THIS MAP OF THE ANCIENT SKY SHOWS THE MINUTE VARIATIONS IN TEMPERATURE AND DENSITY DISCOVERED BY THE TEAM LED BY LBL ASTROPHYSICIST GEORGE SMOOT. OVER BILLIONS OF YEARS, GRAVITY MAGNIFIED THESE SMALL DIFFERENCES INTO THE CLUSTERS OF GALAXIES WE OBSERVE TODAY.
By Jeffery Kahn
From the dawn of the spoken word, humans have devised creation stories. From Easter Island comes the story of a bird god that laid an egg which hatched into the world. Greek mythology describes how the divinity Gaea, or Earth, arose out of a state of emptiness called Chaos. The Old Testament describes the six days of genesis. Modern science, too, has its creation story, the Big Bang.
First proposed by Belgian priest and mathematician Georges Lemaitre in the 1920s, the Big Bang theory has been revised and developed throughout the 20th century. According to the theory -- it is perhaps more difficult to comprehend and believe than any other creation story -- the universe began as a mere pinpoint in a moment prior to the existence of either space or time. In a singular instant, this pinpoint exploded, creating a Universe that has continued to expand and evolve since the beginning, some 15 billion years ago.
Every age has its conceit. Like people from former times, modern thinkers believe their creation story deals with reality whereas previous accounts were mere expressions of whimsy. Yet, even the most confident believers in the Big Bang have had to concede that it too is only a theory. As did our ancestors, the current generation has had to begrudgingly acknowledge that the events of creation ultimately may be unknowable, more within the realm of faith than of human knowledge.
History will judge, but perhaps in our time, this has changed.
On April 23 at an American Physical Society meeting in Washington, D.C., Lawrence Berkeley Laboratory astrophysicist George Smoot announced the discovery of fossil relics from the primeval explosion that began the universe. Smoot said these 15- billion-year-old fossils are the primordial seeds that grew into the galaxies and superclusters of galaxies evident today.
The discovery was made through the use of exquisitely sensitive microwave receivers Smoot's team created for NASA's Cosmic Background Explorer (COBE) satellite. The receivers detected regions of space 100 million light years across and larger, with temperature differences of a hundred-thousandth of a degree. Scientists believe these temperature variations and corresponding variations in density were imprinted at the instant when the universe was born. As the universe expanded, these tiny varying domains inflated, creating vast regions with minuscule variations in temperature, density, and gravitational potential.
Said Smoot, "These small variations are the imprints of tiny ripples in the fabric of space-time put there by the primeval explosion process. Over billions of years, gravity magnified these ripples into galaxies, clusters of galaxies, and the great voids of space."
Smoot, a member of LBL's Physics Division as well as the University of California at Berkeley's Center for Particle Astrophysics and its Spaces Sciences Laboratory, is the principal investigator of the team that looked back into time for our cosmological roots. Spanning 18 years since its inception and involving, at one time or another, an estimated 1000 individuals, the project may forever change the nature of the quest to understand the origin and general structure of the universe.
Up until now, cosmology has been essentially a theoretical field. Smoot is an experimentalist, an approach that produces data. Henceforth, cosmological truth will be revealed not only through theory but also by way of experiment.
Says Smoot, "Human beings have had the audacity to conceive a theory of creation and now, we are able to test that theory. I believe we have discovered the fossil remnants of the progenitors of present day structure in the universe. They tell us that we have a viable theory of the universe back to about 10-30 second. At that time the currently observable universe was smaller than the smallest dot on your TV screen, and less time had passed than it takes for light to cross that dot."
The account of the discovery made by these researchers was front page news around the world. From Science and Nature to the New York Times and Japanese public television, from television and radio talk shows to the cartoon page and Calvin and Hobbes, the Big Bang research finding was the Big Story. Among others, Scientific American described how Smoot's team "may have found the primordial blueprint that determined the structure of the modern universe." It titled its cover article, "The Golden Age of Cosmology."
Smoot recalls that not long ago, when he first started his career, cosmology wasn't even considered a real science. "It was a fringe field," he says. "Back in 1964, you could get all of us in the field into a single room. I remember the teasing from my particle physics colleagues that real physics is done at accelerators. Today, opinions have changed. We have begun to explore the early universe, the original accelerator. The fields of particle physics and cosmology have been joined."
Smoot's COBE team is a large collaboration involving participants from LBL, the University of California at Berkeley, the NASA Goddard Space Flight Center, UCLA, MIT, and Princeton. In addition to Smoot, team members at LBL include LBL/UCB astrophysicist Giovanni De Amici, data analyst Jon Aymon, and Berkeley graduate students Charley (cq) Lineweaver and Luis Tenorio.
Smoot says what cosmologists actually know about the early universe is quite limited. "From my standpoint," he said, "prior to our work, we had perhaps four fundamental pieces of cosmological knowledge."
To start with, notes Smoot, the night sky is dark. As obvious as this seems, it was not until the 1600s that people first began to realize the implications. In 1836, German astronomer Wilhelm Olbers wrote that this presented a seeming paradox. That is, if the universe were infinite in space and time, no matter which direction you looked, there would be a star and the night sky would not be dark but instead flooded with light. Olbers' paradox led to the realization that the universe is finite.
In the 1920s, Edwin Hubble established that the Milky Way is not alone but one among myriad galaxies in the universe. Hubble observed that the galaxies he had discovered are moving further apart as time passes. The Hubble law, that galaxies are receding from one another at velocities directly proportional to the distances separating them, led to Hubble's revelation that our universe is expanding.
In the 1940s, a community of physicists and astronomers -- George Gamow, Ralph Alpher and Robert Herman -- puzzled out how the chemical elements originated. The light elements hydrogen, helium, and lithium were created in the hot, dense stage of the early universe. The remaining elements are literally stardust, created when the three primordial elements are burned in stars and ejected.
In 1964, Bell Laboratories' Arno Penzias and Robert Wilson discovered the cosmic microwave background radiation, the relic thermal radiation from the Big Bang. The researches were using a microwave horn antenna built for satellite communications experiments and encountered interference, a hiss that persisted no matter which direction in the sky the antenna was pointed. This microwave noise -- it had a temperature of 2.7 degrees kelvin -- tipped the scales away from competing theories on the origin of the universe toward the Big Bang. Penzias and Wilson had detected the faint afterglow of the primeval fireball.
Smoot says his team's discovery was presaged by Penzias and Wilson's finding.
The 1964 discovery of the cosmic microwave background had left cosmologists with an unresolved central question. If our universe originated in a Big Bang, how then did it develop its present structure consisting of stars, galaxies, clusters of galaxies, galactic superclusters, and the great voids of space?
As cosmologists wrestled with this question, they realized that the finding of the cosmic microwave background ultimately was a double-edged sword. While it brought the Big Bang theory widespread scientific acceptance, it also meant that the theory would self-destruct unless the question of the origin of structure could be resolved. Penzias and Wilson found a smooth and uniform microwave background. This created a problem, a cosmic contradiction for cosmologists. They had to resolve the question of how a universe with a uniform microwave background could have developed clumps that evolved into structures.
Theorists say any contradiction is only apparent.
As explained by astronomer Martin Rees of Cambridge University, "The early universe cannot have been absolutely smooth and uniform. To give rise to the structures we see, some initial irregularities must have been imprinted (during the primeval explosion). Regions slightly denser than average would have been decelerated more by gravity. They would have lagged more and more behind the cosmic expansion rate; their expansion would eventually halt, and they would condense into gravitationally bound systems."
These early variations did not have to be pronounced. Theory indicates that the very early universe must have had fluctuations in temperature and density on the order of one part in 100,000. That would be sufficient for gravity, working over billions of years, to magnify perturbations of this magnitude into the universe we observe today.
Since 1964, many scientists from around the world have searched unsuccessfully for these variations in the cosmic microwave background. Twenty-eight years passed before Smoot's team became the first to find them.
Smoot, like many scientists, believed the best way to detect these minute differentials would be from a satellite in space. Eighteen years ago, when NASA solicited proposals for a cosmological research satellite, the astrophysicist jumped at the opportunity. His proposal to build instruments that could measure anisotropies (irregularities) in the cosmic microwave background was accepted and the research project began.
Reflected Smoot, "People aren't accustomed to spending 15 years or 20 years of their life on a project. It is hard for the public to comprehend how long this took."
To find the cosmic seeds, Smoot's team designed an instrument they call the Differential Microwave Radiometers (DMR). Rather than measure temperature directly, a differential radiometer uses a pair of antenna to detect the difference in temperature between two separate parts of the sky. This particular DMR measures the difference in microwave power between two regions of the sky separated by 60 degrees.
The DMR actually includes three radiometers, each designed to detect a specific wavelength of microwaves. Specific frequencies were targeted for measurement where the signal is strongest from the deepest, most ancient reaches of space rather than from "local" galactic sources. In fact, at the frequencies selected, the cosmic microwave background is more than 1000 times stronger than the galactic microwave emissions.
After years of work, the DMR was launched aboard NASA's COBE satellite on November 18, 1989. One of three instrument packages aboard COBE, the satellite was placed into a near-polar orbit. Operating 900 kilometers above the Earth, COBE is high enough that the residual effects of the Earth's atmosphere do not affect its instruments, yet low enough to minimize interference by the charged particles in the Earth's radiation belts.
The DMR is incredibly precise, able to measure temperature differences of one part in a million. Three independent techniques have been used to calibrate the instrument. Solid-state noise sources provide in-flight calibration as do measurements of the moon, which are compared to known values. Additionally, the instrument was carefully calibrated inside a cryogenic vacuum chamber prior to launch.
Scientists from the team devised computer models to separate the cosmic microwave background signal from unwanted noise, such as the magnetic effects of the Earth and microwave emissions from within the galaxy. Additionally, the three sets of data from the three radiometers provide redundancy and help to identify any anomalies.
The DMR maps the entire sky twice a year. During that time, each of its six radiometers takes 70 million measurements. When the data are fit together and the extraneous noise eliminated, what emerges are maps of the early universe. One newspaper in Europe published a map developed by the COBE DMR researchers and identified it in headlines as a "baby photo" of the universe. Smoot says that technically, this is an accurate description. In fact, the maps show the universe as it looked when it was about one-ten- thousandth of its current age, or about 300,000 years after its birth.
Scientists say the age of the universe that the COBE maps record is determined, even dictated, simply by the selection of the wavelengths and the temperature of radiation that the DMR targets.
In looking for the relic radiation left over from the Big Bang, researchers had to turn to their understanding of what happens to very hot matter as it cools and the universe expands over billions of years. When matter is hot, it emits electromagnetic radiation (photons). Each temperature produces a unique and corresponding distribution of wavelengths and frequencies known as the blackbody spectrum. As the early universe expanded, photons emitted at torrid temperatures became "red- shifted" by this expansion. That is, photons were shifted toward the lower frequency, long-wavelength end of the electromagnetic spectrum. Today, 15 billion years after the Big Bang, they should exist uniformly, across the breadth of the sky, as cooled microwave radiation with a temperature of about 2.73 kelvin. This is the spectrum and temperature that the DMR targeted, zeroing in to measure any tiny variations within this range.
The maps record a time about 300,000 to one million years after the primeval explosion.
Prior to that time, the universe, though expanding and cooling, was still so dense that no photons could escape. It consisted of a soup of charged particles and radiation. Some time after 300,000 years had elapsed, this primeval mix cooled to about 3000 kelvin, a temperature cool enough to allow the first atoms to form. At this moment, matter and radiation became separate and the first photons were released, beginning their journey through space. The sky maps produced by Smoot's team record this time. The moment is aptly described in the book of Genesis with the simple words, "Let there be light."
In addition to finding the ancient seeds for the modern structure of the universe, Smoot says the research has perhaps even greater significance. The findings help verify the theory of inflation.
Inflationary cosmology is a bizarre elaboration on the Big Bang developed in the 1980s by Alan Guth of MIT. The era of inflation began at the moment of the Big Bang and ended almost instantly, at about 10-30 second. During that era, the universe suddenly expanded at an unimaginable rate, swelling by a quadrillion quadrillion quadrillion quadrillion times. Stated another way, the universe went from subatomic size to larger than the universe we can observe from Earth today.
During inflation, says Smoot, the tremendous forces of expansion created small amplitude ripples in the fabric of space/time. These ripples caused the density to differ slightly from place to place, with a distribution of density that is predicted by quantum mechanics. During this era of inflation, these variations expanded exponentially in size as did the universe.
At the end of the brief inflationary era, the universe continued to expand but at a more modest rate, not exceeding the speed of light. What remains from the era of inflation are the residual fluctuations.
Says Smoot, "Those are the fluctuations that we believe we are mapping here at 300,000 years old. For me, this connection to inflation turns out to be the most exciting result of our research. This is the first time inflation has been tested. Its predictions on the cosmic microwave background have been verified. Though our findings do not prove inflation, they are consistent with it. It is time to start believing in inflation."
Inflationary theory predicts that these varying regions should be distributed evenly no matter the scale on which they are observed. That is exactly what the COBE DMR team detected and measured.
Smoot explains that the fluctuations COBE measured are "scale invariant." That is, no matter whether the scale observed is very small or very large, hot and cold regions were detected and the variations in temperature between them remain the same. Similar variations were measured whether the DMR was viewing at its largest scale, one-quarter of the entire sky, or at its smallest scale, 14 times the apparent diameter of the moon.
Some of the hot and cold regions mapped are so large that they dwarf the largest structures ever observed by astronomers. In fact, a statement of their dimensions is an apparent contradiction in terms. The maps include regions larger than the observable universe. (italics)
The observable universe is what can be seen -- that is, anything close enough that over the lifetime of the universe, its light has had enough time to travel to an observer. By definition, then, the range of our currently observable universe is the sphere around us which has a radius of about 15 billion light years. And the size of the observable universe in the new map of the early universe is a sphere with a radius of 300,000 light years.
"We can only observe a tiny part of the universe," said Smoot. "Inflation blew our universe to a size that, in effect, there may as well be separate, unobservable universes. One part in a trillion trillion trillion trillion is the part we can observe, our observable horizon."
Smoot notes that when his research team began its mission, the theory of inflation did not exist. The same can be said for dark matter, another postulation that the research helps corroborate but which came along after the COBE DMR team was formed.
Physicists say that most of the universe consists of dark matter. Dark matter remains an enigma to science. Invisible, its existence has been inferred but it has yet to be detected by scientists. Dark matter is unlike ordinary matter or anything conceived even in laboratory accelerators. Scientists believe it exists because its gravity influences the motion of ordinary matter.
The research findings of the Smoot team provide additional evidence for the existence of dark matter. The temperature and size of the regions mapped is in agreement with theories that state that anywhere from 10 to 99 percent of the matter in the universe is dark.
Dark matter does not interact with electromagnetic radiation and could have begun accumulating into condensed structures about 10,000 years after the Big Bang. Ordinary matter could not begin this agglomeration process until 300,000 years or so later. The window of time mapped by the Smoot team is this 300,000 year date, and the structures observed are vast. Again, some of these regions of varying temperatures are as large as the observable universe. This finding is consistent not only with inflation but with the existence of dark matter.
"We believe the evidence for dark matter, for the Big Bang and for inflation is fairly conclusive," said Smoot. "Our findings tie our concepts of the very beginning of the universe to what we see today."
The COBE satellite continues to orbit the Earth, and the DMR and Smoot's team persevere with their mission. Researchers are analyzing a second year of data and believe they soon will have refined, higher resolution maps of the cosmic seeds that condensed into today's universe. Already, their findings have electrified the world of science.
"They have found the Holy Grail of cosmology," said physicist Michael Turner of the University of Chicago. "It is the discovery of the century, if not of all time," said Stephen Hawking of Cambridge University. "We are viewing the birth of the universe," said Joseph Silk of the University of California at Berkeley.
The public imagination has been similarly kindled. Evidently, despite the distractions of our times, we, like our forbearers, continue to look up at the starlit heavens and wonder how it all came to be.
"When we made this announcement, I thought that the scientific community would be quite excited. But," said Smoot, "I completely underestimated the degree of public interest. Ordinarily you do science, you publish your result, and very few outside your immediate field ever notice. With this, it was more like a sporting event. There have been people calling and writing from all over the world and from every walk of life, crowds of people watching closely, cheering us on."