A guide to the big picture, fundamental physical law, windows of space and time, the great war, and extremely big numbers.
January 1, 7,000,000,000 A.D., Ann Arbor:
The New Year rings in little cause for celebration. Nobody is present even to mark its passing. Earth's surface is a torrid unrecognizable wasteland. The Sun has swelled to enormous size, so large that its seething red disk nearly fills the daytime sky. The planet Mercury and then Venus have already been obliterated, and now the tenuous outer reaches of the solar atmosphere are threatening to overtake the receding orbit of Earth.
Earth's life-producing oceans have long since evaporated, first into a crushing, sterilizing blanket of water vapor, and then into space entirely. Only a barren rocky surface is left behind. One can still trace the faint remains of ancient shorelines, ocean basins, and the low eroded remnants of the continents. By noon, the temperature reaches nearly 3000 degrees Fahrenheit, and the rocky surface begins to melt. Already, the equator is partly ringed by a broad glowing patchwork of lava, which cools to form a thin gray crust as the distended Sun eases beneath the horizon each night.
A patch of the surface which once cradled the forested moraines of southeastern Michigan has seen a great deal of change over the intervening billions of years. What was once the North American continent has long since been torn apart by the geologic rift which opened from Ontario to Louisiana and separated the old stable continental platform to produce a new expanse of sea floor. The sedimented, glaciated remains of Ann Arbor were covered by lava which arrived from nearby rift volcanos by coursing through old river channels. Later, the hardened lava and the underlying sedimentary rock were thrust up into a mountain chain as a raft of islands the size of New Zealand collided with the nearby shoreline.
Now, the face of an ancient cliff is weakened by the Sun's intense heat. A slab of rock cleaves off, causing a landslide and exposing a perfectly preserved fossil of an oak leaf. This trace of a distant verdant world slowly melts away in the unyielding heat. Soon the entire Earth will be glowing a sullen, molten red.
This scenario of destruction is not the lurid opening sequence from a grade B movie, but rather a more or less realistic description of the fate of our planet as the Sun ends its life as a conventional star and expands to become a red giant. The catastrophic melting of Earth's surface is just one out of a myriad of events that are waiting to occur as the universe and its contents grow older.
Right now, the universe is still in its adolescence with an age of ten to fifteen billion years. As a result, not enough time has elapsed for many of the more interesting astronomical possibilities to have played themselves out. As the distant future unfolds, however, the universe will gradually change its character and will support an ever changing variety of astonishing astrophysical processes. This book tells the biography of the universe, from beginning to end. It is the story of the familiar stars of the night sky slowly giving way to bizarre frozen stars, evaporating black holes, and atoms the size of galaxies. It is a scientific glimpse at the face of eternity.
FOUR WINDOWS TO THE UNIVERSE
Our biography of the universe, and the study of astrophysics in general, plays out on four important size scales: planets, stars, galaxies, and the universe as a whole. Each of these scales provides a different type of window to view the properties and evolution of nature. On each of these size scales, astrophysical objects go through life cycles, beginning with a formation event analogous to birth and often ending with a specific and deathlike closure. The end can come swiftly and violently; for example, a massive star ends its life in a spectacular supernova explosion. Alternatively, death can come tortuously slowly, as with the dim red dwarf stars, which draw out their lives by slowly fading away as white dwarfs, the cooling embers of once robust and active stars.
On the largest size scale, we can view the universe as a single evolving entity and study its life cycle. Within this province of cosmology, a great deal of scientific progress has been accomplished in the past few decades. The universe has been expanding since its conception during a violent explosion -- the big bang itself. The big bang theory describes the subsequent evolution of the universe over the last ten to fifteen billion years and has been stunningly successful in explaining the nature of our universe as it expands and cools.
The key question is whether the universe will continue to expand forever or halt its expansion and recollapse at some future time. Current astronomical data strongly suggest that the fate of our universe lies in continued expansion, and most of our story follows this scenario. However, we also briefly lay out the consequences of the other possibility, the case of the universe recollapsing in a violent and fiery death.
Beneath the grand sweep of cosmology, at a finer grain of detail, are the galaxies, such as our Milky Way. These galaxies are large and somewhat loosely knit collections of stars, gas, and other types of matter. Galaxies are not distributed randomly throughout the universe, but rather they are woven into a tapestry by gravity. Some aggregates of galaxies have enough mass to be bound together by gravitational forces, and these galaxy clusters can be considered as independent astrophysical objects in their own right. In addition to belonging to clusters, galaxies are loosely organized into even larger structures that resemble filaments, sheets, and walls. The patterns formed by galaxies on this size scale are collectively known as the large-scale structure of the universe.
Galaxies contain a large fraction of the ordinary mass in the universe and are well separated from each other, even within their clusters. This separation is so large that galaxies were once known as "island universes." Galaxies also play the extremely important role of marking the positions of space-time. As the universe expands, the galaxies act as beacons in the void that allow us to observe the expansion.
It is difficult to comprehend the vast emptiness of our universe. A typical galaxy fills only about one-millionth of the volume of space that contains the galaxy, and the galaxies themselves are extremely tenuous. If you were to fly a spaceship to a random point in the universe, the chances of landing within a galaxy are about one in a million at the present time. These odds are already not very good, and in the future they will only get worse, because the universe is expanding but the galaxies are not. Decoupled from the overall expansion of the universe, the galaxies exist in relative isolation. They are the homes of most stars in the universe, and hence most planets. As a result, many of the interesting physical processes in the universe, from stellar evolution to the evolution of life, take place within galaxies.
Just as they do not thickly populate space, the galaxies themselves are mostly empty. Very little of the galactic volume is actually filled by the stars, although galaxies contain billions of them. If you were to drive a spaceship to a random point in our galaxy, the chances of landing within a star are extremely small, about one part in one billion trillion (one part in 1022). This emptiness of galaxies tells us much about how they have evolved and how they will endure in the future. Direct collisions between the stars in a galaxy are exceedingly rare. Consequently, it takes a very long time, much longer than the current age of the universe, for stellar collisions and other encounters to affect the structure of a galaxy. As we shall see, these collisions become increasingly important as the universe grows older.
The space between the stars is not entirely empty. Our Milky Way is permeated with gas of varying densities and temperatures. The average density is only about one particle (one proton) per cubic centimeter and the temperature ranges from a cool 10 degrees kelvin to a seething million degrees. At low temperatures, about 1 percent of the material lives in solid form, in tiny rocky dust particles. This gas and dust that fill in the space between stars are collectively known as the interstellar medium.
The stars themselves give us the next smaller size scale of importance. Ordinary stars, objects like our Sun which support themselves through nuclear fusion in their interiors, are now the cornerstone of astrophysics. The stars make up the galaxies and generate most of the visible light seen in the universe. Moreover, stars have shaped the current inventory of the universe. Massive stars have forged almost all of the heavier elements that spice up the cosmos, including the carbon and oxygen required for life. Most of what makes up the ordinary matter of everyday experience -- books, cars, groceries -- originally came from the stars.
But these nuclear power plants cannot last forever. The fusion reactions that generate energy in stellar interiors must eventually come to an end as the nuclear fuel is exhausted. Stars with masses much larger than our Sun burn themselves out within a relatively brief span of a few million years, a lifetime one thousand times shorter than the present age of the universe. At the other end of the range, stars with masses much less than that of our Sun can last for trillions of years, about one thousand times the current age of the universe.
When stars end the nuclear burning portion of their lives, they do not disappear altogether. In their wake, stars leave behind exotic condensed objects collectively known as stellar remnants. This cast of degenerate characters includes brown dwarfs, white dwarfs, neutron stars, and black holes. As we shall see, these strange leftover remnants will exert an increasingly important and eventually dominant role as the universe ages and the ordinary stars depart from the scene.
The planets provide our fourth and smallest size scale of interest. They come in at least two varieties: relatively small rocky bodies like our Earth, and larger gaseous giants like Jupiter and Saturn. The last few years have seen an extraordinary revolution in our understanding of planets. For the first time in history, planets in orbit about other stars have been unambiguously detected. We now know with certainty that planets are relatively commonplace in the galaxy, and not just the outcome of some rare or special event which occurred in our solar system. Planets do not play a major role in the evolution and dynamics of the universe as a whole. They are important because they provide the most likely environments for life to evolve. The long-term fate of planets thus dictates the long-term fate of life -- at least the life-forms with which we are familiar.
In addition to planets, solar systems contain many smaller objects, such as asteroids, comets, and a wide variety of moons. As in the case of planets, these bodies do not play a major role in shaping the evolution of the universe as a whole, but they do have an important impact on the evolution of life. The moons orbiting the planets provide another possible environment for life to thrive. Comets and asteroids are known to collide with planets on a regular basis. These impacts, which can drive global climatic changes and extinctions of living species, are believed to have played an important role in shaping the history of life here on Earth.
THE FOUR FORCES OF NATURE
Nature can be described by four fundamental forces which ultimately drive the dynamics of the entire universe: gravity, the electromagnetic force, the strong nuclear force, and the weak nuclear force. All four of these forces play significant roles in the biography of the cosmos. They have helped shape our present-day universe and will continue to run the universe throughout its future history.
The first of these forces, gravity, is the closest to our everyday experience and is actually the weakest of the four. Since it has a long range and is always attractive, however, gravity dominates the other forces on sufficiently large size scales. Gravity holds objects to Earth, and holds Earth in its orbit around the Sun. Gravity keeps the stars intact and drives their energy generation and evolution. Ultimately, it is the force responsible for forming most structures in the universe, including galaxies, stars, and planets.
The second force is the electromagnetic force, which includes both electric and magnetic forces. At first glance, these two forces might seem different, but at the fundamental level they are revealed to be aspects of a single underlying force. Although the electromagnetic force is intrinsically much stronger than gravity, it has a much smaller effect on large size scales. Positive and negative charges are the source of the electromagnetic force and the universe appears to have an equal amount of each type of charge. Because the forces created by charges of opposite sign have opposite effects, the electromagnetic force tends to cancel itself out on large size scales that contain many charges. On small size scales, in particular within atoms, the electromagnetic force plays a vitally important role. It is ultimately responsible for most of atomic and molecular structure, and hence is the driving force in chemical reactions. At the fundamental level, life is governed by chemistry and the electromagnetic force.
The electromagnetic force is a whopping 1040 times stronger than the gravitational force. One way to grasp this overwhelming weakness of gravity is to imagine an alternate universe containing no charges and hence no electromagnetic forces. In such a universe, ordinary atoms would have extraordinary properties. With only gravity to bind an electron to a proton, a hydrogen atom would be larger than the entire observable portion of our universe.
The strong nuclear force, our third fundamental force of nature, is responsible for holding atomic nuclei together. The protons and the neutrons are held together in the nucleus by this force. Without the strong force, atomic nuclei would explode in response to the repulsive electric forces between the positively charged protons. Although it is intrinsically the strongest of the four forces, the strong force has a very short range of influence. Not by coincidence, the range of the strong nuclear force is about the size of a large atomic nucleus, about ten thousand times smaller than the size of an atom (about 10 fermi or 10-12 centimeters). The strong force drives the process of nuclear fusion, which in turn provides most of the energy in stars and hence in the universe at the present epoch. The large magnitude of the strong force in comparison with the electromagnetic force is ultimately the reason why nuclear reactions are much more powerful than chemical reactions, by a factor of a million on a particle-by-particle basis.
The fourth force, the weak nuclear force, is perhaps the farthest removed from the public consciousness. This rather mysterious weak force mediates the decay of neutrons into protons and electrons, and also plays a role in nuclear fusion, radioactivity, and the production of the elements in stars. The weak force has an even shorter range than the strong force. In spite of its weak strength and short range, the weak force plays a surprisingly important role in astrophysics. A substantial fraction of the total mass of the universe is most likely made up of weakly interacting particles, in other words, particles that interact only through the weak force and gravity. Because such particles tend to interact on very long time scales, they play an increasingly important role as the universe slowly cranks through its future history.
THE GREAT WAR
A recurring theme throughout the life of the universe is the continual struggle between the force of gravity and the tendency for physical systems to evolve toward more disorganized conditions. The amount of disorder in a physical system is measured by its entropy content. In the broadest sense, gravity tends to pull things together and thereby organizes physical structures. Entropy production works in the opposite direction and acts to make physical systems more disorganized and spread out. The interplay between these two competing tendencies provides much of the drama in astrophysics.
Our Sun provides an immediate example of this ongoing struggle. The Sun lives in a state of delicate balance between the action of gravity and entropy. The force of gravity holds the Sun together and pulls all of the solar material toward the center. In the absence of competing forces, gravity would rapidly crush the Sun into a black hole only several kilometers across. This disastrous collapse is prevented by pressure forces which push outward to balance the gravitational forces and thereby support the Sun. The pressure that holds up the Sun ultimately arises from the energy of nuclear reactions taking place in the solar interior. These reactions generate both energy and entropy, leading to random motions of the particles in the solar core, and ultimately supporting the structure of the entire Sun.
On the other hand, if the force of gravity was somehow shut off, the Sun would no longer be confined and would quickly expand. This dispersal would continue until the solar material was spread thinly enough to match the very low densities of interstellar space. The rarefied ghost of the Sun would then be several light-years across, about 100 million times its present size.
The evenly matched competition between gravity and entropy allows the Sun to exist in its present state. If this balance is disrupted, and either gravity or entropy overwhelms the other, then the Sun could end up either as a small black hole or a very diffuse wisp of gas. This same state of affairs -- a balance of gravity and entropy -- determines the structure of all the stars in the sky. The fierce rivalry between these two opposing tendencies drives stellar evolution.
This same general theme of competition underlies the formation of astronomical structures of every variety, including planets, stars, galaxies, and the large-scale structure of the universe. The existence of these astrophysical systems is ultimately due to gravity, which acts to pull material together. Yet in each case, the tendency toward gravitational collapse is opposed by disruptive forces. On every scale, the relentless competition between gravity and entropy ensures that a victory is often temporary, and never entirely complete. For example, the formation of astrophysical structures is never completely efficient. Successful formation events mark local triumphs for gravity, whereas failed incidences of formation represent victories for disorganization and entropy.
This great war between gravity and entropy determines the long-term fate and evolution of astrophysical objects such as stars and galaxies. After a star has burned through all of its nuclear fuel, for example, it must adjust its internal structure accordingly. Gravity pulls the star inwards, whereas the tendency for increasing entropy favors dispersal of the stellar material. The subsequent battle can have many different outcomes, depending on the mass of the star and its other properties (for example, the rate at which the star spins). As we shall see, this drama will be repeated over and over again, as long as stellar objects populate the universe.
The evolution of the universe itself provides an intensely dramatic example of the ongoing struggle between the force of gravity and entropy. The universe is expanding and becoming more spread out with time. Resisting this evolutionary trend is the force of gravity, which tries to pull the expanding material of the universe back together. If gravity wins this battle, the universe must eventually halt its expansion and begin to recollapse some time in the future. On the other hand, if gravity loses the battle, the universe will continue to expand forever. Which one of these fates lies in our future path depends on the total amount of mass and energy contained within the universe.
THE LIMITS OF PHYSICS
The laws of physics describe how the universe works on a wide range of size scales, from the enormously large to the tremendously small. A high-water mark of human accomplishment is our ability to explain and predict how nature behaves in regimes that are vastly disconnected from our everyday Earthbound experience. Most of this expansion of our horizons has occurred within the past century. Our realm of knowledge has been extended from the largest scale structures of the universe all the way down to subatomic particles. Although this domain of understanding may seem large, we must keep in mind that discussions of physical law cannot be extended arbitrarily far in either direction. The very largest and the very smallest size scales remain beyond the reach of our current scientific understanding.
Our physical picture of the largest size scales in the universe is limited by causality. Beyond a certain maximum distance, information has simply not had time to reach us during the relatively short life of the universe. Einstein's theory of relativity implies that no signals that contain information can travel faster than the speed of light. So, given that the universe has lived for only about ten billion years, no information-bearing signals have had time to travel farther than ten billion light-years. This distance provides a boundary to the part of the universe that we can probe with any kind of physics; this causality boundary is often referred to as the horizon scale. Because of the existence of this causality barrier, very little can be ascertained about the universe at distances greater than the horizon scale. This horizon scale depends on the cosmological time. In the past, when the universe was much younger, this horizon scale was correspondingly smaller. As the universe ages, the horizon scale continues to grow.
The cosmological horizon is an extremely important concept that limits the playing field of science. Just as a football game must take place within well defined boundaries, physical processes in the universe are constrained to occur within the horizon at any given time. In fact, the existence of a causal horizon leads to some ambiguity regarding what the term "universe" actually means. The term sometimes refers to only the material that is within the horizon at a given time. In the future, however, the horizon will grow and hence will eventually encompass material that is currently outside our horizon. Is this "new" material part of the universe at the current time? The answer can be yes or no, depending on how you define "the universe." Similarly, there can be other regions of space-time that will never lie within our horizon. For the sake of definiteness, we consider such regions of space-time to belong to "other universes."
On the smallest size scales, the predictive power of physics is also limited, but for an entirely different reason. On size scales smaller than about 10-33 centimeters (this scale is known as the Planck length), the nature of space-time is very different than on larger length scales. At these tiny size scales, our conventional concepts of space and time no longer apply because of quantum mechanical fluctuations. Physics at this scale must simultaneously incorporate both the quantum theory and general relativity to describe space and time. The quantum theory implies that nature has a wavelike character at sufficiently small size scales. For example, in ordinary matter, the electrons orbiting the nucleus of an atom display many wave properties. The quantum theory accounts for this waviness. The theory of general relativity holds that the geometry of space itself (along with time: space and time are intimately coupled at this fundamental level) changes in the presence of large amounts of matter, which produce strong gravitational fields. Unfortunately, however, we do not yet have a complete theory that combines both quantum mechanics and general relativity. The absence of such a theory of quantum gravity greatly limits what we can say about size scales smaller than the Planck length. As we shall see, this limitation of physics greatly inhibits our understanding of the very earliest times in the history of the universe.
In this biography of the universe, the ten billion years already gone by represent an utterly insignificant fragment of time. We must take up the formidable challenge of establishing a time line which depicts the universally interesting events that are likely to transpire over the next 10100 years.
The number 10100 is big. Very big. Written down without the benefit of scientific notation, this number consists of a 1 followed by one hundred zeros and it looks like this:
Not only is the number 10100 rather cumbersome to write out, but it is difficult to obtain an accurate feeling for just how tremendously gigantic it is. Attempts to visualize 10100 by imagining collections of familiar objects are soon thwarted. For example, the number of grains of sand on all of the beaches in the world is often trotted out as an example of an incomprehensibly large number. However, a rough estimate shows that the total number of sand grains is about 1023, a 1 followed by 23 zeros, a big number but still hopelessly inadequate to the task. How about the number of stars in the sky? The number of stars in our galaxy is close to one hundred billion, again a relatively small number. The number of stars in all the galaxies in our observable universe is about 1022, still far too small. In fact, in the entire visible universe, the total number of protons, the fundamental building blocks of ordinary matter, is only 1078, still a factor of ten billion trillion times too small! The number of years between here and eternity is truly immense.
In order to describe the time scales involved in the future evolution of the universe, without becoming completely bewildered, let's use a new unit of time called a cosmological decade. If t is the time in years, then t can be written in scientific notation in the form
t = 10n years,
where n is some number. According to our definition, the exponent n is the number of cosmological decades. For example, the universe is currently only about ten billion years old, which corresponds to 1010<> years or n = 10 cosmological decades. In the future, when the universe is 100 billion years old, the time will be 1011<> years or n = 11 cosmological decades. The power of this scheme is that each successive cosmological decade represents a tenfold increase in the total age of the universe. The concept of a cosmological decade thus provides us with a way to think about immensely long time spans. Our aggressively large example, the number 10100<>, thus corresponds to the rather more tractable hundredth cosmological decade, or n = 100.
We can also use cosmological decades to refer to the very short but very eventful slivers of time which came immediately after the big bang. We simply allow the cosmological decade to be a negative number. With this extension, one year after the big bang corresponds to 100<> years, or the zeroth cosmological decade. One-tenth or 10-1<> years is thus cosmological decade -1, and one-hundredth or 10-2<> years is cosmological decade -2, and so on. The beginning of time corresponds to t = 0 when the big bang itself took place; in terms of cosmological decades, the big bang is understood to have occurred at the cosmological decade corresponding to negative infinity.
FIVE GREAT ERAS OF TIME
We can organize our current understanding of the past and future history of the universe by defining the following distinct eras of time. As the universe passes from one era to the next, the inventory and character of the universe change rather dramatically, and in some ways almost completely. These eras, which are analogous to geological eras, provide a broad outline of the life of the universe. As time unfolds, a series of natural astronomical disasters shape the universe and drive its subsequent evolution. A newsreel of this story might run like this:
The Primordial Era. -50 < n < 5. This era encompasses the early phase in the history of the universe. While the universe is less than 10,000 years old, most of the energy density of the universe resides in the form of radiation and this early time is often called the radiation dominated era. No astrophysical objects, like stars or galaxies, have been able to form.
During this brief early epoch, many important events took place which served to set the future course of the universe. The synthesis of light elements, such as helium and lithium, occurred during the first few minutes of this Primordial Era. Even closer to the beginning, complex physical processes set up a small excess of ordinary baryonic matter over antimatter. The antimatter annihilated almost completely with most of the matter and left behind the small residue of matter which makes up the universe of today.
In turning back the clock even further, our understanding becomes shakier. At extraordinarily early times, when the universe was unbelievably hot, it seems that very high energy quantum fields drove a period of fantastically rapid expansion and produced very small density fluctuations in the otherwise featureless universe. These tiny fluctuations have survived and grown into galaxies, clusters, and the large-scale structures that populate the universe today.
Near the end of the Primordial Era, the energy density of the radiation became less than the energy density associated with the matter. This crossover took place when the universe was about ten thousand years old. Shortly thereafter, another watershed event took place as the temperature of the universe became cool enough for atoms (or more specifically, hydrogen atoms) to exist. The first appearance of neutral hydrogen atoms is known as recombination. After recombination took place, fluctuations in the density of matter in the universe allowed it to grow into clumps without being affected by the pervasive sea of radiation. Familiar astrophysical objects, like galaxies and stars, began forming for the first time.
The Stelliferous Era. 6 < n < 14. Stelliferous means "filled with stars." During this era, most of the energy generated in the universe arises from nuclear fusion in conventional stars. We now live in the middle of the Stelliferous Era, a time period when stars are actively forming, living, and dying.
In the earliest part of the Stelliferous Era, when the universe was only a few million years old, the first generation of stars was born. During the first billion years, the first galaxies appeared and began organizing themselves into clusters and superclusters.
Many freshly formed galaxies experience violent phases in connection with their rapacious central black holes. As the black holes rip apart stars and surround themselves with whirlpool-like disks of hot gas, vast quantities of energy are released. Over time, these quasars and active galactic nuclei slowly die down.
In the future, near the end of the Stelliferous Era, a key role will be played by the universe's most ordinary stars -- the low-mass stars known as red dwarfs. Red dwarf stars have less than half the mass of the Sun, but they are so numerous that their combined mass easily exceeds the mass of all the larger stars in the universe. These red dwarfs are true misers when it comes to fusing hydrogen into helium. They hoard their energy and will still be around ten trillion years from now, after the larger stars have long since exhausted their nuclear fuel and evolved into white dwarfs or exploded as supernovae. The Stelliferous Era comes to a close when the galaxies run out of hydrogen gas, star formation ceases, and the longest-lived (lowest mass) red dwarfs slowly fade away. When the stars finally stop shining, the universe will be about one hundred trillion years old (cosmological decade n = 14).
The Degenerate Era. 15 < n < 39. After star formation and conventional stellar evolution have ended, most of the ordinary mass in the universe is locked up in degenerate stellar remnants which remain after stellar evolution has run its course. In this context, degeneracy connotes a peculiar quantum mechanical state of matter, rather than a state of moral turpitude. The inventory of degenerates includes brown dwarfs, white dwarfs, neutron stars, and black holes. During the Degenerate Era, the universe looks very different from the way it appears now. No visible radiation from ordinary stars can light up the night skies, warm the planets, or endow galaxies with the faint glow they have today. The universe is colder, darker, and more diffuse.
Nevertheless, events of astronomical interest continually sparkle against the darkness. Chance close encounters scatter the orbits of dead stars, and the galaxies gradually readjust their structure. Some stellar remnants are ejected far beyond the edge of the galaxy, while others fall in towards the center. A rare beacon of light can emerge when two brown dwarfs collide to create a new low-mass star, which will subsequently live for trillions of years. On average, at any given time, a few such stars will be shining in a galaxy the size of our Milky Way. Every so often, as two white dwarfs collide, the galaxy is rocked by a supernova explosion.
During the Degenerate Era, the white dwarfs, which are the most common stellar remnants, contain most of the ordinary baryonic matter in the universe. These white dwarfs sweep up dark matter particles, which orbit the galaxy in an enormous diffuse halo. Once trapped in the interior of a white dwarf, these particles subsequently annihilate and thereby provide an important energy source for the universe. Indeed, the annihilation of dark matter replaces conventional nuclear burning in stars as the dominant energy mechanism. By the 30th cosmological decade (n = 30) or sooner, however, the supply of the dark matter particles becomes depleted and this avenue of energy generation comes to an end. The matter inventory of the universe is then limited to white dwarfs, brown dwarfs, neutron stars, and dead, widely scattered planets.
At the end of the Degenerate Era, the mass-energy stored within the white dwarfs and neutron stars dissipates into radiation as their constituent protons and neutrons decay. A white dwarf fueled by proton decay generates approximately 400 watts, enough power to run a few light bulbs. An entire galaxy of these erstwhile stars has a total luminosity smaller than one ordinary hydrogen-burning star like the Sun. As the proton decay process grinds to completion, the Degenerate Era draws to a close. The universe -- ever darker, ever more rarefied -- changes its character yet again.
The Black Hole Era. 40 < n < 100. After the epoch of proton decay, the only stellarlike objects remaining are the black holes. These fantastic objects have such strong gravitational fields that even light cannot escape from their surfaces. The black holes are unaffected by proton decay and survive unscathed through the end of the Degenerate Era.
As the white dwarfs evaporate and disappear, the black holes slowly sweep up material and grow larger. Yet even black holes cannot last forever. They must eventually evaporate away through a very slow quantum mechanical process known as Hawking radiation. In spite of their name, black holes are not completely black. In reality, they shine ever so faintly by emitting a thermal spectrum of light and other decay products. After the protons are gone, the evaporation of black holes, almost by default, provides the universe with its primary source of energy. A black hole with the mass of the Sun lasts for about 65 cosmological decades; a large black hole with the mass of a galaxy takes about 98 to 100 cosmological decades to evaporate. All black holes are thus slated for destruction. The Black Hole Era is over when the largest black holes have evaporated.
The Dark Era. n > 101. After a hundred cosmological decades, the protons have long since decayed and black holes have evaporated. Only leftover waste products from these processes remain: photons of colossal wavelength, neutrinos, electrons, and positrons. An odd parallel exists between the Dark Era and the Primordial Era, when the universe was less than a million years old. In each of these eras, distantly separated in time, no stellarlike objects of any kind are present to generate energy.
In this cold and distant future, activity in the universe has tailed off dramatically. Energy levels are low and the expanses of time are mind-boggling. Electrons and positrons drifting through space encounter one another and occasionally form positronium atoms. These late-forming structures are unstable, however, and their constituent particles must eventually annihilate. Other low-level annihilation events can also take place, albeit very slowly.
Copyright © 1999 by Fred C. Adams and Gregory Laughlin