when he discovered Cepheid variable stars in the Great Nebula in the Andromeda constellation Using these stars to measure the distance to the nebula, Hubble showed that they were too far away to be located in our own galaxy The Andromeda nebula then became known as the Andromeda galaxy, shown in Figure 30.10 New Vocabulary dark matter supercluster Hubble constant radio galaxy active galactic nucleus quasar FOLDABLES Incorporate information from this section into your Foldable Properties of galaxies Masses of galaxies range from the dwarf ellipticals, which have masses of approximately million times the mass of the Sun; to large spirals, such as the Milky Way, with masses of around 100 billion times the mass of the Sun; to the largest galaxies, called giant ellipticals, which have masses as high as trillion times that of the Sun Measurements of the masses of many galaxies indicate that they have extensive halos containing more mass than is visible, just as the Milky Way does Figure 30.10 shows a large spiral and several elliptical and dwarf galaxies ■ Figure 30.10 Andromeda is a spiral galaxy like the Milky Way The bright elliptical object and the sphere-shaped object near the center are small galaxies orbiting the Andromeda galaxy Section • Other Galaxies in the Universe 869 John Chumack/Photo Researchers Active Galaxies Radio-telescope surveys of the sky have revealed a number of galaxies that are extremely luminous These galaxies, called radio galaxies, are often giant elliptical galaxies that emit as much or more energy in radio wavelengths than they in wavelengths of visible light Radio galaxies have many unusual properties The radio emission usually comes from two huge lobes of very hot gas located on opposite sides of the visible galaxy These lobes are linked to the galaxy by jets of very hot gas The type of emission that comes from these regions indicates that the gas is ionized, and that electrons in the gas jets are traveling nearly at the speed of light Many radio galaxies have jets that can be observed only at radio wavelengths One of the brightest of the radio galaxies, a giant elliptical called M87, shown in Figure 30.17, also has a jet of gas that emits visible light extending from the galactic center out toward one of the radio-emitting lobes In some unusual galaxies, some sort of highly energetic object or activity exists in the core This object or activity emits as much or more energy than the rest of the galaxy The output of this energy often varies over time, sometimes as little as a few days The cores of galaxies where these highly energetic objects or activities are located are called active galactic nuclei (AGN) Figure 30.17 In addition to radio lobes, M87 has a jet that emits visible light ■ Reading Check Describe the unusual properties of a radio galaxy Quasars In the 1960s, astronomers discovered another new type of object These objects looked like ordinary stars, but some emitted strong radio waves Most stars not The spectra of these new objects were completely different from the spectra of normal stars Whereas most stars have spectra with absorption lines, these new objects had mostly emission lines in their spectra These starlike objects with emission lines in their spectra were called quasars Two quasars are shown in Figure 30.18 At first, astronomers could not identify the emission lines in the spectra of quasars Finally, they realized that the emission lines were spectral lines of common elements, such as hydrogen, shifted far toward longer wavelengths Soon, astronomers also discovered that many quasars vary in brightness over a period of a few days Once astronomers had identified the large spectral-line shifts of quasars, they wondered whether they could have redshifts caused by the expansion of the universe Figure 30.18 Quasars are old and distant celestial objects that emit several thousand times more energy than does our entire galaxy Recall What other objects emit jets of matter? ■ Section • Other Galaxies in the Universe 875 (t)AFP/CORBIS, (b)Atlas Photo Bank/Photo Researchers ■ Figure 30.19 An interstellar gas cloud (A) collapses gravitationally (B) on its way to forming a galaxy The nucleus (C) forms a black hole as the gas there is compressed Magnetic fields of the rapidly rotating disk surrounding the black hole form two highly energetic jets (D) that are perpendicular to the disk’s equatorial plane A B C D Careers In Earth Science Computer Programmer Many astronomers use equipment that does not observe light A computer programmer writes programs astronomers can use to observe spectra, calculate, and decipher the data collected by telescopes To learn more about Earth science careers, visit glencoe.com 876 Chapter 30 • Galaxies and the Universe Quasar redshift The redshift of quasars was much larger than any that had been observed in galaxies up to that time, which would mean that the quasars were much farther away than any known galaxy At first, some astronomers doubted that quasars were far away, but in the decades since quasars were discovered, more evidence supports the hypothesis that quasars are distant One piece of supporting evidence indicates that those quasars associated with clusters of galaxies have the same redshift, verifying that they are the same distance away Another more important discovery is that most quasars are nuclei of very dim galaxies, shown in Figure 30.19 The quasars appear to be extra-bright active galactic nuclei — so much brighter than their surrounding galaxies that astronomers could not initially see those galaxies Reading Check Explain how astronomers determined distances to quasars Looking back in time Because quasars are distant, it takes their light a long time to reach Earth Therefore, observing a quasar is seeing it as it was a long time ago For example, it takes light from the Sun approximately minutes to reach Earth When you observe the Sun, you are seeing it as it was minutes earlier When you observe the Andromeda galaxy, you see the way it looked million years earlier The most remote quasars are several billion light-years away, which indicates the stage you see is from billions of years ago If quasars are extra-bright galactic nuclei, then the many distant ones are nuclei of galaxies as they existed when the universe was young This suggests that many galaxies went through a quasar stage when they were young Consequently, today’s active galactic nuclei might be former quasars that are not as energetic as they were long ago Looking far back into time, the early universe had many quasars Current theory suggests that they existed around supermassive black holes that pulled gas into the center, where in a violent swirl, friction heated the gas to extreme temperatures resulting in the bright light energy that was first detected Chandra X-Ray Observatory/NASA/Photo Researchers Source of power The AGN and quasars emit far more energy than ordinary galaxies, but they are as small as solar systems This suggests that all of these objects are supermassive black holes Recall that the black hole thought to exist in the core of our own galaxy has a mass of about million Suns The black holes in the cores of AGN and quasars are much more massive — up to hundreds of millions of times the mass of the Sun The beams of charged particles that stream out of the cores of radio galaxies and form jets are probably created by magnetic forces As material falls into a black hole, the magnetic forces push the charged particles out into jets There is evidence that similar beams or jets occur in other types of AGN and in quasars In fact, radiolobed quasars have jets that are essentially related to radio galaxies Figure 30.20 shows a supermassive black hole In modeling a supermassive black hole of this magnitude, the mass of nearly billion Suns would be needed to pull the stars in this galaxy into the center A plasma jet, ejected from the nucleus, extends nearly 5000 lightyears into space Section 0.2 ■ Figure 30.20 A jet of energetic X-ray particles is emitted from the AGN, which probably hides a supermassive black hole The other white areas are probably X-ray-emitting neutron stars or black hole binaries Assessment Section Summary Understand Main Ideas ◗ Galaxies can be elliptical, diskshaped, or irregular ◗ Galaxies range in mass from million Suns to more than a trillion Suns Summarize why astronomers theorize that most of the matter in galaxies and clusters of galaxies is dark matter ◗ Many galaxies seem to be organized in groups called clusters ◗ Quasars are the nuclei of faraway galaxies that are dim and seen as they were long ago, due to their great distances ◗ Hubble’s law helped astronomers discover that the universe is expanding MAIN Idea Explain how astronomers discovered that there are other galaxies beyond the Milky Way Explain why it is difficult for astronomers to accurately measure a value for the Hubble constant, H Once a value is determined, describe how it is used Explain the differences among normal spiral, barred spiral, elliptical, and irregular galaxies Think Critically Deduce how the nighttime sky would look from Earth if the Milky Way were an elliptical galaxy Infer what similarities between AGN and quasars are due to black holes MATH in Earth Science Convert the distance across the Milky Way to Mpc if the diameter of the Milky Way is 100,000 ly What is the distance in Mpc across a supercluster of galaxies whose diameter is 200 million ly? (1 Mpc = 3,260,000 ly) Self-Check Quiz glencoe.com Section • Other Galaxies in the Universe 877 Section 3 Objectives ◗ Distinguish the different models of the universe ◗ Compare and contrast how expansion is relative to each of the models ◗ Explain the importance of the Hubble constant Cosmology MAIN Idea The Big Bang theory was formulated by comparing evidence and models to describe the beginning of the universe Real-World Reading Link Manipulating a magnet and iron filings can help you model Earth’s magnetic field Cosmologists use particle accelerators to help create models of the early universe Review Vocabulary radiation: the process of emitting radiant energy in the form of waves or particles New Vocabulary cosmology Big Bang theory cosmic background radiation ■ Figure 30.21 The universe is either open, flat, or closed, depending on whether gravity or the momentum of expansion dominates Momentum of expansion Force of gravity 878 Chapter 30 • Galaxies and the Universe Big Bang Model The study of the universe—its nature, origin, and evolution—is called cosmology The mathematical basis for cosmology is general relativity, from which equations were derived that describe both the energy and matter content of the universe These equations, combined with observations of density and acceleration, led to the most accurate model so far—the Big Bang model The fact that the universe is expanding implies that it had a beginning The theory that the universe began as a point and has been expanding since is called the Big Bang theory Although the name might seem to imply explosion into space, the theory describes an expansion of space itself while gravity holds matter in check Review the effects of expansion by checking results from the MiniLab in Section 30.2 Outward expansion Similar to a star’s internal fusion pressure opposing the effort of a gravitational force to collapse the star, the universe has two opposing forces In the Big Bang model, the momentum of the outward expansion of the universe is opposed by the inward force of gravity acting on the matter of the universe to slow that expansion, as illustrated in Figure 30.21 What ultimately will happen depends on which of these two forces is stronger When the rate of expansion of the universe is known, it is possible to calculate the time since the expansion started and determine the age of the universe When the distance to a galaxy and the rate at which it is moving away from Earth are known, it is simple to calculate how long ago that galaxy and the Milky Way were together In astronomical terms, if the value of H, the expansion (Hubble) constant, is known, then the age of the universe can be determined Corrections are needed to allow for the fact that the expansion has not been constant—it has slowed since the beginning and is now accelerating Based on the best value for H that has been calculated from Hubble Space Telescope data and the data on the cosmic background radiation, the age of the universe can be pinpointed to 13.7 billion years This fits with what astronomers know about the age of the Milky Way galaxy, which is estimated to be between 12 and 14 billion years old, based on the ages of the oldest star clusters Possible outcomes Based on the Big Bang theory, there are three possible outcomes for the universe, as shown in Figure 30.22 The average density of the universe is an observable quantity with vast implications to the outcome Open universe An open universe is one in which the expansion will never stop This would happen if the density of the universe is insufficient for gravity to ever halt the expansion Open universe Closed universe A closed universe will result if the expansion stops and turns into a contraction That would mean the density is high enough that eventually the gravity caused by the mass will halt the expansion of the universe and pull all of the mass back to the original point of origin Flat universe A flat universe results if the expan- sion slows to a halt in an infinite amount of time, but never contracts This means that while the universe would continue to expand, its expansion would be so slow that it would seem to stop Critical density All three outcomes are based on the premise that the rate of expansion has slowed since the beginning of the universe, but the density of the universe is what is unknown At the critical density, there is a balance, so that the expansion will come to a halt in an infinite amount of time The critical density, about × 10-27 kg/m3, means that, on average, there are only two hydrogen atoms for every cubic meter of space When astronomers attempt to count the galaxies in certain regions of space and divide by the volume, they get an even smaller value So they would conclude that the universe is open, except that the dark matter has not been included But even the best estimates of dark matter density are not enough to conclude that the universe is a closed system Closed universe Flat universe ■ Figure 30.22 There are three possible outcomes for the future of the universe It could continue to expand forever and be open, it could snap back at the end and be a closed system, or it could be flat and just die out like a glowing ember The green squares show the estimated cosmic background radiation necessary for each result See Figure 30.24 Cosmic Background Radiation Scientists hypothesize that if the universe began in a highly compressed state before the Big Bang, it would have been extremely hot Then as the universe expanded, the temperature cooled After about 750,000 years, the universe was filled with electromagnetic radiation in the form of shortwave radio radiation With continued expansion, the wavelengths became longer Today this radiation is in the form of microwaves Section • Cosmology 879 Discovery In 1965, scientists discovered a persistent background noise in their radio antenna, shown in Figure 30.23 This noise was caused by weak radiation, called the cosmic background radiation, that appeared to come from all directions in space and corresponded to an emitting object having a temperature of about 2.735 K (–270°C) This was very close to the temperature predicted by the Big Bang theory, and the radiation was interpreted to be from the beginning of the Big Bang ■ Figure 30.23 The cosmic background radiation was discovered by accident with this radio antenna at Bell Labs in Holmdel, New Jersey VOCABULARY SCIENCE USAGE V COMMON USAGE Cosmic Science usage: of or relating to the universe in contrast to Earth alone Common usage: characterized by greatness of thought or intensity ■ Figure 30.24 Temperature differences of one-millionth of a degree can be noted in the WMAP of cosmic background radiation Write one-millionth of a degree in scientific notation 880 Chapter 30 • Galaxies and the Universe (t)Bettmann/CORBIS, (b)NASA/WMAP Science Team Mapping the radiation Since the discovery of the cosmic background radiation, extensive observations have confirmed that it matches the properties of the predicted leftover radiation from the early, hot phase in the expansion of the universe Earth’s atmosphere blocks much of the radiation, so it is best observed from high-altitude balloons or satellites An orbiting observatory called the Wilkinson Microwave Anisotropy Probe (WMAP), launched by NASA in 2001, mapped the radiation in greater detail, as shown in Figure 30.24 The peak of the radiation it measured has a wavelength of approximately mm; thus, it is microwave radiation in the radio portion of the electromagnetic spectrum Reading Check Identify what discovery helped solidify the Big Bang theory Acceleration of the expansion The data produced by WMAP have provided enough detail to refine cosmological models In particular, astronomers have found small wiggles in the radiation representing the first major structures in the universe This helped to pinpoint the time at which the first galaxies and clusters of galaxies formed and also the age of the universe According to every standard model, the expansion of the universe is slowing down due to gravity However, the debate about the future of the universe based on this model came to a halt with the surprising discovery that the expansion of the universe is now accelerating Astronomers have labeled this acceleration dark energy Although they not know its cause, they can determine the rate of acceleration and estimate the amount of dark energy NASA/Reuters/CORBIS Contents of the Universe All the evidence is now pointing in the same direction, and astronomers can say with a high degree of precision of what the universe is composed Their best clue comes from the radiation left in space from the universe’s beginning The ripples left during the time of cooling of the universe’s beginning radiation set the density at that point of time and dictated how matter and energy would separate This in turn laid the groundwork for future galaxies Figure 30.25 gives one view into the universe Dark matter and energy Cosmologists estimate that the universe is composed of dark matter (21 percent), dark energy (75 percent), and luminous matter If you compare the universe to Earth, dark energy is like the water covering the surface of Earth That would be like saying that 70 percent of Earth is covered with something that is not identified What is unknown today is the nature of the dark matter and dark energy Dark matter is thought to consist of subatomic particles, but of the known particles, none display the right properties to explain or fully define dark matter And although scientists recognize the effects of dark energy, they still not know what it is Section 0.3 ■ Figure 30.25 Astronomers estimate that only percent of the universe is composed of luminous matter Assessment Section Summary Understand Main Ideas ◗ The study of the universe’s origin, nature, and evolution is cosmology ◗ The Big Bang model of the universe came from observations of density and acceleration Describe how the age of the universe can be calculated using the Big Bang model ◗ The critical density and the amount of dark energy of the universe will determine whether the universe is open or closed Explain why the cosmic background radiation was an important discovery ◗ Cosmic background radiation gives support to the Big Bang theory of the universe ◗ Mapping the cosmic background radiation has indicated the existence of dark matter and dark energy MAIN Idea Compare and Contrast What are the differences among the three possible outcomes of the universe? Explain why dark matter is important in determining the density of matter in the universe Think Critically Determine What does dark matter have to with the critical density of the universe? Analyze All of the models tell us that the universe should be slowing down, but instead it is speeding up How does this affect our model of the universe? Earth Science Write one paragraph summarizing the evidence for the Big Bang model of the universe Self-Check Quiz glencoe.com Section • Cosmology 881 NASA/ESA/A M Koekemoer/M Dickinson/The GOODS Team Black holes seem to come straight from the pages of a science fiction book They are incredibly dense cosmic bodies from which nothing — not even light — can escape The gravitational pull attracts whatever ventures close enough Finding black holes Black holes are extremely difficult to see because they not emit light, and those that are produced by a collapsed massive star can be very small (only to times the mass of the Sun) Astronomers know where black holes might be located due to the effects of the matter falling into them Supermassive black holes In the center of some galaxies exist a different kind of black hole— a supermassive one These black holes are huge ; they can consist of more mass than a million, even a billion, Suns Scientists think that supermassive black holes are created when large volumes of interstellar gases collapse in on themselves Once matter passes into a spherical boundary surrounding the black hole, called the event horizon, it is pulled into the black hole, never to escape Energy Before the matter gets pulled into the event horizon, however, it gathers energy through friction and from the magnetic field of the black hole That energy is released in the form of diffuse light or focused jets The jets release about 1000 times more energy than the diffuse light, either in the form of radio waves or energetic X rays The jets race outward from the black holes almost at the speed of light, creating empty bubbles in their wake These bubbles can span thousands of lightyears Scientists used these bubbles to discover the fuel efficiency of the supermassive black holes 882 Chapter 30 • Galaxies and the Universe In this image taken by the Chandra X-Ray Telescope, X rays shine from heated material falling into a black hole Black holes are “green” Recent research into supermassive black holes has uncovered an interesting fact: They are the most fuel-efficient engines in the entire universe In fact, a physicist at Stanford University reported that “if you could develop a car that was as energy efficient as a supermassive black hole, it would get about one billion miles per gallon of gas!” Astronomers think that the energy released from supermassive black holes actually prevents star formation The heat that they produce prevents gases from cooling and potentially forming billions of new stars, effectively limiting the size of each galaxy Earth Science Summary Visit glencoe.com to learn more about black holes Summarize what you learn in a newspaper article about black holes that is interesting and scientifically accurate Jean-Charles Cuillandre/Canada-France-Hawaii Telescope/Photo Researchers, COBE/NASA, CORBIS, Space Telescope Science Institute/NASA/Photo Researchers INTERNET: CLASSIFY GALAXIES Background: Edwin Hubble developed rules for classifying galaxies according to their telescopic image shapes Modern astronomers are also interested in the classification of galaxies Information used for classification can indicate whether a certain type of galaxy is more likely to form than another and helps astronomers unravel the mystery of galaxy formation in the universe Using the Internet and sharing data with your peers, you can learn how galaxies are classified Galaxy Data Galaxy Name Image or Sketch of Galaxy NGC 3486 Classification Notes Sc Question: How can different galaxies be classified? Materials Analyze and Conclude internet access to glencoe.com or galaxy images provided by your teacher Visit a local library or observatory to gather images of galaxies and information about them Differentiate Which galaxy classes were the most difficult to find? Identify How many of each galaxy class did you find? Calculate the percentages of the total number of galaxies of each type Do you think this reflects the actual percentage of each type in the universe? Explain Discuss Were there any galaxies that didn’t fit the classification scheme? If so, why? List What problems did you have with galaxies seen edge-on? Illustrate Reconstruct the tuning fork diagram with images that you find Procedure Read and complete the lab safety form Find a resource with multiple images of galaxies and, if possible, names or catalog numbers for the galaxies Visit glencoe.com for links to sites that have galaxy images Choose one of the following types of galaxies to start your classification: spiral, elliptical, or irregular galaxies Sketch or gather images and information, such as catalog numbers and names of galaxies Sort the images by basic types: spiral, elliptical, or irregular galaxies Complete the data table Add any additional information you think is important INQUIRY EXTENSION Share Your Data With your classmates, calculate the percentage of each type of galaxy Based on the results, decide if your results are typical or atypical Determine how your class might find actual percentages of galaxies by type GeoLab 883 Download quizzes, key terms, and flash cards from glencoe.com BIG Idea Observations of galaxy expansion, cosmic background radiation, and the Big Bang theory describe an expanding universe that is 13.7 billion years old Vocabulary Key Concepts Section 30.1 The Milky Way Galaxy • Cepheid variable (p 863) • halo (p 863) • Population I star (p 866) • Population II star (p 866) • RR Lyrae variable (p 863) • spiral density wave (p 868) • variable star (p 862) Stars with varying light output allowed astronomers to map the Milky Way, which has a halo, spiral arms, and a massive galactic black hole at its center The discovery of variable stars aided in determining the shape of the Milky Way RR Lyrae and Cepheid are two types of variable stars used to measure distances The nuclear bulge and halo of the Milky Way is a globular cluster of old stars The spiral arms of the Milky Way are made of younger stars and gaseous nebulae Population I stars are found in the spiral arms, while Population II stars are in the central bulge and halo MAIN Idea • • • • • Section 30.2 Other Galaxies in the Universe • active galactic nucleus (p 875) • dark matter (p 870) • Hubble constant (p 874) • quasar (p 875) • radio galaxy (p 875) • supercluster (p 873) Finding galaxies with different shapes reveals the past, present, and future of the universe Galaxies can be elliptical, disk-shaped, or irregular Galaxies range in mass from million Suns to more than a trillion Suns Many galaxies seem to be organized in groups called clusters Quasars are the nuclei of faraway galaxies that are dim and seen as they were long ago, due to their great distances Hubble’s law helped astronomers discover that the universe is expanding MAIN Idea • • • • • Section 30.3 Cosmology • Big Bang theory (p 878) • cosmic background radiation (p 880) • cosmology (p 878) • • • • • 884 Chapter 30 X ••Study StudyGuide Guide The Big Bang theory was formulated by comparing evidence and models to describe the beginning of the universe The study of the universe’s origin, nature, and evolution is cosmology The Big Bang model of the universe came from observations of density and acceleration The critical density and the amount of dark energy of the universe will determine whether the universe is open or closed Cosmic background radiation gives support to the Big Bang theory of the universe Mapping the cosmic background radiation has indicated the existence of dark matter and dark energy MAIN Idea Vocabulary PuzzleMaker glencoe.com Vocabulary PuzzleMaker biologygmh.com Vocabulary Review The sentences below are false Correct each sentence by replacing the italicized words with the correct vocabulary term from the Study Guide Surrounding the central bulge, this spherical region of a galaxy is known as the Hubble constant Radio emissions coming from two huge lobes of very hot gas located on opposite sides of the visible galaxy are evidence for cosmology Understand Key Concepts 15 Which are the oldest objects in the Milky Way? A globular clusters B spiral arms C Cepheid variables D Population I stars Use the diagram below to answer Question 16 100,000 ly Population I is the weak radiation that appears to come from all directions in space and corresponds to an object having a temperature of about 2.735 K These gigantic quasars are hundreds of millions of light-years in size and can be observed only when astronomers map out the locations of many galaxies ranging over large distances This Cepheid variable is the invisible substance that makes up to 21 percent of the universe One theory of how galaxy arms are maintained involves the RR Lyrae variables Radio galaxy is the study of the origin and history of the universe A supercluster is a star whose magnitude changes are produced by expansion and shrinking of their outer layers Distinguish between the terms in each of the following pairs RR Lyrae variable, Cepheid variable 10 quasar, radio galaxy 11 dark matter, cosmic background radiation 12 halo, active galactic nucleus 13 cosmology, Hubble constant In the set of terms below, select the term that does not belong and explain why it does not belong 14 RR Lyrae, Cepheid, Population II, quasar Chapter Test glencoe.com 28,000 ly Nuclear bulge Disk Sun Globular clusters Halo 16 Where in the Milky Way are new stars being formed? A in the nuclear bulge B in globular clusters C in the spiral arms of the disk D in the halo 17 Where does the energy emitted by AGN and quasars most likely originate? A material falling into a supermassive black hole B a neutron star C a supernova explosion D a pulsar 18 What is the origin of the cosmic background radiation? A It is emitted by stars B It is a remnant of the Big Bang C It is emitted by radio galaxies D It is dark energy 19 In the Big Bang model, which describes a universe that will stop expanding and begin to contract? A open C closed B flat D elliptical Chapter 30 • Assessment 885 20 Which does the existence of cosmic background radiation support? A critical density B Hubble constant C the inflationary model D the Big Bang theory 21 Which two measurements are required to determine the Hubble constant? A distance and speed B distance and absolute magnitude C apparent magnitude and speed D apparent and absolute magnitudes 22 Without doing any calculations, what can astronomers determine from a variable star’s period of pulsation? A distance B apparent magnitude C luminosity D age Use the diagram below to answer Questions 23 and 24 Use the diagram to answer Question 25 Momentum of expansion Force of gravity 25 Which would cause the universe to collapse in on itself to make a closed universe? A force of gravity B critical density C momentum of outward expansion D Hubble constant Constructed Response 26 Interpret the relationship between mass and density and the expansion of the universe 27 Discuss Why are pulsating variable stars useful for finding distances to globular clusters? 28 Explain How astronomers observe the spiral structure of the Milky Way? 23 Which kind of galaxy is illustrated above? A spiral B barred spiral C elliptical D irregular 24 Which designation would the tuning fork diagram assign this galaxy? A S0 B SB C Sa D E3 886 Chapter 30 • Assessment 29 Careers in Earth Science Why astronomers think that there is a great amount of mass in the halo of the Milky Way? 30 Explain Why are the stars in globular clusters classified as Population II stars? 31 Relate the classification of a galaxy to its shape 32 Compare active galactic nuclei with quasars 33 Explain What redshifts and Hubble’s law tell us about the motion of galaxies? 34 Discuss how astronomers determined that dark matter exists Chapter Test glencoe.com Think Critically 35 Infer How would a star that forms in the Milky Way a few billion years in the future would compare with the Sun? Use the graph below to answer Question 36 Additional Assessment 41 Earth Science Write an essay explaining the necessity for continuing spacebased satellite telescope use and development Document–Based Questions The Inflationary Model of the Universe 10 40 10 30 The graph below shows the changes in the strength of the major forces in the universe from the Big Bang until the present 10 20 1010 Size (cm) Data obtained from: Silk, J The SETI module: cosmology 1998 Syracuse University Radius of observable universe 10–10 Time after the Big Bang (s) 10–20 10–30 10 Now 10 –40 10 10–50 10–35 10–25 10–15 10–5 Time after the Big Bang (s) 36 Explain what happened to the universe during the 10–35 portion of the graph 37 Compare the importance of variable stars and cosmic background radiation to the determination of the shape of the universe 38 Identify the cause-and-effect relationship between Population I and Population II stars Concept Mapping 39 Use the following terms to construct a concept map to organize the major ideas in this chapter: cosmic background radiation, quasars, Hubble’s law, black holes, galaxy clusters, and Big Bang theory Challenge Question 40 Infer the difficulty of determining the outcome of the universe relative to the presence of dark matter Chapter Test glencoe.com Force strength 10–45 Inflationary epoch 10–60 10 -10 10 -20 10 -30 10 -40 Big Bang Strong nuclear 10 -10 Electromagnetic 10 -20 Weak nuclear 10 -30 10 Gravity -40 1010 10 20 10 30 Temperature (K) 42 At what time and temperature did gravity and the strong-electro-weak forces separate? 43 What has happened to the force of gravity since the Big Bang? To the other forces? 44 One-billionth of a second (10-9) after the Big Bang initiation, atoms began to form At what temperature did this occur? Cumulative Review 45 Which two basic stellar properties are displayed on an H-R diagram? (Chapter 29) 46 Why is it unwise to try to forecast weather by simply extrapolating current conditions beyond a few hours? (Chapter 12) Chapter 30 • Assessment 887 Standardized Test Practice Multiple Choice Use the table below to answer Questions to Stellar Magnitudes Star Apparent Magnitude Absolute Magnitude Procyon +0.38 +2.66 Altair +0.77 +2.22 Becrcux +1.25 -3.92 Bellatrix +1.64 -1.29 Denebola +2.14 +1.54 Which is the brightest star as seen from Earth? A Procyon C Bellatrix B Becrux D Denebola Which is the brightest star as seen from 10 parsecs? A Procyon C Bellatrix B Becrux D Denebola Which is the dimmest star as seen from 10 parsecs? A Bellatrix C Procyon B Altair D Becrux What two measurements are required to determine the Hubble constant? A distance and speed B distance and absolute magnitude C apparent magnitude and speed D apparent and absolute magnitude What does Kepler’s first law state? A Each planet revolves around the Sun in a circular path B Each planet revolves around the Sun in an elliptical path C Planets closer to the Sun move faster than planets farther away D Planets closer to the Sun move slower than planets farther away Use the graph below to answer Questions to Solid Waste Recycled 100,000 80,000 60,000 40,000 20,000 Tons What is a piece of interplanetary material that burns up in Earth’s atmosphere called? A a meteorite C a meteor B an asteroid D a meteoroid 1994 1995 1996 1997 1998 1999 2000 Year What can be implied about the graph above? A Before 1994, recycling did not exist B As people became more aware of the benefits of recycling, the amount of waste being recycled increased C Less waste was consumed in 1997, causing a decrease in the amount of waste recycled D Recycling interests began to decrease in 1998 What could not account for the sharp increase in recycling between 1995 and 1996? A implementation of recycling laws B increased public awareness C more convenience for recycling D less production of materials that need recycling Which of the following years had the greatest increase in amount of material recycled? A 2000-2001 C 1996-1997 B 1998-1999 D 1995-1996 10 Carbon-14 has a radioactive decay half-life of 5730 years Which item would carbon-14 be most useful for dating? A a rock from the Moon B a Native American fire pit C a jawbone from a triceratops D a granite rock from the Canadian Shield 11 In which region of the Milky Way galaxy is 90 percent of its mass located? A spiral arms B halo C nuclear bulge D disk Standardized Test Practice glencoe.com 888 Chapter 30 • Assessment Short Answer Use the illustration below to answer Questions 11 to 13 The original stars formed from gas and dust in the void of space and are thought to have been many times more massive than today’s stars The ancient stars remain invisible to telescopes and have never before been detected Using NASA’s orbiting Spitzer Space Telescope the stars have been identified indirectly by measuring the enduring energy that they once radiated into the void of space As the universe expands, starlight is stretched into longer, redder wavelengths Most emissions from the first stars in the universe would appear today as infrared light The universe is filled with background radiation known as the cosmic infrared background (CIB) This includes radiation from all stars— young and old, near and far If these earliest stars were massive and formed in the standard cosmological mode, they should have left a signature in the fluctuations of the CIB Earth Sun Venus Mars Mercury Jupiter 12 What the lines through the planets represent? 13 Name and describe the material located between Mars and Jupiter 14 Explain why this material did not form into a planet Article obtained from: Handwerk, B First stars in universe detected? National Geographic News November 2, 2005 15 Compare and contrast refracting and reflecting telescopes Which one is used more widely today? Why? 18 What can be inferred from this passage? A These stars are still present in space B Telescopes are not a good way to view stars C These first stars formed at the same time as the big bang D The first stars no longer exist, but we are just now seeing their radiation 16 Describe the geocentric model of the solar system 17 Why is Earth’s Moon unique among all moons in the solar system? Reading for Comprehension First Stars in the Universe NASA researchers say they have detected what might be the faint infrared glow of the first stars in the universe Known as population III stars, the distant bodies are thought to have formed just 200 million years after the big bang 19 What are scientists seeing that confirms the existence of these stars? A their visible light finally reaching Earth B the faint infrared glow from their emissions C the gas and dust particles of the stars D radiation from the existing stars 20 Hypothesize why basic telescopes are not able to find these stars but the Spitzer Space Telescope can NEED EXTRA HELP? If You Missed Question 10 11 12 13 14 15 16 17 Review Section 28.1 29.2 29.2 29.2 30.2 28.1 26.1 26.1 26.1 21.3 30.1 28.1 28.4 28.4 27.1 28.1 27.2 Standardized Test Practice glencoe.com Chapter 30 • Assessment 889 ... describe the beginning of the universe The study of the universe s origin, nature, and evolution is cosmology The Big Bang model of the universe came from observations of density and acceleration The. .. density and the amount of dark energy of the universe will determine whether the universe is open or closed Cosmic background radiation gives support to the Big Bang theory of the universe Mapping the. .. the star, the universe has two opposing forces In the Big Bang model, the momentum of the outward expansion of the universe is opposed by the inward force of gravity acting on the matter of the