| Mercury, January/February 2000 Table of Contents 
The present epoch is important to us for obvious reasons. But what of those in the far distant future? Can the excitement and structure of now be perpetuated into the dark emptiness of then? Fred Adams and Greg Laughlin At this dawn of a new century, the latest advances in physics and astronomy allow us to understand our Universe with unprecedented clarity. The entire life story of the cosmos, from its singular inception at the Big Bang to its long and gradual slide into the far future, now rests on a basic but nonetheless solid foundation. A continuing theme within this intricately detailed biography of the Universe is the underlying conflict between the attractive force of gravity and the tendency for physical systems to evolve toward more disorganized conditions. Entropy provides a measure of disorder within a physical system-whenever entropy is generated, the amount of disorder increases. In the broadest sense, gravity pulls things together and thereby organizes physical structures. Entropy production opposes this order 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 and ultimately drives the evolution of the entire Universe, including the life cycles of its constituent stars and galaxies. Our Universe is now ten to fifteen billion years old. In the grander perspective provided by this long-running cosmic saga, the past history of our Universe represents an utterly insignificant fragment of time. To face the formidable challenge of establishing a time line for the far future, we need a convenient way to denote aggressively large time intervals. For this role, we use a time unit called a "cosmological decade." When an interval of time, in years, is expressed in scientific notation, say 10h years, the exponent h (the Greek letter Eta) is called the cosmological decade. Since the Universe is just over ten billion or 1010 years old, we are now living in the 10th cosmological decade. When the Universe is ten times older, 1011 years, the Universe will experience its 11th cosmological decade. For this journey into the future, we report on the battle between gravity and entropy over the next 100 cosmological decades. The past and future history of the Universe can be organized into five distinct eras of time. As the Universe passes from one era to the next, its inventory and character change dramatically, and in many ways almost completely. These eras are analogous to the geological eras that describe the history of our planet, and they delineate a broad general outline for the life of our Universe. As time unfolds, a series of natural astronomical disasters punctuates this cosmic newsreel and shapes the subsequent development of the Universe. And throughout this evolutionary history, both past and future, the force of gravity continually engages in a cosmic battle with entropy. The Primordial Era We cannot actually describe the beginning of time, when our Universe burst into existence (we would need a theory that incorporates both quantum mechanics and general relativity at the same time). However, we can pick up the story just after our expanding Universe is violently launched on its trajectory towards the future. During its first moments of existence, the Universe experienced a period of fantastically rapid expansion. The rate at which space was created was so great that regions which were initially in causal contact found themselves being separated much faster than they could be connected by light signals. This inflationary epoch (see "Armchair Astrophysics," Nov/Dec 1999, p. 6, and in this issue, p. 8) explains many of the observed properties of the Universe, including its large size, its striking uniformity, and the precise flatness of its space-time geometry. During these earliest instants of creation, entropy and disorder initially gain the upper hand. When the Universe inflates, the force of gravity loses major ground as everything in the Universe flies apart at fantastic speeds. This first engagement between gravity and entropy is over long before the Universe is even a nanosecond old, but this initial one-sided outcome ensures that gravity cannot recollapse the Universe for at least billions of years, and most likely, not ever.
Development of structure in an early epoch of the Universe. Image courtesy of Michael Norman, NCSA.
After inflation runs its course, the Universe settles down into a state of more leisurely expansion, and complex physical processes set up a tiny excess of ordinary baryonic matter over antimatter. At extreme temperatures, matter lives in the form of microscopic particles called quarks and antiquarks, rather than the more familiar protons and neutrons we know today. The asymmetry between matter and antimatter is quite modest-for every 30 million antiquarks in its inventory, the Universe has 30 million and one quarks of "ordinary" matter. But the antimatter annihilates almost completely with most of the matter and prodigious amounts of entropy are produced. Only a small residue of ordinary matter survives to build the stars and galaxies in the Universe today. A few minutes later, the synthesis of light elements-such as helium, deuterium, and lithium-helps shape the future nuclear inventory of our Universe. During this short-lived epoch, the strong nuclear force acts as a surrogate for gravity and gains an important victory in the name of consolidation. The strong force combines about one fourth of the available protons and neutrons into nuclei of elements heavier than hydrogen. This advantage is soon compromised, however, when entropy is produced by the annihilation of electrons and positrons, and by the radioactive decay of the remaining neutrons. After these cosmic birth throes are completed, the Universe enters an extended phase of smooth and peaceful expansion, which continues for many uneventful millennia. During the first 10,000 years, most of the Universe resides in the form of radiation. The radiation fields are so pervasive and energetic that the formation of astronomical structures is seriously inhibited; no planets, stars, or galaxies grace the cosmos during this early era. As the Universe ages, the density of radiation grows more diffuse relative to the matter, which then begins to consolidate into astronomical entities. When the temperature of the Universe grows cool enough for electrons to attach themselves to atomic nuclei, neutral atoms spring into existence and budding cosmic structures can grow larger without being affected by the background sea of radiation. Gravity thus begins its organizational efforts and makes vital progress in its continuing struggle against entropy. The Primordial Era ends as stars and galaxies start to form for the first time. The Stelliferous Era Stelliferous means "filled with stars." Most of the energy generated in our Universe today arises from nuclear fusion in conventional stars. At the current cosmological age of ten billion years, we now live in the middle of the Stelliferous Era, when stars are actively forming, living, and dying. As the Universe reaches its adolescence in the early Stelliferous Era, gravity finally makes some headway against the universal tendencies toward disorganization. During the first billion years, galaxies are created as gravity overcomes the background expansion of the Universe. Gravity also organizes these galaxies into bound clusters and cosmic structures on even larger size scales. Many freshly formed galaxies experience violent early phases in connection with their rapacious central black holes, which represent gravitational forces so strong that even light cannot escape their wrath. As these black holes rip apart stars and surround themselves with whirlpool-like disks of hot gas, vast quantities of energy and entropy are released. Inside the galaxies, huge clouds of gas are brought together by gravity and new stars are forged within them. Gravity wins yet another battle as planets coalesce within nebular disks that orbit about the nascent stars. The Stelliferous Era thus witnesses the formation of many types of cosmic structures-planets, stars, galaxies, and clusters-all the result of gravity's relentless and constructive efforts. The general theme of competition between gravity and entropy underlies the formation of each of these astronomical structures. The very 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 and the formation of astrophysical structures is never completely efficient. On every scale, the indefatigable competition between gravity and entropy ensures that victory is often temporary, and never absolute. A successful formation event marks a local triumph for gravity, whereas failed attempts at formation represent victories for disorganization and entropy. When star formation takes place within an interstellar cloud of gas and dust, for example, only a small fraction of the available material is incorporated into a new generation of stars. Young and forming stars produce intense jets of hot gas which inject energy into the parent cloud and prevent most of its mass from making new stars.
A galactic arrangement. This image, only a small section of one collected by the first telesope of ESO's VLT project, details life in the vicinity of quasar PB5763; most notable is the presence of this peculiar, distant cluster of galaxies. Indeed, in the Universe's hierarchy, galaxy clusters are the overwhelming norm. Image courtesy of the European Southern Observatory.
After an astronomical body forms, entropy production and the force of gravity regroup and continue their struggle on a redrawn field of battle. Our own Sun provides an immediate example of this ever-present competition (see "Of Solar Matters"). The great war between gravity and entropy guides the future evolution and the ultimate fate of all stellar objects; indeed, the interplay between these two competing tendencies drives most of stellar evolution. After a star burns through its nuclear fuel and reaches the end of its conventional life, the ongoing conflict reaches a more desperate level of competition. The impoverished stellar body must face astronomical death with radical adjustments to its internal structure. Gravity pulls the star inwards, whereas the tendency for increasing entropy favors dispersal of the stellar material. This stellar endgame can have many different outcomes, depending on the mass of the star and its other properties. For the vast majority of stars, the two warring parties reach a kind of armistice by producing a degenerate stellar remnant called a white dwarf. Within these dense stellar objects, the inward pull of gravity is exactly balanced by pressure forces arising from Heisenberg's uncertainty principle acting on electrons. The resulting white dwarfs live in this deadlock for many cosmological decades to come. The creation of a massive star, with more than ten times the mass of the Sun, is a rare event. Soon after its birth, a massive star quickly depletes its store of nuclear fuel and then explodes in a fiery burst called a supernova (see "Unveiling Black Holes in a Supernova Cauldron," Nov/Dec 1999, p. 8). During these highly dramatic death scenes, another type of compromise is negotiated. A sizable majority of the massive star is dispersed across interstellar space by a shock wave resulting from the supernova detonation. The remaining material is tightly concentrated into a dense stellar remnant bound by a strong gravitational field. In most cases, the resulting remnant is a neutron star supported by the degeneracy pressure of its constituent neutrons. Under special circumstances, a space-warping black hole is forged and gravity achieves a more decisive victory. In still other cases, explosive stellar death leaves behind no remnant whatsoever and thermodynamics claims a clean decision.
Spiral stellar show. In this VLT UTI image of the spiral galaxy NGC 2997 in the southern constellation Antlia (The Air Pump), the spiral arms are clearly overdencse with bright stars and are, in fact, regions of vigorous star formation activity. Such is true of all spiral galaxies. Image courtesy of the European Southern Observatory.
The future portion of the Stelliferous Era belongs to the low mass stars known as red dwarfs, by far the most common stars. Although they contain less than half the mass of the Sun, red dwarfs are so numerous that their combined mass easily exceeds that of all the larger stars in the Universe. These red dwarfs are the utmost misers in fusing their hydrogen into helium. They hoard their energy and will still be around ten trillion years from now, long after larger stars have exhausted their nuclear fuel reserves and condensed 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 red dwarfs slowly fade away. The stars finally stop shining during cosmological decade 14, when the Universe is about 100 trillion years old. The Degenerate Era Although gravity is comfortably winning its battle at the current cosmological epoch, thermodynamics and entropy production catch up as the Universe continues to age. After the end of star formation and conventional stellar evolution, most of the ordinary mass in the Universe is locked up in degenerate stellar remnants. The resulting cast of degenerate characters includes brown dwarfs, white dwarfs, neutron stars, and black holes. In an astronomical context, degeneracy connotes a peculiar state of matter, rather than a state of moral depravity. In the dense interior of a degenerate stellar remnant, the quantum mechanical uncertainty principle (due to Heisenberg) compels particles to experience a kind of quantum claustrophobia, which induces particle motions that, in turn, provide the pressure that supports these bizarre objects. During this Degenerate Era, the Universe grows colder, darker, and more diffuse. A dearth of stellar radiation remains to light up the night skies, warm the planets, or endow galaxies with the faint glow they have today. Nevertheless, gravity continues to induce events of astronomical interest that sparkle against the darkness. A rare beacon of light emerges when two brown dwarfs collide to create a new low-mass star. The resulting red dwarf subsequently lives as an "ordinary" hydrogen-burning star 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. Still other collisions can fabricate strange new types of stars that burn helium and carbon. Stars fill up an extraordinarily tiny volume of interstellar space. The density of our local galactic environment is akin to that of individual sand grains surrounded by miles of empty space. And because stars traverse the interstellar gulfs at a glacial pace, close passages between stars are exceedingly rare. In the vast and desolate stretches of time available during the Degenerate Era, however, chance close encounters will scatter the orbits of dead stars, and the Galaxy must gradually readjust its dynamical structure. As scattering events redistribute the Galactic wealth of energy, most stellar remnants are ejected far beyond the Galaxy, while an unfortunate few fall towards the center. The apparent gravitational victory represented by galaxy formation thus turns out to be fleeting when viewed within the grand scheme of time. During the 19th and 20th cosmological decades, as the scattering of stellar remnants approaches completion, galaxies evaporate most of their stars into intergalactic voids. Eventually, the galaxies cease to exist as distinct astronomical entities, and gravity's work is largely undone on the galactic scale. White dwarfs-the most common stellar remnants-contain most of the ordinary baryonic matter during the Degenerate Era. While the galaxy remains intact, these white dwarfs sweep up dark matter particles, which orbit the galaxy in an enormous diffuse halo. Once trapped within the interior of a white dwarf, the dark matter subsequently annihilates and thereby provides an important power source for the Universe. In this dim future, the annihilation of dark matter replaces nuclear burning as the dominant source of energy in stars. But as the galaxies are destroyed and the dark matter supply becomes depleted, this line of energy generation and entropy production shuts down its operations.
The creation of a white dwarf. The expanding cloud of gas shown in this Hubble Space Telescope image surrounds a dying star and its brighter companion. near the end of its life, the now fainter star ejected much of its outer layers to reveal its hot core, which will become a white dwarf. The layers of gas surrounding the stellar "remnant" are energized by the intense radiation emitted by the bared core. Image courtesy of the Hubble Heritage Team (STScI/AURA/NASA).
In the ongoing war between gravity and thermodynamics, white dwarfs and other degenerate objects represent a stalement of stellar evolution. But even these tightly bound remnants are transient. At the end of Degenerate Era, the mass-energy stored within white dwarfs and neutron stars dissipates into radiation as their constituent protons and neutrons decay into smaller particles. The idea of proton decay is predicted in general terms by theoretical physics, but experiments have thus far only set lower bounds on the process. For a reasonable proton lifetime of 37 cosmological decades, 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 shines with less light than one ordinary hydrogen-burning star like our Sun. The net result of proton decay is that stellar remnants evaporate away and colossal amounts of entropy are generated. As proton decay grinds to completion, gravity loses this conflict in the end and the Degenerate Era quietly draws to a close. The Universe grows darker and more rarefied. The Black Hole Era After the protons decay, the only stellar-like objects remaining are the black holes, which doggedly push forward into the next era. These fantastic objects have such strong gravitational fields that even light cannot escape from their surfaces. As a result, black holes are unaffected by proton decay and survive unscathed through the end of the previous Degenerate Era. As white dwarfs evaporate and disappear, black holes slowly sweep up material and grow larger. Although the formation of these dark stellar corpses apparently marks a definitive score for the gravitational forces, this victory also turns out to be illusory. Even black holes cannot last forever. They eventually evaporate away through a painstakingly slow quantum mechanical effect known as Hawking radiation. In spite of their name, black holes are not completely black. In reality, they shine ever so faintly by emitting photons, neutrinos, gravitons, 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 "only" about 65 cosmological decades. A larger black hole with the mass of a million Suns will be destroyed in 83 cosmological decades. Even an enormous black hole with the mass of an entire galaxy evaporates into oblivion within 98 to 100 cosmological decades. All black holes are thus slated for destruction. When the mass-energy of these bodies radiates away, large quantities of entropy are generated, and gravity loses further ground. As black holes evaporate, their effective temperatures rise, and their demise accelerates. In the final moments, the Hawking evaporation process releases so much energy in such a short time that black holes leave the cosmos with a veritable explosion. These spectacular events put an exclamation point on the eventual defeat of gravity on this black hole battlefield. After the largest black holes have made their explosive exits from the Universe, the Black Hole Era is over and gravity's magnificent work is completely obliterated.
The Dark Era After 100 cosmological decades, protons have long since decayed and black holes have evaporated. In the final enveloping desolation of the Dark Era, only the leftover waste products from previous astrophysical processes remain: photons of colossal wavelength, neutrinos, electrons, and positrons. Perhaps weakly interacting dark matter particles and other exotica are present as well. The Universe is empty, dark, and diffuse. In this frigid and far-distant future, astrophysical activity in the Universe tails off dramatically. Energy levels are low and the expanses of time are mind boggling. Electrons and positrons drifting through the barren wasteland of space occasionally encounter one another. A last-minute rally by the forces of organization can then lead to the formation of positronium atoms, which consist of electrons and positrons in orbit about each other. Following the now familiar theme of non-permanence, these late-forming structures are unstable. The giant atoms slowly spiral into ever smaller configurations until the two particles annihilate in a burst of radiation. Other low-level annihilation events can also take place and generate more entropy, albeit very slowly. Gravity thus fumbles its final opportunity for achieving order and its fourth quarter comeback is destined to fail. As the Universe reels through the shadows of the Dark Era, entropy wins yet again. Compared to its profligate past, the Universe now lives a conservative and low-profile existence. Or does it? The apparent poverty of this distant epoch could be due to our limited powers of extrapolation, rather than a true case of deterioration and dilapidation. It remains possible that this uncertain future could allow for the development of new types of complexity, and perhaps even new types of life. As our dying Universe cascades through its five cosmological eras, only time will tell. FRED ADAMS and GREG LAUGHLIN are co-authors of the recent book The Five Ages of the Universe: Inside the Physics of Eternity (New York: The Free Press, 1999). Adams is a physics professor at the University of Michigan in Ann Arbor, and Laughlin is a staff scientist at NASA Ames Research Center in Moffett Field, California.Their email addresses are fca@umich.edu and gpl@ism.arc.nasa.gov, respectively. Of Solar Matters 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 a few kilometers across. This disastrous collapse is prevented by pressure forces which push outward to support the Sun. This pressure that holds up the Sun arises from the energy of nuclear reactions taking place deep within 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.
Image courtesy of SoHO/EIT consortium. SoHO is a project of international cooperation between ESA and NASA.
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 lightyears 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 compromised, and either gravity or entropy overwhelms the other, the Sun would end up either as a small black hole or a very diffuse wisp of gas. This same state of affairs—a finely-tuned balance between gravity and entropy—determines the structure of all the stars in the sky. Universal Drama The expansion of the Universe itself provides an intensely dramatic example of the ubiquitous struggle between the force of gravity and entropy. As the Universe expands and becomes more spread out, gravity resists this trend and tries to pull the expanding Universe back together. The particular fate which our future holds depends on whether gravity wins or loses this cosmic battle, whose outcome depends on the total amount of mass and energy contained within the Universe. Current astronomical data strongly suggest that gravity has already lost this critical conflict and our fate will be determined by a continued and unending expansion. |
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