Synopsis
Is it possible to know the complete life story of the universe from beginning to end? A study has defined the essential mechanisms of astrophysics so that science can determine the five ages the universe will go through. This text investigates the nature of complexity in each of the five eras.
Extrait
Introduction
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. Becau...
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