COSMOLOGY

The night sky is a primal wonder whose infinite nature spurs a longing to understand human existence. The realization that they are beneath a vastness and majesty beyond their personal experience impels people to attempt to know themselves and their place in all that there is. This is a religious impulse and is also the impulse behind cosmology.

From Astronomy to Cosmology

Cosmology is, however, a uniquely modern science of the history, structure, and dynamics of the universe. Although astronomy is a transliteration from the Greek, the word cosmology is a seventeenth-century coinage from an imaginary Greek term. It thus denotes a new, uniquely scientific way to deal with primal wonder about the night sky that was designed to replace the myths that represented primordial efforts to respond to that wonder.

The myths on which traditional societies were built were inspired by and speak to the origins of humankind and its place in the universe. Because the nature of the firmament is unknowable by the direct senses, until recently those myths were untestable and therefore perennial. The birth of technology changed that situation. Tools that take advantage of natural laws and allow humankind to manipulate those laws changed what was knowable. Systematic observations of the motion of the planets that were motivated by Tycho Brahe's (1546–1601) desire to find God's perfection in the sky led Johannes Kepler (1571–1630) to devise a model of the solar system with the sun at its center. Timepieces and levers set the stage for Isaac Newton's (1643–1727) grasp of gravity and its implications for the cosmos. Newton's calculus, a kind of conceptual technology, captured physical law with a generality and precision of unprecedented scope.

Today fossil light from the beginning of time is collected by immense machines both on the earth and in space and analyzed electronically to reveal the most intimate details of the universe and its beginnings. Modern cosmology weaves a creation story that passes the tests of science. The same methodology that has laid out physical truth and made possible the ability to control nature has allowed humankind to know the extent and origin of all that there is. In the process the inevitable imperial nature of science has taken over, displacing the old myths with cold certainty and weakening the ground beneath religions, belief systems, and structures of morality. As science replaces older foundational beliefs, it becomes complicit in the moral confusion of the modern age.

Can heaven survive the heat death of the universe? Will the cherished views of earlier cultures on the origin and meaning of human existence be another casualty of modern science? As astronomers divine the mysteries of the origin and evolution of the universe, are they culpable for the elimination of worldviews that may have had legitimate purposes but did not stand up to the scrutiny of scientific methodology?

The Emergence of the Big Bang Theory

In 1929 the astronomer Edwin Hubble (1889–1953) announced that the recessional velocities of galaxies are proportional to how far away they are. The farthest galaxies were said to be receding the fastest, as measured by the Doppler shifts of their emitted light. The Doppler shift is the stretching of light waves from objects that are receding from the earth at high velocity. Hence, distant galaxies appear redder. The constant of proportionality (between distance and recession velocity) became known as the Hubble constant. The implications of this relationship are profound. The simplest explanation of it is that at some time in the very distant past all the galaxies were packed together. The reciprocal of the Hubble constant is approximately the age of the universe: about 14 billion years.

How far back in time is it possible to see? What immense, sophisticated, and expensive instruments are required to see something as esoteric as the first light of the universe? In fact, one can see the radiation from the explosion of the Big Bang in almost every living room in the United States and almost any household in the world. All that it is necessary to do is to unplug the cable from a television set and set it to a channel where there is no broadcast. Part of that chaotic, somewhat disturbing pattern known as snow is the microwave echo of the Big Bang, which was released when the universe became transparent 200,000 years after it was born.

In 1965 Arnio Penzias (b. 1933) and Robert Wilson (b. 1936) of Bell Laboratories were working on a state-of-the-art antenna for the emerging technology of satellite telecommunications. Wherever they pointed their antenna in the sky, they heard a constant hum. In one of the most serendipitous discoveries in the history science, the cosmic microwave background (CMB) radiation had been found, and at a frequency exactly in agreement with the theory of the Big Bang (Sciama 1973). (Penzias and Wilson won the 1978 Nobel Prize for their discovery.) Since the Big Bang space has been cooling as it expands. If one runs the movie of the evolution of the universe backward to the point where all the galaxies coalesce, one finds that the "primeval egg" began expanding at nearly the speed of light 14 billion years ago. From the inferno of creation to the present the science of thermodynamics predicts that space should have cooled to 2.7 degrees Celsius above absolute zero. The frequencies Penzias and Wilson heard in the CMB correspond exactly to that temperature.

Cosmology and nuclear physics began to merge when scientists started to consider the first three minutes of the universe, a point made clear in Steven Weinberg's The First Three Minutes: A Modern View of the Origin of the Universe (Weinberg 1977). During that time all the fundamental particles—the neutrons, protons, and electrons that make up atoms and the rest of the fundamental particle zoo—were formed. As the universe expanded and cooled, mostly hydrogen nuclei were formed, but a fraction of them teamed with neutrons to make helium, deuterium, and lithium. According to nuclear physics, the relative amounts of each of these elements are quite sensitive to the conditions of the early universe. From that period of nucleosynthesis right after the Big Bang nuclear physics predicted that the universe should have been formed with about 76 percent hydrogen, 24 percent helium, and less than 1 percent heavier elements. In an affirmation of the Big Bang theory spectroscopists have shown that wherever one looks in the universe those ratios prevail.

With the evidence provided by Hubble's observation that the universe is expanding, the measurement of the CMB, and the correct prediction of nucleosynthesis during the first three minutes of the universe the Big Bang has been accepted as the real story of the universe. However, adjustments have been made to it.

The Structure of the Universe

A map of the universe as it is currently understood is shown in Figure 1. The bottom of the chart shows the center of the earth, and the top represents the farthest that can be seen: the CMB. The scale is logarithmic so that any quarter inch on the chart represents ten times the distance of the quarter inch below it. Two populations of artificial satellites populate space immediately above the earth: low orbit satellites at about 200 miles and geostationary satellites at 23,000 miles. The planets, asteroid belt, and Kuiper belt can be seen in the bottom half of the chart. The Kuiper belt is a vast ring of large comets that orbit the sun outside Pluto. Midway on the chart is the Oort cloud, a much larger spherical shell of comets that are bound loosely to the sun. Nearby stars, galactic stars, and the center and edge of the galaxy follow as one moves outward. The Milky Way is part of the local group, a loose collection of about two dozen galaxies that are gravitationally bound. Beyond that is the large-scale structure of the universe. Galaxies fill the heavens in these vast reaches, but they are not randomly placed. Not only do they form clusters, there are coherent structures that are significant fractions of the size of the universe. The Great Wall is one such structure: a long filament of galaxies that is 300 million light-years from the earth.

In fact, the large-scale structure of the universe is foamy and filamentary, as shown in Figure 2 (Gott et al. 2004). In this figure each point represents a galaxy: The foamy nature of the universe can be seen out to 2.7 billion light-years in this diagram. The foam seems to become less dense farther from the earth or, equivalently, farther back in time. In fact, it extends as far back as can be seen. The blank wedge-shaped regions are places in the sky where it is impossible to see out of this galaxy. This is the plane of the Milky Way.

The foamy structure of the universe must be indicative of the small, quantum asymmetries that were imparted during the Big Bang. One can imagine that a perfectly spherical explosion would result in a smooth, uniform universe with no structure. However, somehow small asymmetries must have been present and were amplified by the force of gravity as the universe evolved and expanded. The structure that is seen is not consistent with the amount of matter and energy observed in the universe. There does not appear to be enough gravity to hold it all together, and this is where dark matter comes in.

Dark Matter

The direct evidence for dark matter is simple. Galaxies usually exist in gravitationally bound clusters of a few to several dozen. The motion of the galaxies around their common center, a matter of Newtonian physics, is completely inconsistent with the amount of matter that is seen. The motion of individual galaxies within a cluster can be explained only by the existence of an additional strong gravitational field. In fact, every galaxy or cluster must have a spherical halo of matter around it that is undetectable with electromagnetic radiation but is five times more abundant than the matter in the galaxies themselves. Little else is known about this mysterious cold dark matter, but its existence is generally accepted and there is an ongoing effort to detect it directly.

The Cosmic Microwave Background and Dark Energy

The microwave background also has structure. If the universe began as a microscopic primeval egg, it must have undergone vigorous quantum fluctuations in energy, shape, and even dimensionality. The imprint of those quantum fluctuations is seen in the spatial structure of the microwave background. To an incredible degree, however (about one part in a million), the microwave background is uniform. This implies that at one time the universe was small enough that it could come to thermal equilibrium but then grew rapidly, freezing in both the large-scale isotropy and the quantum fluctuations. This freezing in would have happened during an inflationary period when the universe accelerated outward at an exponential rate.

This is a decidedly nonintuitive move for a universe to make. What caused the universe to accelerate in the first place? In the old standard model of the Big Bang, without inflation, a prime mover is required, but only at the instant of creation. The explosion casts matter and energy outward, expanding under this initial, unimaginable force but eventually slowing down as gravity pulls everything back to the center. The central question in cosmology at the start of the twenty-first century has been, What is the density of the universe? If the density is too low, gravity will never win and the universe will expand forever. If the density is high, beyond a critical point, the universe eventually will slow to a stop and begin to fall in on itself. The end is the Big Crunch, perhaps followed by reincarnation as the cycle begins all over again.

Neither of these scenarios appears to be the likely fate of the universe, however, based on the smooth nature of the microwave background radiation. Instead, the universe appears to exist in a state in between these scenarios, like a penny that has landed on its edge. It seems that the universe is flat, a spacetime geometry that means that the universe will continue to expand forever, although more and more slowly, approaching a stop at t equals infinity. The problem is that when one adds up all the mass and energy and dark matter, the universe is shy of the total amount required for a flat geometry by a factor of two.

This is where two problems are solved at once by the inflationary theory. There are quantum mechanical reasons to suspect that the vacuum itself has energy. That is, there is some underlying fabric that wildly undulates, popping fundamental particles into existence from nothing and swiftly returning them to the weave. Those particles have been observed, although the nature of the fabric and the energy it imparts to the vacuum remain mysterious. At one time the physicist Albert Einstein (1879–1955) postulated that energy, which he inserted into his equations as a cosmological constant. His goal was to produce a model of a steady-state universe, infinite and isotropic in time and space, largely because he felt that that was more aesthetically reasonable than a universe that began with a Big Bang. Although Alexander Friedman (1888–1925) showed that the Big Bang was a valid solution to Einstein's equations, Einstein abhorred that theory. However, he abhorred the ad hoc adjustment to his equations even more, and when the empirical evidence for a Big Bang could not be ignored, he declared the cosmological constant his biggest mistake. On new empirical grounds it must be included again, although a fundamental theory of its origins probably will require the achievement of a grand unified theory, a theory of everything, that string theory seems to promise for the future (Greene 2003).

This quantum vacuum energy is called the dark energy, and there is twice as much of it as there is of everything else that can be seen and measured. The dark energy has been implicated in the inflationary era of the universe and may have been the driving force for it. Still, aside from problems with identifying the quantum vacuum energy with the missing energy of the universe, the invention of the dark energy seems contrived.

There has, however, been an important recent discovery whose status has increased steadily. By very carefully measuring the red shifts, and hence the recessional velocities of galaxies deep into the universe, cosmologists have been able to map the evolution of the expansion rate of the universe. They have found that although the universe slowed down steadily after inflation, as a result of gravity, about 5 billion years ago it began to speed up again (Greene 2003). Today not only is the universe expanding, its expansion rate is increasing. The universe is accelerating, and something must be causing that. The culprit is the dark energy that permeates the vacuum.

The Story of The Creation

The newest creation story is surely not the final answer. A final theory will emerge only when there is a full understanding of how gravity is related to the other three forces and when the theories of gravitation and quantum mechanics are united. Enormous conceptual progress has been made with the development of string theory and its big brother, M (membrane) theory. String theory envisions particles as one-dimensional strings that vibrate not only in the known universe but also within six other hidden dimensions that are curled too small to be seen but that exist at every point in space (Greene 2003). A majority of cosmologists and theoretical physicists consider string theory the most promising and testable avenue for developing a true "theory of everything."

In the beginning there was an incredibly hot multidimensional nugget that was about one Planck scale (10^?33 centimeters) in length. According to string theory, this Planckian egg is the smallest that anything can be. Squeezing it tighter makes it bigger and cooler. String theory avoids the singularity of the conventional Big Bang theory by considering the behavior of matter and energy at the very finest scales. It cannot say, however, what may have existed before this state, although this is an area of ongoing research.

The nugget had the entire mass of the universe in it, and it underwent transitions in its topography rapidly and randomly. Between 10^?36 and 10^?34 seconds after the start of time three dimensions suddenly broke free of their confining strings and inflated ferociously in a violent, exponential expansion. Alan Guth (b. 1947) of the Massachusetts Institute of Technology first showed that inflationary expansion of the universe represents a particular solution to Einstein's equations and can explain a deeply perplexing aspect of the CMB: its overall isotropy. The remaining dimensions stayed curled together, fundamentally influencing the nature of the particles and forces that became manifest in the three macroscopic dimensions. At one-hundred-thousandth of a second quarks began to clump into protons and neutrons.

Meanwhile, as the universe cooled, something strange was happening to the force within it. It was born with only one force, but as it cooled, it underwent phase transitions by which new forces were cleaved from the original one. Ultimately, for reasons that are not understood, the universe ended up with four forces: gravitation, electromagnetism, and the weak and strong nuclear forces. From a hundredth of a second to three minutes after the Big Bang the elements were formed.

At 200,000 years the universe had cooled enough for stable atoms to form. In other words, the universe cooled from a plasma to a gas and became transparent. The photons streaming outward at that time are the blips seen on television sets.

Perhaps a billion years after the Big Bang galaxies began to form. The universe continued to expand at close to the speed of light, but the relentless action of gravity caused its expansion to slow. However, 9 billion years after the origin of the universe its expansion began to accelerate, most likely as a result of the repulsive force of the quantum vacuum energy. If this trend continues, the acceleration of the universe will cause galaxies to fly ever more rapidly away from one another. Some day even the closest galaxy will be too far away to see; the galaxies will be beyond the light horizon. Some day all the fuel for the stars will be used up, first hydrogen and then helium, carbon, and oxygen, until the last sun flickers and the universe is plunged into eternal darkness.

The Ethical and Political Dimensions of Cosmology

For many scientific disciplines the cause-and-effect relationship between scientific outcomes and the well-being of people is of great importance: Scientific results and their technological progeny are the dominant forces shaping the future of the world. The role science will play in determining the quality of life for every human being on the planet is of course determined by the elite that funds science. In this way all scientific enterprise is embedded in the greater moral problem of how individuals and groups should conduct themselves. Is it better for the powerful to channel their efforts solely for competitive self-benefit or to distribute knowledge and technology among all people? What are the consequences of pushing technologies on societies that may not want them? In some fields these issues spring directly from contemplation of the promise and implications of their projects. If it is possible to choose the human qualities of a person through genetic engineering, who will decide what those qualities will be, and to whose progeny will they go? Other subjects may be further afield, but the conceptual shift forced on science by the quantum nature of the infinitesimal in the 1920s has led to the most transforming technology in history: electronics.

Cosmology evokes a sense of the most benign and pure of sciences. The fascination of contemplating what is out there, combined with the fact that humankind cannot do anything to it, lends the study of space its alluring innocence. That of course is the old view. Cosmology is coming dangerously close to asking God rather direct questions.

To some degree scientific disciplines can be categorized by how influential ethics is thought to be in a particular field. Indeed, the ethical weight of astronomy, compared with that of genetics, lends it a kind of lightness and purity that is perceived by the people who fund it. Virtually everyone on the planet has gazed up and rested briefly in that human space where one wonders what it all is and what it all means. The pursuit of these wonders feels ennobling, partly because of the human space it comes from and partly because it is difficult to imagine how contemplation of the stars could alter the fate of humankind.

The modern science of cosmology is perhaps as far removed from the day-to-day concerns of humanity as any human endeavor can be. Futurists may conjure colorful uses for the discoveries of scientific research on the nature and origin of the universe, but this is not a matter of dealing with transistors or life-extending drugs. No one argues that cosmology is studied because of its economic impact. However, this does not mean that the study of the universe lacks an economic impact. The latest discoveries in astronomy have always depended on progress in computer, space, and detector technology (Tegmark 2002). Synergism between the astronomical sciences and industrial and military concerns is strong and growing, and both enterprises benefit.

Philosophical Issues

As self-aware beings people share a special, emergent property of the universe: consciousness. Is the quality of this aspect of nature in some way different from, say, the way space is curved as a result of the distribution of mass in the universe? What is special about the way living, replicating systems employ available resources to thrive, evolve, and produce beings that are capable of studying the deepest questions about their existence? Is mind a statistically unlikely property to have emerged from a universe with 1,000,000,000,000,000,000,000 solar systems? Or is the quality of mind ubiquitous and unifying like gravitation or other universal physical laws? Science is engaged in exploring the origin and nature of the universe as it never has before, along with the role of life and consciousness within it.

Every culture has a cosmology. Science has become the sine qua non of truth, and its revelations are taken as gospel. The insights of science into the nature of the universe therefore are assumed to or allowed to subsume all prior knowledge. It is incumbent on all scientists to ask whether their work leads to living together in harmony or interferes with that harmony. Where is the role of heart or spirit in the exploration of the cosmos or, for that matter, in any scientific endeavor? The scientific study of the origin and structure of the universe is a journey that has begun to yield answers to questions that once were the purview of religion and myth. What is done with this knowledge and what its ultimate meaning may be should be an essential component of the science of cosmology.

MARK A. BULLOCK

SEE ALSO Astronomy.

BIBLIOGRAPHY

Greene, Brian. (2003). The Elegant Universe: Superstrings, Hidden Dimensions, and the Quest for the Ultimate Theory, 2nd edition. New York: Vintage. A superb popular treatment of the physics of the ultra-small.

Gott, J. Richard; Mario Juric; David Schlegel; et al. (2004). A Map of the Universe. Princeton, NJ: Princeton University Press. Available from http://arxiv.org/abs/astro-ph/?0310571. An innovative approach to mapping the universe at all scales.

Sciama, D. W. (1973). Modern Cosmology. Cambridge, UK: Cambridge University Press. A classic text in cosmology.

Tegmark, Max. (2002). "Measuring Spacetime: From the Big Bang to Black Holes." Science 296: 1427–1433. A technical article by one of the leaders in cosmology.

Weinberg, Steven. (1977). The First Three Minutes: A Modern View of the Origin of the Universe. New York: Basic Books. A widely-read popular account of the role of nuclear physics in the origin of the universe.