Astronomy and Cosmology: Big Bang Theory and Modern Cosmology
Cosmologists, who are scientists who study the nature and history of the universe as a whole, state that the universe began in a state of infinite or near-infinite density about 13.7 billion years ago and has been expanding and cooling ever since. The rapid beginning of this expansion is termed the big bang. Although many questions about the big bang remain, and alternative explanations of the universe's history have been proposed, most cosmologists agree that the big bang theory is strongly supported by many observations. These include the movements of distant galaxies and the nature of the cosmic microwave background, a sky-filling bath of radio waves that can be explained as the afterglow of the big bang. The big bang began the process of synthesizing (putting together) the nuclei of all the elements of which Earth and we ourselves are made. It is the organizing or central concept of most modern cosmology.
Historical Background and Scientific Foundations
Astronomers observed in the 1910s that the spiral nebulae are receding from us. Spiral nebulae are actually galaxies—vast wheeling clouds of stars, dust, and gas—but this was not known at the time, because the spiral nebulae were visible only as pinwheel-shaped smears of light in even the best telescopes. The recession of the spiral nebulae is seen not because the spiral nebulae are shrinking, as a car driving into the distance appears to shrink (they are too far away for such an effect to be visible except over very long periods of time), but because their light is shifted toward the red end of the spectrum. This redshifting was at first interpreted as being caused by the Doppler effect, a phenomenon named after its discoverer, Johann Christian Doppler (1803–1853). A commonly noticed form of the Doppler effect occurs when the sound of a police siren is pushed to higher frequencies as the vehicle approaches and then stretched to lower frequencies as it recedes. Light being emitted by a source moving away from the observer is stretched to lower frequencies (redder colors), hence “redshifted.”
The redshifting of light from distant spiral nebulae was interpreted as evidence that the galaxies are moving away from us, like shards of a bomb moving away in all directions from the center of an explosion. This, in turn, hinted that all the matter and energy in the universe were once together at a single point. However, all the spiral nebulae observed were found to be moving away from us, regardless of their location in the sky, and it seemed an impossible coincidence that Earth would just happen to be at the exact center of the universe.
In the early 1920s, astronomers realized that the spiral nebulae are actually distant galaxies. In 1927 Belgian priest Georges-Henri Lemaître (1894–1966) proposed that the universe originated as a primeval atom that exploded and began expanding until it assumed its present-day form, a dark void sprinkled with matter. Lemaître cast his theory in the equations of general relativity, which had been proposed by German scientist Albert Einstein (1879–1955) in 1915. The theory of general relativity describes the shape of space and the nature of gravity.
Two years after Lemaître proposed his primeval-atom idea, further evidence supporting the expansion of the universe was produced by American astronomer Edwin Hubble (1889–1953). Hubble found that distant galaxies are not only receding, but that the farther away any galaxy is the faster it is receding.
General relativity allowed astronomers to make sense of both Lemaître's theory and Hubble's observations without requiring Earth to be at the center of everything. In general relativity's picture of the expandingPage 12 | Top of Article universe, the big bang was not an explosion in space, like a bomb going off in an empty room; it was more like an explosion of space itself. Space, in general relativity, can be curved and finite. An analogy is often drawn between the universe expanding and a spherical balloon being inflated. In this analogy, three-dimensional space is like the surface of the balloon: finite in extent (there are only so many square inches of area on the balloon, only so many cubic miles of space in the universe), but unbounded (a flea crawling on the balloon's surface would never find an end or edge, likewise a space traveler in the universe). The illusion that Earth is at the center is explained as follows: Imagine that a number of fleas are sitting on the balloon's surface as it is in-flated. Each flea will observe that all its fellow fleas are getting farther and farther away from it, and that the farther away a fellow flea is the faster it is receding (because there is more expanding balloon surface between fleas that are farther apart). Yet the fleas are not crawling or hopping away from each other; that is, their physical velocity with respect to the surface of the balloon is zero in every case. Rather, it is the space they inhabit (the balloon's surface) that is expanding. As a result, each flea on the balloon will appear to itself to be at the center of a universal expansion. Yet no flea is at the center, for the balloon's surface has no center.
Likewise, to an observer in any galaxy, all other galaxies are seen to be receding in all directions, and the farther away another galaxy is, the faster it recedes. Yet this recession is not a physical velocity like that imparted to the pieces of an exploding bomb; it is caused by the expansion of the space between the galaxies.
Another aspect of the balloon-universe analogy is that if an expanding balloon is at a certain size right now, it must have been smaller in the past. This reasoning can be applied to the balloon at every stage of its growth, extending all the way back to some small, initial size, a time when all the fleas would have been crowded together in one dense mass. Likewise, the matter and energy of the universe must once have been crowded together at a single point. The sudden beginning of the expansion of this mass was the big bang.
The expansion of space following a big bang explains the redshifting of light from distant galaxies. This redshifting, today termed cosmological redshifting, is not, however, due to the Doppler effect as originally thought; distant galaxies are not redshifted because they are receding from us with actual, physical velocities. Rather, the light from those distant galaxies has been stretched during the many millions of years it took to reach us by the expansion of the space through which it traveled. This effect is called the cosmological redshift.
The big bang theory is not the only explanation of the universe that has been offered by astronomers. In fact, the term “big bang” was coined in 1949 in mockery of the big bang theory by English astronomer Fred Hoyle (1915–2001), who proposed an alternative theory called continuous creation. Hoyle suggested that hydrogen atoms spontaneously appear in empty space everywhere in the universe at a certain rate. Space expands, but the average density of matter throughout it remains Page 13 | Top of Articleconstant through infinite time because of this universal drizzle of new hydrogen atoms. Several cosmologists have defended continuous creation and other nonstandard cosmologies up to the present day, but these are in a small minority.
Beside the recession of the distant galaxies, the first strong observational evidence for the big bang theory was discovered by accident in 1965 when radio engineers working for Bell Telephone in Holmdel, New Jersey, found that a new microwave-receiving antenna they were building seemed to detect a weak microwave signal no matter what part of the sky they pointed it at. As soon became apparent, they had detected the radio afterglow of the big bang, which had been predicted a few years earlier on theoretical grounds by American scientists George Gamow (1904–1968) and Ralph Apher (1921–2007) and which scientists at nearby Princeton University had begun building an antenna to detect in 1964. The phone-company engineers discovered the microwave radiation before the Princeton scientists could make their first observations.
The cosmic microwave background (CMB) radiation discovered in 1965 is not just any microwave signal; it has a spectrum (variation of power over a range of frequencies) that corresponds to the spectrum predicted for a perfect black body (perfectly absorbing or radiating object) at 2.725 Kelvin, close to absolute zero. Black-body radiation filling the entire universe is exactly what was predicted on the basis of the big bang theory. The CMB is still one of the strongest pieces of evidence for the big bang.
However, it is not the only such evidence, because big bang theory also predicts other phenomena beside the CMB. In particular, it predicts that within the first three minutes after the beginning of the universe, temperatures would have dropped low enough to permit the formation of certain atoms. Three light elements—hydrogen, helium, and lithium—would have formed under these conditions. Big bang theory predicts that about 24% of the ordinary matter in the universe should consist of helium atoms—and so it does. All elements heavier than lithium were synthesized later in the history of the universe in the hearts of stars, where the nuclei of lighter atoms can be welded together by fast collisions. This process is termed stellar nucleosynthesis. All the carbon, iron, oxygen, nitrogen, and other elements of which Earth and our own bodies are composed were forged by stellar nucleosynthesis.
Stellar nucleosynthesis began inside stars 100–1,000 times the mass of our sun, which began to form about 100 million years after the big bang, flooding the universe with its first starlight. These first stars eventually exploded, scattering the universe's first heavy elements into space as clouds of dust and gas. Some of these clouds eventually clumped again under the pull of gravity and formed second-generation stars. Many of these second-generation stars have also lived out their life cycle and exploded, synthesizing further heavy elements, and the debris from these explosions has collected into a third generation of stars. Our own sun (including its planets) is one of these third-generation systems. Confusingly, first-generation stars are termed Population III, second-generation stars Population II, and third-generation stars Population I. No Population III stars exist in the nearby universe, since they existed primarily during the first billion years after the big bang (13.7 billion years ago). Astronomers are striving to detect traces of these earliest stars in the most distant universe but have not yet unambiguously succeeded.
Further confirmation of the big bang theory has come from data collected by space probes, especially NASA's Cosmic Background Explorer (COBE, launched in 1989) and Wilkinson Microwave Anisotropy Probe (WMAP, launched in 2001). Specifically, the modern big bang theory includes a feature called inflation. Inflation is an extremely rapid expansion supposedly undergone by the universe from 10‒ 35 to 10‒ 32 seconds after the beginning of the big bang. Inflation theory predicts that subatomic-scale quantum field fluctuations—subatomic in scale, but as large as the whole universe during the brief inflationary period—would have seeded the new universe with irregularities that grew as the universe grew, eventually spanning the sky. (Similarly, small dots marked on an uninflated balloon become large when the balloon is inflated.) These irregularities should appear as a pattern of slight unevenness in the CMB, and their pattern, according to the theory, should have a certain random character. The CMB is generally quite even in all directions, to about 1 part in 100,000, but it does vary slightly from place to place, and its variations have been mapped by the COBE and WMAP satellites. The random character of its blotches or brightness variations, termed its angular power spectrum, closely matches the predictions of inflation theory.
A controversial recent development of big bang theory has been the hypothesis that there may be an infinite number of big bangs and an infinite number of universes, isolated from each by immense distances. Although direct physical evidence for the existence of other universes is not available, some interpretations of quantum physics—which is intimately related to the big bang—seem to call for a many-worlds reality or multiverse. By the early 2000s, a variety of multiverse theories were being debated by cosmologists, but none had yet been put to any observational test. Such tests are not impossible in principle, even though other universes canPage 15 | Top of Article never be directly observed: equations describing the formation of such universes might predict certain characteristics of our own universe, which might then be either observed or ruled out by observation. However, so far, all multiverse theories remain speculative.
A startling advance in modern cosmology occurred in 1998 as a result of observations of distant supernovae (exploding stars). This was the discovery that not only is the universe expanding, but its expansion is accelerating—getting faster. This has forced scientists to conclude that some unknown form of energy is driving the acceleration. The nature of this energy is still unknown as of 2008, though several possibilities have been proposed. Because it is unseen and its nature obscure, it has been dubbed “dark energy.”
In recent years most astronomers have also been convinced of the existence of a form of matter that is scattered unevenly throughout the universe but does not reflect or absorb light, hence it is called “dark” matter. The nature of the dark matter is also unknown, but by examining the CMB measurements from WMAP and other data, cosmologists have concluded that the universe consists of 4% ordinary matter (atoms, photons, neutrinos), 22% dark matter (nature unknown, but exerting gravitational pull that shapes galaxies and clusters of galaxies), and 74% dark energy (nature completely unknown). Matter and energy can be lumped and compared in this way because, as Einstein's theory of special relativity showed in 1905, the two are interchangeable.
Modern cosmology has thus reached a peculiar state. It has confirmed its answers to basic questions about the history of the universe yet raised for itself other questions that are at least as great. Inflationary big bang theory has passed several observational tests with close agreement, and most cosmologists have strong confidence in the theory; we therefore seem to have a good working account of the history of the universe thus far, starting with its still-mysterious origin at time zero. But starting a mere decade or so ago, cosmologists have confronted a universe that consists mostly (about 96%) of substances (dark matter, dark energy) whose nature is unknown. They also confront the possibility that the observable universe may be either all that exists, or an infinitely tiny fraction of all that exists (one universe in an infinity of universes). Also, modern cosmology is uncertain about what the ultimate future of the universe will be. Accelerating expansion rules out an eventual slow-down of the post-big bang expansion with ultimate re-collapse into an infinitely dense point. This hints that the universe will grow infinitely old, spending eternity in an almost perfectly dark, cold, diffuse state; however, this is not certain. Quantum fluctuations may re-ignite a newPage 16 | Top of Article big bang many trillions of years from now, for example, or the universe may be annihilated by a “big rip” caused by unbounded growth of dark energy. These ideas and others have been proposed, and all have supporters and detractors. Therefore, there is still no consensus view among cosmologists, as of 2008, of the universe's ultimate fate. Several cosmological theories have been offered as alternatives to the big bang, and these are taken seriously by scientists, but so far none has shown the big bang model's fruitfulness in correctly predicting cosmic observations. Most cosmologists are convinced that some version of the big bang (including early expansion) is correct.
Modern Cultural Connections
Scientific cosmology has, in the last half century, given at least partial answers to questions that for thousands of years addressed by religious creation myths: What is the universe, how big is it, and how is it shaped? When did it begin, and how long ago? The big bang model, which has been strongly supported by recent observations, replies that the universe is an expanding, finite, unbounded volume of space containing a mixture of ordinary matter, dark matter, and dark energy that came into being about 13.7 billion years ago. In the beginning it was much smaller than today—sub-atomic in size—and in a state of extremely high temperature and density. The big bang was, however, not an expanding event in space: It was an expansion of space itself.
Since the big bang, the universe has spread out and cooled. Stars and galaxies have coalesced under the pull of gravity. Heavier elements have been welded together in stellar explosions and expelled into space and have recoalesced into second- and third-generation stars. Human beings have evolved on a planet orbiting one such third-generation star in a typical galaxy. There are over 100 billion galaxies in the observable universe; most galaxies contain several hundred billion stars.
Although many or most religious believers around the world accept all of these statements, which are supported by multiple, independent, convergent lines of scientific evidence, some persons have found it difficult to adjust to conflicts between modern big bang cosmology and ancient cosmologies based upon religious beliefs or cultural traditions. The most prominent group of this type in the industrialized world is that of the creationists. All creationists deny at least some part of the scientific account of biological evolution. Many, termed young earth creationists, also deny that the universe is ancient (13.7 billion years for the whole universe, 4.5 billion years for Earth itself), preferring a much shorter chronology derived from their interpretation of the Hebrew Bible (which Christians term the Old Testament). On the basis of this interpretation they affirm the rapid, miraculous creation of the world from nothingness about 6,000–10,000 years ago. Some young earth creationists have attacked the big bang theory, arguing that it is fatally flawed, contradicted by evidence, and doubted even by many astronomers. However, these claims are not scientifically or historically accurate. The age of the universe and its origin in a big bang-type event are attested by many forms of physical evidence.
Political, legal, theological, and social struggles between young earth creationists and other persons continue, primarily in the United States, but increasingly in other countries such as Australia, Germany, Turkey, and the United Kingdom. Although these struggles often focus primarily on biological evolution, Page 17 | Top of Articlebig bang cosmology and its explanation of the great age of the universe are also frequently attacked by advocates of creationism.
Ironically, although some religious believers now view modern cosmology as a challenge to their beliefs, in the mid-twentieth century some scientists originally viewed the big bang theory with suspicion because it seemed to bear an uncomfortable resemblance to the Genesis account of sudden creation from nothing. These scientists would have preferred an eternal “static” universe with no dramatic first moment. However, the big bang theory does not necessarily posit the creation of the universe out of true nothingness; even the uniqueness of the big bang is questioned in multiverse cosmology. The big bang is a scientific concept, not a religious one.
Primary Source Connection
One of the longest lasting and deepest conflicts in American culture has occurred over the teaching of the evolution of man in public schools. Often, scientific and religious communities assume opposing sides in the debate, which is frequently waged before wavering school boards who struggle to enact policy that reflects the belief system of the local community as well as established scientific fact. The following newspaper article, written for the New York Times by James Glanz in 1999, describes a movement that gained notoriety in the 1990s, whereby some states called into question their schools' curriculum for teaching the evolution of the entire universe.
James Glanz serves as the Baghdad bureau chief for the New York Times.
SCIENCE VS. THE BIBLE: DEBATE MOVES TO THE COSMOS
Scientific lessons about the origins of life have long been challenged in public schools, but some Bible literalists are now adding the reigning theory about the origin of the universe to their list of targets.
Nearly overlooked in the furor over the Kansas school board's vote in August to remove evolution from its education standards was a decision on the teaching of the science of the cosmos. Influenced by a handful of scientists whose literal faith in the Bible has helped convince them that the universe is only a few thousand years old, the board deleted from its standards a description of the Big Bang theory of cosmic origins, the central organizing principle of modern astronomy and cosmology.
The Big Bang theory, based on decades of astronomical observations and physics research, suggests that the universe originated in a colossal explosion of matter and radiation some 15 billion years ago.
But “young Earth creationists,” as they are generally known, have come up with their own theories to explain how cosmic history could be condensed into mere thousands of years. They are making this case in books, pamphlets and lectures, as well as on a number of Web sites.
Mainstream scientists consider their theories to be wildly out of line with reality, even though books describing them are often liberally sprinkled with references to authorities like Albert Einstein and Stephen Hawking.
As a result, physical scientists now find themselves in a fight in which they have seldom played a public role. They have responded with a mixture of disdain, disbelief and consternation, and the reactions have not been limited to physicists and cosmologists in Kansas.
“It's the denial of what understanding we have of the origin of the universe in terms of modern science,” said Jerome Friedman, a physicist at the Massachusetts Institute of Technology who was awarded a Nobel Prize in 1990 for collaborating in the discovery of the subatomic particles called quarks and is the president of the American Physical Society. “That's a terrible loss,” Dr. Friedman said.
Hume A. Feldman, a cosmologist at the University of Kansas in Lawrence who has worked at Princeton University and the Canadian Institute for Theoretical Astrophysics, called the matter “frightening.”
“When I went into cosmology,” Dr. Feldman said, “I never thought I would get involved in anything like that.”
Dr. Feldman said that developments in his state bore a distant resemblance to the difficulties of political scientists under Communist regimes in Eastern Europe, and that he feared that such pressures could impair the educational system.
But advocates of the creationist view say alarm over their theories is overblown. Steve Abrams, a member of the Kansas board and a veterinarian in Arkansas City who was among the leaders of the push to make the changes, said there were legitimate scientific doubts about whether the universe was more than several thousand years old.
“There is sufficient data to lend credibility to the idea that we do not have all the answers for teaching the origin of our universe,” he said.
That sentiment was echoed by John W. Bacon, a board member from Olathe who also voted with a narrow 6–4 majority for the changes.
“I can't understand what they're squealing about,” Mr. Bacon said of scientists who oppose the board's action. Millions or billions of years ago, Mr. Bacon said, “I wasn't here, and neither were they. Based on that, whatever explanation they may arrive at is a theory and it should be taught that way.”
Those objections closely mirror criticisms leveled at evolution by its opponents. Alabama biology textbooks, for example, must carry a warning that reads in part: “No one was present when life first appeared on earth. Therefore, any statement about life's origins should be considered as theory, not fact.”
The Kansas challenge to the teaching of the Big Bang is not the first public objection to the theory on religious or political grounds. Three years ago, the school superintendent of a conservative county in western Kentucky ordered two pages that explained the Big Bang in grade-school textbooks to be glued together. The superintendent said that the Big Bang should not have been explained without including the biblical version of creation as well.
The change in the Kansas standards does not preclude the teaching of mainstream biology, physics or cosmology, allowing teachers to present alternative viewpoints if they choose to do so. But because the standards are used as the basis for state tests, the changes will probably have a practical effect on what is taught, said Dr. Bill Wagnon, a professor of history at Washburn University in Topeka and a board member who voted in the minority. Students' scores on those tests help determine whether a school receives accreditation from the state.
“The curriculum standards describe that process of what needs to be covered,” Dr. Wagnon said.
So radical were the Kansas board's recommendations that it has been unable to publish its own standards, or even to display them on its Web site. That is because the standards include long extracts from a book on education standards that was published by the National Research Council. Because of its disapproval of the board's revised standards, the Council has refused permission for them to be reprinted.
Beyond the expunging of evolution, the board also took out references to the hundreds of millions of years of Earth's geologic ages and modified sections on using the slow decay of radioactive elements to measure the ages of fossils and other rocks.
Among the most striking changes was the removal of passages in the original standards dealing with the Big Bang. Cosmologists see ample evidence for that explosion in the present expansion of the Universe, in a diffuse afterglow in space called the cosmic background radiation, and in the precise abundances of light elements like hydrogen and helium that were left over from the cataclysm.
Cosmologists have also calculated the way in which stars, galaxies and clusters of galaxies coalesced from slight ripples in the primordial soup that emerged from the big bang. To date, the results of those calculations match the precise observations of such structures in the heavens. Of course, for all its success in accounting for observations, the Big Bang is indeed just a theory, although it is one with few scientific dissenters.
The biggest problem for the young Earth creationists is explaining the time that has apparently passed since the light we see from distant galaxies was emitted. Given the constancy of the speed of light and estimates of the distance between Earth and faraway galaxies it is difficult to explain how Earth and the cosmos could be young.
But D. Russell Humphreys, a nuclear weapons engineer at Sandia National Laboratory who is also an adjunct professor at the Institute for Creation Research near San Diego, thinks he has an answer. In an interview, he said that Einstein's equations of relativity, the basis of the Big Bang theory, could be used to construct a universe in which the Earth is only a few thousand years old.
Dr. Abrams said that in thinking about the Kansas standards he had been struck by Dr. Humphreys's book, “Starlight and Time: Solving the Puzzle of Distant Starlight in a Young Universe” (Master Books, fifth printing in 1998).
Dr. Humphreys's ideas “seem to be right there on the cutting edge, so to speak,” Dr. Abrams said.
But most cosmologists say they are simply out of left field.
The theory relies on a peculiar feature of Einstein's equations, which predict that powerful gravitational fields can speed the progress of time and, in effect, make clocks run at different rates in different places. So Dr. Humphreys assumes that the Earth is close to the center of a structure related to a black hole, in which gravity is especially intense, so that billions of years could pass in deep space while only a few thousand years went by on Earth.
Such a universe “has clocks clicking at drastically different rates in different parts,” Dr. Humphreys said in an interview.
Dr. Edward L. Wright, vice chairman for astronomy at the University of California at Los Angeles, said that there is no evidence that the Earth is at the center of the universe, or that such tremendous gravitational fields exist outside of ordinary black holes.
Moreover, Dr. Wright said, the acceleration of time would alter the vibrations of waves of light, shortening its wave length and turning it into deadly gamma rays. Bombarded by such radiation, he said, “the Earth would be sterilized.”
Dr. Humphreys, whose research in cosmology is unrelated to his work at the lab, said other features of his model would prevent the frequency increase.
Dr. Abrams also cited a theory that the speed of light was almost infinitely fast in the past, meaning that the light from distant galaxies could have reached Earth quickly and would not be billions of years old.
He referred to writings on this subject by Dr. Danny Faulkner, a professor of astronomy at the University of South Carolina's Lancaster campus and an adjunct professor at the Institute for Creation Science. In a telephone interview, Dr. Faulkner cautioned that he had merely been describing ideas put forth by other scientists in the creationist movement and was not certain that the changing speed of light was correct. Indeed, high-precision measurements of the speed of light and other crucial physical constants have revealed no detectable change in their values over recent time.
The debate over the age of the universe has exposed intense disagreements not just in schools but also among evangelical Christians.
“Often young-universe and old-universe creationists focus more energy on defending their respective positions than on reaching out to nonbelievers,” wrote Dr. Hugh Ross, a former radioastronomer who is an evangelical Christian, in “Creation and Time: A Biblical and Scientific Perspective on the Creation-Date Controversy” (NavPress, 1994).
Dr. Ross thinks that a literal reading of the Bible can be reconciled with the Big Bang, but says that his views are distinctly in the minority among evangelical Christians. The six days of Genesis could stand for “six consecutive long periods of time,” Dr. Ross said.
The importance of the issue for many Bible literalists means that cosmologists could face the pressures that biologists have dealt with since John Scopes was convicted of violating a Tennessee law against the teaching of evolution in 1925, said Dr. Eugenie C. Scott, executive director of the National Center for Science Education Inc., in El Cerrito, Calif.
“I don't think physical scientists are going to be immune to this,” Dr. Scott said. “It would be very unwise for them to brush this off.”
GLANZ, JAMES. “SCIENCE VS. THE BIBLE: DEBATE MOVES TO THE COSMOS.” NEW YORK TIMES (OCTOBER 10, 1999).
Weinberg, Steven. The First Three Minutes. New York: Basic Books, 1988.
Bennett, Charles L. “Cosmology from Start to Finish.” Nature. 440 (2006): 1126–1131.
Cho, Adrian. “Gravity Distorts Big Bang Afterglow, Opening New Window on Cosmos.” Science. 316 (2007): 1407.
Cowan, John J., and Christopher Sneden. “Heavy Element Synthesis in the Oldest Stars and the Early Universe.” Nature. 440 (2006): 1151–1156.
Glanz, James. “Science vs. the Bible: Debate Moves to the Cosmos.” New York Times (October 10, 1999).
Hogan, Craig J. “The Beginning of Time.” Science. 295 (2002): 2223–2225.
National Aeronautics and Space Administration (U.S.). “Cosmology: The Study of the Universe.” September 26, 2006. http://map.gsfc.nasa.gov/m_uni.html (accessed February 6, 2008).