Neil F. Comins • William J. Kaufmann III Discovering the Universe Tenth Edition CHAPTER 18 Cosmology This painting indicates how astronomers visualize the burst of star formation that occurred within a few hundred million years after the Big Bang. The arcs and irregular circles represent interstellar gas illuminated by supernovae. (Adolf Schaller, STScI/NASA/K. Lanzetta, SUNY)

In this chapter, you will discover… cosmology, which seeks to explain how the universe began, how it evolves, and its fate the best theory that we have for the evolution of the universe—the Big Bang how astronomers trace the emergence of matter and the formation of galaxies how astronomers explain the overall structure of the universe our understanding of the fate of the universe

Cosmological Redshift FIGURE 18-1 Cosmological Redshift Just as the waves drawn on this rubber band are stretched along with the rubber band, so too are the wavelengths of photons stretched as the universe expands. Just as the waves drawn on this rubber band are stretched along with the rubber band, so too are the wavelengths of photons stretched as the universe expands.

Cosmology and Age of the Universe The universe consists of all matter, energy, and spacetime that we can ever detect or that will ever be able to affect us. Cosmology is the study of the large-scale structure and evolution of the universe. The photons we observe from galaxies and superclusters are all expanding in what is called the cosmological redshift. Assuming the universe began expanding when it came into existence and that it has always been expanding at a constant rate we can calculate the age of the universe by dividing the separation distance by the recessional velocity resulting in an age of approximately 13.8 billion years. Information taken from sections 18-1 through 18-3.

Bell Labs Horn Antenna FIGURE 18-2 Bell Labs Horn Antenna This Bell Laboratories horn antenna at Holmdel, New Jersey, was used by Arno Penzias (right) and Robert Wilson in 1965 to detect the cosmic microwave background. (Roger Ressmeyer/CORBIS) This Bell Laboratories horn antenna at Holmdel, New Jersey, was used by Arno Penzias (right) and Robert Wilson in 1965 to detect the cosmic microwave background.

In Search of Primordial Photons FIGURE 18-3 In Search of Primordial Photons (a) The Wilkinson Microwave Anisotropy Probe (WMAP) satellite, launched in 2001, improved upon the measurements of the spectrum and angular distribution of the cosmic microwave background taken by the COBE satellite. (NASA/WMAP Science Team) The Wilkinson Microwave Anisotropy Probe (WMAP) satellite, launched in 2001, improved upon the measurements of the spectrum and angular distribution of the cosmic microwave background taken by the COBE satellite.

In Search of Primordial Photons FIGURE 18-3 In Search of Primordial Photons (b) The balloon-carried telescope BOOMERANG orbited above Antarctica for 10 days, collecting data used to resolve the cosmic microwave background, with 10 times higher resolution than that of COBE. All of these experiments found local temperature variations across the sky but no overall deviation from a blackbody spectrum. (BOOMERANG/NASA/NSF) The balloon-carried telescope BOOMERANG orbited above Antarctica for 10 days, collecting data used to resolve the cosmic microwave background, with 10 times higher resolution than that of COBE. All of these experiments found local temperature variations across the sky but no overall deviation from a blackbody spectrum.

Spectrum of the Cosmic Microwave Background FIGURE 18-4 Spectrum of the Cosmic Microwave Background The little squares on this graph are COBE’s measurements of the brightness of the cosmic microwave background plotted against wavelength. To a remarkably high degree of accuracy, the data fall along a blackbody curve for 2.73 K. The peak of the curve is at a wavelength of 1.1 mm, in accordance with Wien’s law. (E. Cheng; NASA COBE Science Team) The little squares on this graph are COBE’s measurements of the brightness of the cosmic microwave background plotted against wavelength. To a remarkably high degree of accuracy, the data fall along a blackbody curve for 2.73 K. The peak of the curve is at a wavelength of 1.1 mm, in accordance with Wien’s law.

The Big Bang Astronomers believe that the universe began as an exceedingly dense cosmic singularity that expanded explosively in an event called the Big Bang. The Hubble law describes the ongoing expansion of the universe and the rate at which superclusters of galaxies move apart. The observable universe extends about 13.8 billion light-years in every direction from Earth to what is called the cosmic light horizon. We cannot see any objects that may exist beyond the cosmic light horizon because light from these objects has not had enough time to reach us. Observations show that the universe is flat and that the cosmic microwave background is almost perfectly isotropic, resulting from a brief period of very rapid expansion (the inflationary epoch) in the very early universe. Inflation explains the isotropic and homogeneous appearance of the universe. Information taken from sections 18-5 through 18-10.

Microwave Sky This map of the microwave sky was produced from data taken by instruments on board COBE. The galactic center is in the middle of the map, and the plane of the Milky Way runs horizontally across the map. Color indicates Doppler shift and temperature: Blue is where the microwave background is blueshifted and warmer, while red is where it is redshifted and appears cooler. This Doppler shift across the sky is caused by Earth’s motion through the microwave background. The resulting variation in background temperature is quite small, only 0.0033 K above the average radiation temperature of 2.726 K. FIGURE 18-5 Microwave Sky This map of the microwave sky was produced from data taken by instruments on board COBE. The galactic center is in the middle of the map, and the plane of the Milky Way runs horizontally across the map. Color indicates Doppler shift and temperature: Blue is where the microwave background is blueshifted and appears warmer, while red is where it is redshifted and appears cooler. This Doppler shift across the sky is caused by Earth’s motion through the microwave background. The resulting variation in background temperature is quite small, only 0.0033 K above the average radiation temperature of 2.726 K (which we have rounded to 2.73 K in the text). (NASA)

Our Motion Through the Microwave Background FIGURE 18-6 Our Motion Through the Microwave Background Because of the Doppler effect, we detect shorter wavelengths in the microwave background and a higher temperature of radiation in that part of the sky toward which we are moving. This part of the sky is the area shown in blue in Figure 18-5. In the opposite part of the sky, shown in red in Figure 18-5, the microwave radiation has longer wavelengths and a cooler temperature. Because of the Doppler effect, we detect shorter wavelengths in the microwave background and a higher temperature of radiation in that part of the sky toward which we are moving. This part of the sky is the area shown in blue in the previous figure. In the opposite part of the sky, shown in red, the microwave radiation has longer wavelengths and a cooler temperature.

Unification of the Four Forces FIGURE 18-7 Unification of the Four Forces The four physical forces were initially a single force. This symmetry was broken as the universe expanded and cooled. This figure shows the time and temperature of the universe when the forces separated from each other. The four physical forces were initially a single force. This symmetry was broken as the universe expanded and cooled. This figure shows the time and temperature of the universe when the forces separated from each other.

Separation of the Four Physical Forces Four basic forces—gravity, electromagnetism, the strong nuclear force, and the weak nuclear force—explain the interactions observed in the universe. According to current theory, all four forces were identical just after the Big Bang (although our understanding of the nature of matter and energy is extremely limited). At the end of the Planck time (about 10−43 s after the Big Bang), gravity became a separate force. A short time later, the strong nuclear force became a distinct force. A final separation created the electromagnetic force and the weak nuclear force.

Early History of the Universe FIGURE 18-8 Early History of the Universe Current theory holds that, as the universe cooled, the four forces separated from their initially unified state. The inflationary epoch lasted from 10–36 s to 10–32 s after the Big Bang. Quarks became confined together, thereby creating neutrons and protons 10–6 s after the Big Bang. The universe became transparent to light (that is, photons decoupled from matter) when the universe was about 1.2 × 1013 s (380,000 years) old. The physics of the Planck era is presently unknown. Current theory holds that, as the universe cooled, the four forces separated from their initial unified state. The inflationary epoch lasted from 10–36 s to 10–32 s after the Big Bang. Quarks became confined together, thereby creating neutrons and protons 10–6 s after the Big Bang. The universe became transparent to light (that is, photons decoupled from matter) when the universe was about 1.2 × 1013 s (380,000 years) old. The physics of the Planck era is presently unknown.

Cause of Inflation FIGURE 18-9 Cause of Inflation The universe formed in an unstable energy state that (b) began to transition to a stable configuration. (c) This transition provided the energy that caused inflation. (a) The universe formed in an unstable energy state that (b) began to transition to a stable configuration. (c) This transition provided the energy that caused inflation.

Observable Universe Before and After Inflation FIGURE 18-10 Observable Universe Before and After Inflation Shortly after the Big Bang, the universe expanded by a factor of about 1050 due to inflation. This growth in the size of the presently observable universe occurred in a very brief time (shaded interval). Shortly after the Big Bang, the universe expanded by a factor of about 1050 due to inflation. This growth in the size of the presently observable universe occurred in a very brief time (shaded interval).

Radiation During Planck Time and Inflation According to the theory of inflation, early in its existence, the universe expanded very rapidly for a short period, spreading matter that was originally far from our location (and hence at different temperatures and densities) throughout a volume of the universe so large that we cannot yet observe it. The observable universe today is thus a growing volume of space containing matter and radiation that was in close contact with our matter and radiation during the first instant after the Big Bang (and hence at the same temperature, pressure, and density). Information taken from sections 18-5 through 18-10.

Observable Universe This diagram shows why we only see part of the entire universe. As time passes, this volume grows, meaning that light from more distant galaxies reaches us. The farthest galaxies we see (inset) as they were within a few hundred million years after the Big Bang. These galaxies, formed at the same time as the Milky Way, appear young because the light from their beginnings is just now reaching us. While the light from the most distant galaxies we see was traveling toward us, the universe has been growing. Therefore, objects that appear 13 billion light-years away from us today are actually about 3 times farther away today, or nearly 46 billion light-years in radius. (Inset) This image shows some of the most distant galaxies we have seen. FIGURE 18-11 Observable Universe This diagram shows why we see only part of the entire universe. As time passes, this volume grows, meaning that light from more distant galaxies reaches us. The farthest galaxies we see appear as they were within a few hundred million years after the Big Bang. These galaxies, formed at the same time as the Milky Way, appear young because the light from their beginnings is just now reaching us. While the light from the most distant galaxies we see was traveling toward us, the universe has been growing. Therefore, objects that appear 13 billion light-years away from us are actually about 3 times farther away today. Put another way, if we could see it all as it is today, rather than as it was when photons from distant objects started their journeys, the visible universe would be nearly 46 billion light-years in radius. Inset: This image shows some of the most distant galaxies we have seen. (inset: Robert Williams and the Hubble Deep Field Team (STScI) and NASA)

Pair Production and Annihilation FIGURE 18-12 Pair Production and Annihilation (a) A particle and an antiparticle can be created when a high-energy photon collides with a nucleus. (b) Conversely, a particle and an antiparticle can annihilate each other and emit energy in the form of gamma rays. (The processes are more complex and are only summarized in these drawings.) (a) A particle and an antiparticle can be created when a high-energy photon collides with a nucleus. (b) Conversely, a particle and an antiparticle can annihilate each other and emit energy in the form of gamma rays. (The processes are more complex and are only summarized in these drawings.)

Change from a Radiation-Dominated Universe to a Matter-Dominated Universe In its first 30,000 years, the universe was radiation- dominated, during which time photons prevented matter from forming clumps. Then it was matter-dominated, during which time superclusters and smaller clumps of matter formed. Today, it is dark-energy-dominated. Dark energy of some sort supplies a repulsive gravitational force that causes superclusters to accelerate away from one another. During the first 380,000 years of the universe, matter and energy formed an opaque plasma, called the primordial fireball. Cosmic microwave background radiation is the greatly redshifted remnant of the universe as it existed about 380,000 years after the Big Bang. Information taken from sections 18-5 through 18-10.

Evolution of Mass/Energy Density For approximately 30,000 years after the Big Bang, the gravitational effects from photons (ρrad, shown in red ) exceeded the effects of all the matter in the universe (ρm, shown in blue). This early period is said to have been radiation dominated. Later, however, continued expansion of the universe caused ρrad to become less than ρm, at which time the universe became matter dominated. FIGURE 18-13 Evolution of Mass/Energy Density For approximately 30,000 years after the Big Bang, the gravitational effects from photons (ρrad, shown in red) exceeded the attraction of all the matter in the universe (ρm, shown in blue). This early period is said to have been radiation-dominated. Later, however, continued expansion of the universe caused ρrad to become less than ρm, at which time the universe became matter-dominated.

Era of Recombination (a) Before recombination, the energies of photons in the cosmic background were high enough to prevent protons and electrons from forming hydrogen atoms. FIGURE 18-14 Era of Recombination (a) Before recombination, the energies of photons in the cosmic background were high enough to prevent protons and electrons from forming hydrogen atoms. (b) As soon as the energy of the background radiation became too low to ionize hydrogen, neutral atoms came into existence. (b) As soon as the energy of the background radiation became too low to ionize hydrogen, neutral atoms came into existence.

Recombination About 380,000 years after the Big Bang, spacetime expansion caused the temperature of the universe to fall below 3000 K, allowing protons and electrons to combine and thereby form neutral hydrogen atoms. This period is called the era of recombination. The universe became transparent during the era of recombination, with the photons that existed back then still traveling through space today. In other words, the microwave background radiation is composed of the oldest photons in the universe. Information taken from sections 18-5 through 18-10.

Structure of the Early Universe FIGURE 18-15 Structure of the Early Universe (a) This microwave map of the entire sky, produced from data taken by the Wilkinson Microwave Anisotropy Probe (WMAP), shows temperature variations in the cosmic microwave background. Red regions are about 0.00003 K warmer than the average temperature of 2.73 K; blue regions are about 0.00003 K cooler than the average. (Inset) These tiny temperature fluctuations, observed by BOOMERANG, are related to the large-scale structure of the universe today, indicating where superclusters and voids grew. The radiation detected to make this map is from a time 380,000 years after the Big Bang. (NASA/WMAP Science Team; inset: NSF/NASA) This microwave map of the entire sky, produced from data taken by the Wilkinson Microwave Anisotropy Probe (WMAP), shows temperature variations in the cosmic microwave background. Red regions are about 0.00003 K warmer than the average temperature of 2.73 K; blue regions are about 0.00003 K cooler than the average. (Inset) These tiny temperature fluctuations, observed by BOOMERANG, are related to the large-scale structure of the universe today, indicating where superclusters and voids grew. The radiation detected to make this map is from a time 380,000 years after the Big Bang.

Structure of the Early Universe FIGURE 18-15 Structure of the Early Universe (b) Acoustic peaks show the sizes of the hot spots on the inset map in (a), along with overtones that provide information about the kinds of matter in the universe. (NASA/WMAP Science Team) Acoustic peaks show the sizes of the hot spots on the inset map in the previous figure, along with overtones that provide information about the kinds of matter in the universe.

Galaxies Forming by Combining Smaller Units FIGURE 18-16 Galaxies Forming by Combining Smaller Units (a) This painting indicates how astronomers visualize the burst of star formation that occurred within a few hundred million years after the Big Bang. The arcs and irregular circles represent interstellar gas illuminated by supernovae. (Adolf Schaller, STScI/NASA/K. Lanzetta, SUNY) This painting indicates how astronomers visualize the burst of star formation that occurred within a few hundred million years after the Big Bang. The arcs and irregular circles represent interstellar gas illuminated by supernovae.

Galaxies Forming by Combining Smaller Units FIGURE 18-16 Galaxies Forming by Combining Smaller Units (b) Using the Hubble and Keck telescopes, astronomers discovered two groups of stars (arrows) 13.4 billion light-years away that are believed to be protogalaxies from which bigger galaxies grew. These protogalaxies were discovered because they were enlarged by the gravitational lensing of an intervening cluster of galaxies. (b: NASA, ESA, A. Fruchter and the ERO Team (STScI, ST-ECF) Using the Hubble and Keck telescopes, astronomers discovered two groups of stars (arrows) 13.4 billion light-years away that are believed to be protogalaxies from which bigger galaxies grew. These protogalaxies were discovered because they were enlarged by the gravitational lensing of an intervening cluster of galaxies.

Galaxies Forming by Combining Smaller Units FIGURE 18-16 Galaxies Forming by Combining Smaller Units (c) The Chandra X-ray telescope imaged gravitationally bound gas around the distant galaxy 3C 294. The X-ray emission from this gas is the signature of an extremely massive cluster of galaxies, in this case, at a distance of about 11.2 billion light-years from us. (c: NASA/IoA/A. Fabian et al.) The Chandra X-ray Observatory imaged gravitationally bound gas around the distant galaxy 3C 294. The X-ray emission from this gas is the signature of an extremely massive cluster of galaxies, in this case, at a distance of about 11.2 billion light-years from us.

Stellar Birth Rates FIGURE 18-17 Stellar Birth Rates (a) This figure shows that star formation started quickly in the life of the universe and has been tapering off ever since. This figure shows that star formation started quickly in the life of the universe and has been tapering off ever since.

Stellar Birth Rates FIGURE 18-17 Stellar Birth Rates (b) Most of the stars in an elliptical galaxy are created in a brief burst of star formation when the galaxy is very young. In spiral galaxies, stars form at a more leisurely pace that extends over billions of years. Most of the stars in an elliptical galaxy are created in a brief burst of star formation when the galaxy is very young. In spiral galaxies, stars form at a more leisurely pace that extends over billions of years.

Creation of Spiral and Elliptical Galaxies FIGURE 18-18 Creation of Spiral and Elliptical Galaxies A galaxy begins as a huge cloud of primordial gas that collapses gravitationally. (a) If the rate of star birth is low, then much of the gas collapses to form a disk, and a spiral galaxy is created. (b) If the rate of star birth is high, then the gas is converted into stars before a disk can form, resulting in an elliptical galaxy. A galaxy begins as a huge cloud of primordial gas that collapses gravitationally. (a) If the rate of star birth is low, then much of the gas collapses to form a disk, and a spiral galaxy is created. (b) If the rate of star birth is high, then the gas is converted into stars before a disk can form, resulting in an elliptical galaxy.

Mapping Dark Matter (Top) The Hubble Space Telescope observed that galaxies in the same direction, but at different distances from Earth, undergo different amounts of gravitational lensing. (Bottom) Much of this effect is due to dark matter. By subtracting out the lensing effects of intervening galaxies, the distorted shapes of the galaxies at various distances enable astronomers to determine the distribution of dark matter. FIGURE 18-19 Mapping Dark Matter (Top) The Hubble Space Telescope observed that galaxies in the same direction, but at different distances from Earth, undergo different amounts of gravitational lensing. (Bottom) Much of this effect is due to dark matter. By subtracting out the lensing effects of intervening galaxies, the distorted shapes of the galaxies at various distances enable astronomers to determine the distribution of dark matter. (NASA, ESA, and R. Massey [CIT])

Possible Geometries of the Universe The shape of space (represented here as two-dimensional for ease of visualization) is determined by the matter and energy contained in the universe. The curvature is either (a) positive, (b) zero, or (c) negative, depending on whether the average matter and energy density throughout space is greater than, equal to, or less than a critical value. The lines on each curve are initially parallel. They converge, remain parallel, or diverge, depending on the curvature of space. FIGURE 18-20 Possible Geometries of the Universe The shape of space (represented here as two-dimensional for ease of visualization) is determined by the matter and energy contained in the universe. The curvature is either (a) positive, (b) zero, or (c) negative, depending on whether the average matter and energy density throughout space is greater than, equal to, or less than a critical value. The lines on each curve are initially parallel. They converge, remain parallel, or diverge, depending on the curvature of space.

The Shape and the Fate of the Universe The average density of matter and dark energy in the universe determines the curvature of space and the ultimate fate of the universe. Based on observations, the universe has negative curvature (hyperbolic) and displays an outward acceleration extending without limit. This means it should expand forever and is called an open universe.

Cosmic Microwave Background and the Curvature of Space FIGURE 18-21 Cosmic Microwave Background and the Curvature of Space Temperature variations in the early universe appear as “hot spots” in the cosmic microwave background. The apparent sizes of these spots depend on the curvature of space. (a) In a closed universe with positive curvature, light rays from opposite sides of a hot spot bend toward each other. Hence, the hot spot appears larger than it actually is (dashed lines). (b) The light rays do not bend in a flat universe. (c) In an open universe, light rays bend apart. The dashed lines show that a hot spot would appear smaller than its actual size. (a-c: NASA/ a b c WMAP Science Team) Temperature variations in the early universe appear as “hot spots” in the cosmic microwave background. The apparent sizes of these spots depend on the curvature of space. (a) In a closed universe with positive curvature, light rays from opposite sides of a hot spot bend toward each other. Hence, the hot spot appears larger than it actually is (dashed lines). (b) The light rays do not bend in a flat universe. (c) In an open universe, light rays bend apart. The dashed lines show that a hot spot would appear smaller than its actual size.

Dimmer Distant Supernova SN 1997ff , more than 10 billion light-years away, was dimmer than expected, indicating that the distance to it is greater than the distance it would have if the universe had been continually slowing down since the Big Bang. This supports the notion that an outward (cosmological) force is acting over vast distances in the universe. The arrow on the first inset shows the galaxy in which the supernova was discovered. The bright spot on the right inset shows the supernova by subtracting the constant light emitted by all the other nearby objects. FIGURE 18-22 Dimmer Distant Supernova (a) These Hubble Space Telescope images show the galaxy in which the Type Ia supernova SN 1997ff occurred. This supernova, more than 10 billion light-years away, was dimmer than expected, indicating that the distance to it is greater than the distance it would have if the universe had been continually slowing down since the Big Bang. This supports the notion that an outward (cosmological) force is acting over vast distances in the universe. The arrow on the left inset shows the galaxy in which the supernova was discovered. The bright spot on the right inset shows the supernova by subtracting the constant light emitted by all the other nearby objects. (Adam Riess, Space Telescope Science Institute/NASA)

Dimmer Distant Supernova FIGURE 18-22 Dimmer Distant Supernova (b) The distances and brightnesses of many very distant supernovae are plotted on this diagram. The locations of the most distant supernovae in the upper region strongly indicate that the universe has been accelerating outward for the past 6 billion years. The distances and brightnesses of many very distant supernovae are plotted on this diagram. The locations of the most distant supernovae in the upper region strongly indicate that the universe has been accelerating outward for the past 6 billion years.

Big Picture of the Evolution of the Universe FIGURE 18-23 Big Picture of the Evolution of the Universe This figure shows our current thinking about the evolution of star and galaxy formation in the early universe, as well as the present-day acceleration of the universe’s expansion. (NASA/WMAP Science Team) This figure shows our current thinking about the evolution of star and galaxy formation in the early universe, as well as the present-day acceleration of the universe’s expansion.

Big Picture of the Evolution of the Universe Clusters of galaxies and individual galaxies formed from pieces of enormous hydrogen and helium clouds, each of which became a separate supercluster of galaxies. All of the superclusters and some of the clusters of galaxies within each supercluster are moving away from one another. Supermassive black holes appear to have “seeded” the formation of most galaxies. During the matter-dominated era, structure formed in the universe. As the universe goes farther into the dark-energy-dominated era, the large-scale structure of superclusters of galaxies will fade away. Dark energy is the repulsive gravitational effect that is causing the universe to accelerate outward and provides the majority of the energy needed to account for the universe being flat. Information taken from sections 18-5 through 18-15.

Percentages of the Major Components of the Universe FIGURE 18-24 Percentages of the Major Components of the Universe

Summary of Key Ideas

The Big Bang Astronomers believe that the universe began as an exceedingly dense cosmic singularity that expanded explosively in an event called the Big Bang. The Hubble law describes the ongoing expansion of the universe and the rate at which superclusters of galaxies move apart. The observable universe extends about 13.8 billion light-years in every direction from Earth to what is called the cosmic light horizon. We cannot see any objects that may exist beyond the cosmic light horizon because light from these objects has not had enough time to reach us.

The Big Bang According to the theory of inflation, early in its existence, the universe expanded very rapidly for a short period, spreading matter that was originally far from our location (and hence at different temperatures and densities) throughout a volume of the universe so large that we cannot yet observe it. The observable universe today is thus a growing volume of space containing matter and radiation that was in close contact with our matter and radiation during the first instant after the Big Bang (and hence at the same temperature, pressure, and density). Inflation explains the isotropic and homogeneous appearance of the universe.

A Brief History of Spacetime, Matter, Energy, and Everything Four basic forces—gravity, electromagnetism, the strong nuclear force, and the weak nuclear force—explain the interactions observed in the universe. According to current theory, all four forces were identical just after the Big Bang (although our understanding of the nature of matter and energy is extremely limited). At the end of the Planck time (about 10−43 s after the Big Bang), gravity became a separate force. A short time later, the strong nuclear force became a distinct force. A final separation created the electromagnetic force and the weak nuclear force. Observations show that the universe is flat and that the cosmic microwave background is almost perfectly isotropic, resulting from a brief period of very rapid expansion (the inflationary epoch) in the very early universe.

A Brief History of Spacetime, Matter, Energy, and Everything In its first 30,000 years, the universe was radiation- dominated, during which time photons prevented matter from forming clumps. Then it was matter dominated, during which time superclusters and smaller clumps of matter formed. Today, it is dark-energy-dominated. Dark energy of some sort supplies a repulsive gravitational force that causes superclusters to accelerate away from each other. During the first 380,000 years of the universe, matter and energy formed an opaque plasma, called the primordial fireball. Cosmic microwave background radiation is the greatly redshifted remnant of the universe as it existed about 380,000 years after the Big Bang.

A Brief History of Spacetime, Matter, Energy, and Everything About 380,000 years after the Big Bang, spacetime expansion caused the temperature of the universe to fall below 3000 K, allowing protons and electrons to combine and thereby form neutral hydrogen atoms. This period is called the era of recombination. The universe became transparent during the era of recombination, with the photons that existed back then still traveling through space today. In other words, the microwave background radiation is composed of the oldest photons in the universe.

A Brief History of Spacetime, Matter, Energy, and Everything Clusters of galaxies and individual galaxies formed from pieces of enormous hydrogen and helium clouds, each of which became a separate supercluster of galaxies. All of the superclusters and some of the clusters of galaxies within each supercluster are moving away from one another. Supermassive black holes appear to have “seeded” the formation of most galaxies. During the matter-dominated era, structure formed in the universe. As the universe goes farther into the dark- energy-dominated era, the large-scale structure of superclusters of galaxies will fade away.

The Fate of the Universe The average density of matter and dark energy in the universe determines the curvature of space and the ultimate fate of the universe. The universe is accelerating outward and it will expand forever.

Superstring Theories To combine gravitation and the other three fundamental forces in nature into one comprehensive Theory of Everything, scientists have had to consider a universe that contains more than the four dimensions we know about today (three of space and one of time). The theories that mathematically describe this new formulation of the universe are called superstring theories. There are basically five such theories, which allow all of the particles we have been studying—such as quarks, protons, neutrons, electrons, and photons—to exist. Spacetime in superstring theories has 10 dimensions, of which 6 are everywhere rolled up into such tiny volumes that we cannot detect them directly. The other 4 dimensions are our normal spacetime. Superstring’s more general spacetime carries with it properties that allow scientists to combine all four forces in nature into one set of equations. The difficulty in reconciling quantum mechanics (describing the weak, strong, and electromagnetic forces) and general relativity (describing gravitation) is that the three forces in quantum mechanics are quantized, whereas general relativity is not.

Superstring and M-Theories Superstring theories begin with a different assumption about all particles and their interactions than do either quantum mechanics or general relativity. The new theories assert that each particle is actually a tiny vibrating string, with different types of particles vibrating at different rates, like different strings on a guitar. Superstring and M-theories are, so far, consistent with observations, but it remains to be seen if they will continue to maintain consistency with future observations and, very importantly, if they will make predictions that can be tested. Without being able to do that, they will remain truly elegant mathematical formalisms, but not science.

Key Terms isotropy Big Bang isotropy problem (horizon problem) matter-dominated universe open universe pair production Planck era Planck time primordial fireball primordial nucleosynthesis quark quintessence radiation-dominated universe strong nuclear force superstring theories Theories of Everything universe weak nuclear force Big Bang closed universe confinement cosmic light horizon cosmic microwave background cosmological constant cosmological redshift cosmology dark ages dark energy decoupling era of recombination expanding universe Grand Unified Theory (GUT) homogeneity inflation inflationary epoch