We are the first generation of human beings to glimpse the full sweep of cosmic history, from the universe's fiery origin in the Big Bang to the silent, stately flight of galaxies through the intergalactic night. Humankind continues its own journey into the future with a new depth of understanding and appreciation for the forces that shape our destiny.
What Is Cosmology?
Cosmologists work to understand how the universe came into being,why it looks as it does now, and what the future holds. They makeastronomical observations that probe billions of years into the past,to the edge of the knowable universe. They seek the bases of scientificunderstanding, using the tools of modern physics, and fashion theoriesthat provide unified and testable models of the evolution of theuniverse from its creation to the present, and into the future.
What's All the Excitement About?
For 70 years, astronomers have known that the universe is expanding,that galaxies (gigantic collections of billions of stars) appearto be flying away from one another. Measurements of the speed anddistance of other galaxies show that the more distant a galaxy isfrom the Milky Way (our galaxy), the faster it recedes. This phenomenonis called the Hubble expansion. Working backwards from their data,astronomers infer that the universe must have been created at a definitetime, between 8 billion and 15 billion years ago.
In its early stages, the universe must have been enormously denseand hot—so hot that at one point it consisted mostly of radiation.As called the hot Big Bang model. For decades it remained untestedand controversial.
Over the last three decades, new technologies and ideas have drivencosmology forward at a rapidly increasing pace. Once a science ofdata-starved speculation, cosmology is now a full-blown race betweentheory and observations—the hallmark of a vigorous physical science.This race began in earnest in 1964, when scientists at Bell Laboratories,while attempting to understand radio antenna noise, discovered thatsome of that noise was a signal received from all directions in outerspace. It was soon realized that this signal might be the cooled-downremnant of the radiation predicted by the Big Bang model. Cosmologyfinally had an observational foothold—a measurable remnant of theearly universe, a probe to test the various cosmological models.The radiation was dubbed the cosmic microwave background radiation(CMBR). In 1990, early data from the Cosmic Background Explorer (COBE,pronounced ko-bee) satellite showed that the CMBR had precisely theprofile of intensity versus frequency to be consistent with the hotBig Bang model of the universe (see Figure 1).
In 1992, the COBE satellite produced another remarkable discovery.Data from a second experiment aboard the satellite showed slightvariations of the CMBR intensity with direction in the sky. The searchfor these variations had spanned 25 years. This discovery causedgreat excitement among cosmologists,
because lumps in the CMBR are believed to be the ancestors of lumpsof matter in our universe today. Making large surveys of the sky,astronomers are now able to locate the positions of thousands ofgalaxies in space and have found to their surprise that galaxiesare far from uniformly distributed. Enormous sheets of galaxies enclosehuge empty voids and form a structure that resembles a sponge orsoap foam. Similarly, large-scale studies of galaxy motions showthat huge regions of the universe are involved in high-speed bulkmotion relative to the CMBR. The complexity and enormous scale ofstructure in the universe surprised cosmologists. But an eleganttheory has been proposed to explain the formation of the recentlydiscovered large-scale structure. In this theory, puny irregularitiesin the CMBR, similar to those detected by the COBE satellite, areamplified over the eons by gravitational forces to become the lumpsand sheets of matter astronomers see in the universe today. As thistheory is tested by new data, new ideas, and new calculations, cosmologistsmay at long last understand, in broad outline, the mechanism forstructure formation in the universe.
The details of the statistical properties of the structure expectedin the universe are dependent on the type and quantity of “dark matter” that dominates the universe. Cosmologists look forward to anotherexciting discovery—the identification of this mysterious dark matter.Over the decades, astronomers have gathered evidence of unseen massbinding together galaxies and clusters of galaxies, but the natureof the dark matter still remains a mystery. Is it something thatwe already know about, like the stuff that makes up our Earth andSun? Or has nature concealed some completely new kind of matter fromour earthbound physics experiments? Theoretical particle physicsoffers many exotic candidates, raising the possibility of not onlydiscovering a major component of the universe but triggering a newera in particle physics as well. The search is on, in physics laboratoriesand at telescopes. A recent development is the search for dark matterby using its ability to bend light from distant stars or galaxies(gravitational lensing). Dark matter in clusters of galaxies andin the halo of our own galaxy is being studied with this elegantnew technique.
A dream of cosmologists is to be able to make detailed studies ofgalaxies at great distances and early times. What did galaxies looklike soon after formation? How do they evolve? When did they form?With the Hubble Space Telescope (HST) and giant (8- to 10-meter)new ground-based optical telescopes just commissioned and being built,the astronomical exploration of deep space (astronomers say “highredshift”) starts now. The repaired Hubble and the new Keck telescopehave racked up major discoveries in their first few months of operation,leaving no doubt that a new era is beginning for observational cosmology.
But what about even earlier, simpler cosmic times, before the galaxiesformed? The CMBR can bring us news from a time when the universewas only 150,000 years old, or 0.001 percent of its current age.To find out what happened at even earlier times cosmologists mustrely more on theoretical calculations, based on the physics we havelearned on Earth. Our current understanding of conditions in theuniverse penetrates to remarkably early times, because theoreticalpredictions of early events have measurable consequences today. Forexample, nuclear reactions predicted to occur when the universe wasonly about a minute old should have produced helium, deuterium, andother light elements. The predicted abundances agree exquisitelywith currently measured abundances of these elements! On less firmground, but even more amazing, is the idea that the same CMBR lumpsthat acted as seeds for the formation of the large-scale structurecan be traced back to quantum fluctuations occurring in the firstbillion billion billion billionth of a second after the Big Bang!If confirmed by measurements now under way, this knowledge will standamong the major triumphs of human ingenuity and imagination.
These are some of the current research frontiers that challenge andexcite cosmologists. The main body of this report discusses current
work on these issues in more depth. But before that, it describessome of the key questions that cosmologists are trying to answer.These give a broader perspective on the long-range goals of cosmologyresearch.
The Cosmic Questions
If you are unfamiliar with the basic ideas of the Big Bang model,this might be a good time to read the sidebars—“The Cosmic Picture” (p. 5) and “The Early Universe” (p. 8). They outline the various epochs of the evolving universeand put the following discussion into a larger context.
Spanning enormous ranges of time, space, radiation wavelength, matterdensity, and temperature, cosmology research (like most science)must be subdivided in order to match the scales of human activityand instrumentation. Individual researchers may work in a relativelysmall area of the subject, yet each has in sight a set of large questionsthat his or her results might address. It is the need to find answersto questions like the ones below that motivates the cosmologist.
When did the universe start and how will it end? The universe is expanding; galaxies are moving apart. Imagine thisprocess in reverse, like a movie run backwards. The universe wouldappear to collapse. So there must have been a time in the past whenthe universe was concentrated to high density. This moment is theorigin of the universe, the Big Bang. Measurements indicate thatthe universe is between 8 billion and 15 billion years old, onlya few times the age of Earth; intense research is under way to increasethe accuracy of this number. As for the end of the universe, thereare two possibilities: the present expansion may go on forever, orthe universe may stop expanding and come to a smooth halt, followedby collapse. Determining which will happen is harder than measuringthe age, but there are two ways to do it: (1) by measuring the averagemass density of the universe and finding out whether there is enoughgravitational force to stop the current expansion (i.e., whetherthe receding galaxies have “escape velocity”), or (2) by observing the expansionvelocities of galaxies at greater distances and earlier times tomeasure the rate at which the universe is slowing down. Both methodsare being aggressively pursued, and answers are possible before theend of this decade.
What is the dark matter and what is its cosmological role? Astronomers have been able to demonstrate that most of the matterin the universe cannot be observed directly. Dark matter does notshine like a star, and so astronomers cannot see it. But it mustbe present, because astronomers can observe its effect on other matterthat can be seen. The obvious candidate for this dark matter wouldbe ordinary matter in the form of old, burned-out stars or starsthat are too small to shine (“Jupiters”). But this idea seems to be ruledout by calculations of the synthesis of light elements (for example,helium), which occurred when the universe was between 1 and 100 secondsold. As noted above, these calculations predict abundances that arein good agreement with the measured abundances of light elements.The agreement holds, however, only on the assumption that the amountof ordinary matter present today is small, less than 20 percent ofthe amount of dark matter deduced from gravitational effects observedin today's universe. Something other than ordinary matter must bepresent, something quite exotic and revolutionary—new elementaryparticles, perhaps, that fit into an attractive theoretical schemeof things but have not yet been detected on Earth. The dominant formof matter in our universe is unknown to us!
How did the large-scale structure of matter form, and how large is it? When averaged over very large scales, the CMBR shows that the universeis quite smooth (homogeneous).
However, surveys of galaxies out to just 5 percent of the distancerepresented by the CMBR show the universe to be clumpy and uneven(see Figure 2). How much farther must astronomers go to find smoothness, on average?Assuming that the homogeneous scale is finally found, what accountsfor the complex structure graded from smooth to rough, and how didthe universe evolve to this state? Cosmologists must rely heavilyon special instrumentation to make very large sky surveys (encompassinga million galaxies!) to obtain the data to test theoretical predictions. Those predictions are based on theidea that feeble variations (“bumps”) in the CMBR show us the seeds ofthe structure-formation process. Important data on the bumps willcome from new CMBR maps showing even finer details than those revealedby the COBE satellite. Understanding the connections between CMBRbumps and the large-scale structure seen now will challenge observersand theorists for some time to come.
What can we learn about physical laws from relics of the Big Bang? When the universe was only about a minute old, the density and temperaturewere just right to quickly generate most of the helium and all ofthe deuterium present in the universe today. The remarkable thingis that our physics seems to work in a time and place so distantfrom us, a striking confirmation that physics as we know it can beused to study the early universe, and vice versa. There are otherways to improve our understanding of physics using other relics ofthe Big Bang. A current example is the effort of elementary-particlephysicists to understand the observed ratio (with a value of 1010) of photons from the CMBR to hydrogen atoms in the universe. Theanswer may be found in elementary-particle theory.
Did the universe undergo inflation at a very early stage? Despite many successes, the standard, simplest Big Bang model hasserious problems. For example, large regions of the universe observedtoday (using the CMBR) were not in causal contact at the time theearly universe was dense enough to settle down to a uniform temperature.That is, they were not close enough together to have ever exchangedlight signals, or information of any sort. If the regions were neverin causal contact, how did they manage to reach the same temperature,as indicated by the all-sky CMBR measurements? Also, why is the densityof the universe so close (within a
The Cosmic Picture
The pie-shaped figure on the next page shows a slice of the universewith the present-day Earth at the vertex, looking out and thereforelooking back in time. Properties of the nearby universe can be measureddirectly by telescopes, and so the picture is more accurate nearthe vertex. Farther out, our knowledge is based less on direct observationand more on calculations and our knowledge of physics.
An important feature of the universe is that, as astronomers lookat galaxies at greater and greater distances, they are seeing fartherand farther back in time, because the speed of light is finite andit takes time for light to travel from a distant galaxy to us. Thus,astronomers see galaxies as they were in the past—the more distantthe galaxy, the younger it was when the light left it, and thus theyounger it appears. Labeled on the figure are the ages (measuredfrom the Big Bang) of features as astronomers see them.
Our neighbors in the universe are other galaxies, each consistingof billions of stars. (The Milky Way Galaxy is relatively large,with about 100 billion stars.) Because many galaxies are “nearby” (the greatAndromeda Galaxy is only 2 million light years away) and can be seenby the naked eye, they are relatively bright and easy to study. However,astronomers are eager to gather the vast amount of information availablefrom more distant galaxies. This is the primary reason that astronomerswant to build larger, more sensitive telescopes and detectors. Bylooking deeper and deeper into the universe, astronomers hope towitness the birth and aging processes of galaxies, and to study theirdistribution in space—the large-scale structure of the universe.
Going deeper into the universe, cosmologists imagine a period whenstars and galaxies were forming for the first time. The age of theuniverse when stars and galaxies started to form is uncertain—somewherebetween 15 million years and 1 billion years. The first stars andgalaxies may have formed quickly, or the process may have been gradual.Cosmologists can only speculate because the data at this depth areso sparse. Paradoxically, cosmologists know more about the universewhen it was only 150,000 years old. This early time is explored usingthe cosmic microwave background radiation (CMBR) that fills the universe.As the universe aged and expanded, the CMBR cooled, ultimately reachingits present temperature of T = 2.73 K (K = degrees kelvin above absolute zero). When the universewas 15 million years old, the CMBR had a temperature of about 300K, which is close to room temperature (radiation temperatures areindicated at various times in the figure). An important epoch forthe universe occurred when the temperature was about 4,000 K (t ≈ 150,000 years). At higher temperatures (earlier times) atoms couldnot have existed because energetic collisions stripped electronsoff the nuclei. All matter in the universe was electrically charged,and this charged matter interacted strongly with the CMBR. As theuniverse cooled below 4,000 K, atoms formed, the matter became neutral,and the radiation-matter interaction ceased to be cosmologicallysignificant. Just slightly before this, gravitational clumping beganto prevail over the dispersing tendency of radiation as the radiationcooled and weakened. Gravity pulled matter together to form the firststars, galaxies, and large-scale structure. The seeds for this structurecan still be seen as tiny variations (anisotropy) in the intensityof the CMBR across the sky. These are vital clues to the structureformation process, coming from very early times.
Atoms formed during the epoch of photon decoupling, when the CMBRand matter first stopped interacting strongly. During this epoch,the universe was about a thousand times smaller, a billion timesdenser, and a thousand times hotter than it is now; it was filledwith visible light instead of the microwaves astronomers detect now.(Neutral gas is essentially transparent to light.) Sitting in spaceduring this epoch of the early universe would have been like sittinginside the Sun today—the light would have been blindingly brightin all directions.
The Early Universe
Before the epoch of photon decoupling, the CMBR was hotter yet, sohot that it constituted most of the energy in the universe at thattime. Labeled the “radiation era” in the figure, this period can be studied byapplying known laws of physics and by measuring the intensity ofthe CMBR at different wavelengths.
At t = 1 to 100 seconds after the Big Bang, something remarkable happened(as shown in the figure on p. 7). Calculations show that the temperature (T = 1 billion K) and density were just right for free protons and neutronsto combine via nuclear reactions, forming helium and other lightelements. The calculations use data measured in high-energy and nuclearphysics laboratories, and extrapolations of the density and temperatureof the universe today. Accurate agreement between the predicted andthe measured abundances of these elements is one of the great successesof the Big Bang model.
Going back to even earlier times tests particle physics theoriesat very high energies. Based on the best theories and observationsof relics from this high-energy physics era, particle physicistshave outlined a complex picture. The behavior of fundamental forcesand elementary particles dominates the processes in this era of extremelyhigh temperatures and densities. Forces merge and unify; particlesappear and disappear. Perhaps the most exotic hypothesis posits inflation,a huge, sudden acceleration in the expansion of the universe, drivenby a phase transition in a yet unknown field. Here cosmologists encounterthe limits of present knowledge.
The edge of the accessible universe is the causal horizon, a sphericalboundary centered on Earth with a radius of about 15 billion light-years(the speed of light × the age of the universe). Information frombeyond the causal horizon cannot reach us because there has not beenenough time since the Big Bang for any signal to travel so far, evenat the speed of light. But as the universe gets older, the horizonmoves out, bringing more of the unseen universe into view. What isthe nature of the “stuff” beyond the horizon? Lacking information and anadequate physical theory of the Big Bang itself, cosmologists canonly speculate. Observers on a galaxy a billion light-years awayfrom the Earth could draw a similar causal horizon around themselves,but their horizon would include parts of the universe that astronomerson Earth would not be able to see. Given the assumption that observerssee a similar universe regardless of where they are, the part thatwe call unseen must be similar to the part just inside our horizon.From this argument cosmologists conclude that the part of the universewe can see is embedded in a much larger universe of the same stuff,possibly extending to infinity, or possibly not.
The cosmological picture gets much fuzzier and more speculative asone tries to understand more distant, earlier conditions and events.But cosmology is a young science; most of our data and theories areless than 30 years old. It will be fascinating to see how this picturechanges and gets filled in over the next three decades.
factor of 10) to the critical density required to stop the expansion?For the density of the universe to be so close to the critical densitywould seem to require extraordinary coincidences in the values ofthe cosmological constants in the initial universe. Such fine tuningof the initial values is necessary to achieve the delicate balancebetween the kinetic energy of expansion and the potential energyof gravity and presents a problem for cosmologists. Theoretical physicistshave proposed a brilliant solution to these and other cosmologicalpuzzles. A concept called inflation proposes that the universe wentthrough a huge, rapidly accelerating expansion at extremely earlytimes (somewhere between 10−43 and 10−32 seconds). This proposal, which is based in elementary particle theory,solves the temperature uniformity problem by expanding a tiny uniformpiece of the universe into a region much larger than the region astronomerscan see today (see the sidebars). Inflation also adjusts the densityto precisely the critical value needed to balance the expansion.Like all worthwhile scientific theories, the idea of inflation istestable. For example, it predicts a particular size distributionfor the bumps in the CMBR. The initial results from the COBE satelliteare encouraging, but a worldwide effort is under way to extend thoseresults to smaller angles where unique signatures of the early universemay be found. The concepts of inflation and dark matter have revolutionizedmodern cosmology.
Do physics and cosmology offer a plausible description of creation? As cosmologists and physicists push the boundary of our understandingof the universe ever closer to its beginning, one has to wonder whetherthe creation event itself is explainable by physics as we know it,or can ever know it. Though such a program still seems quite fantastic,not so long ago it seemed utterly unthinkable. A few theoreticalphysicists have started to work on the problem. One approach, stillhighly speculative, is to consider our entire universe as the resultof a tiny quantum fluctuation in the vacuum. Under the right circumstancessuch a fluctuation could expand to scales unimaginably larger thanthe entire observable universe.
Clearly, these questions are at the heart of humankind's quest to understand our place in the cosmos. They involve someof the most fundamental unanswered questions of physical science.But why, in a time of great national needs and budget deficits, shouldthe U.S. taxpayer support such seemingly impractical research asthat described above?
Why Do Research in Cosmology?
In fact, far from being impractical, cosmological research producesimportant benefits for the nation and the world. First, it has uniquetechnical spinoffs. Forefront research in cosmology drives developmentsin instrumentation for the collection, manipulation, and detectionof radiation at radio, infrared, visible, ultraviolet, x-ray, andγ-ray wavelengths. The understanding and application of such typesof radiation are the foundation for many important technologies,such as radar, communications, remote sensing, optics, medical radiology,and many more. Modern astronomical instruments are usually one-of-a-kinddevelopments pioneered by teams of specialists who set out to achievethe best possible performance from their instrumentation. Instrumentteams involve astronomers, physicists, and engineers from observatories,universities, and industry. This process produces a high return innew ideas, devices, and methods in the general areas of radiationtechnology. Some of these projects are models for effective technologytransfer.
Another technical driver in cosmology is large-scale computing. Theoristspush the state of the art by demanding the largest, fastest
machines to run programs that model the universe. Such computer programs,or codes, model the evolution of systems of millions of gravitationallyinteracting particles. Codes for following the hydrodynamics of galaxyformation are the among the largest such codes in existence. Computersare severely taxed by the gigabytes of data streaming in from modernastronomical sensors. Indeed, large cosmological projects are nowdriving innovative hardware and software developments. For example,a new sky survey joins university astronomers with physicists atFermi National Accelerator Laboratory, the latter contributing theirspecial expertise in, and computer capability for, high-speed dataprocessing.
A second benefit of cosmology is its unique ability to probe matterunder extremes of density and temperature that can never be achievedin laboratories on Earth. The conditions in the early universe canbe used to test physical theories against the measured results ofnature's highest-energy experiment—the Big Bang. Pressures, temperatures, and densities in theearly universe are extremes beyond our experience, but not beyondour imagination and physical theories.
A third reason to pursue cosmology is its tremendous intellectualappeal. Few areas of the human endeavor excite the human imaginationas much as curiosity about the universe. How did it start? Why isit here? What is our role? How will it end? Throughout human historyone finds this desire for knowledge about the heavens and human existence,and virtually all periods of enlightenment and progress have beentimes of rapid discovery in astronomy and physical science. Futuregenerations will look back and evaluate our era's contributions similarly.
Finally, our cosmology—every culture's cosmology—serves as an ethical foundation stone, rarely acknowledged but vitalto the long-term survival of our culture. Cosmological knowledgeaffects religious beliefs, ethical choices, and human behavior, whichin turn have important long-term implications for humanity. For example,the notion of Earth as a limitless, indestructible home for humanityis vanishing as we realize that we live on a tiny spaceship of limitedresources in a hostile environment. How can our species make thebest of that? Cosmological time scales also offer a sobering perspectivefor viewing human behavior. Nature seems to be offering us millions,perhaps billions, of years of habitation on Earth. How can we increasethe chances that humans can survive for a significant fraction ofthat time? Cosmology can turn humanity's thoughts outward and forward,to chart the backdrop against which the possible futures of our speciescan be measured. This is not irrelevant knowledge; it is vital.
Over the past three decades, cosmologists have built up a base ofknowledge that can now be used to develop new ideas and better experimentsfor more exploration. Besides that knowledge base, a thriving scienceneeds a technology base to open new possibilities in instrumentationand analysis, and it needs a dedicated, excited work force. But aboveall, a scientific revolution needs a subject rich in undiscoveredknowledge.
These elements have just now come together for cosmology. A solidknowledge base has been developed, though it is meager compared towhat is possible. The technology base is superb, and developmentis accelerating, especially in the areas of large telescopes anddetectors with high sensitivity and resolution. The ability to getinstruments into space, onto balloons, and to unique sites like mountaintops and the South Pole has proved to be a boon for observers. Overthe last three decades, the evident possibilities of the field haveattracted more and more young astronomers and physicists. There nowis a new wave of well-trained young cosmologists, full of ideas andeager to push the field forward. If we recognize and exploit thisunique opportunity, the early 21st century may witness a revolutionin
cosmology as exciting as the revolution in physics that took placein the early 20th century.
The past 30 years have seen seminal discoveries in cosmology. TheBig Bang model is now established as the best description of theevolution of our universe. Observed remnants of its early stagesof high density and high temperature offer persuasive evidence thatcosmologists understand, in broad outline, the history of our evolvinguniverse. Yet urgent questions remain. Cosmologists have vastly improvedtheir overall understanding of the evolution of the universe andthe formation of structure within it. However, the age of the universeand its ultimate destiny are still not known accurately. Cosmologistsare uncertain of the timing of the epoch of galaxy formation andof the details of this complex process. Many candidates for the ubiquitousdark matter have been proposed, but none has yet been observed.
Cosmology is one of the most exciting disciplines in all of physicalscience. The discoveries and insights of the last three decades havefueled interest and ideas, attracting outstanding young people tothe field. The need for better data has driven major improvementsin technology, especially detectors and telescopes, and the volumeof new data has led cosmologists to press hard on the boundariesof computation. The combination of basic knowledge, highly motivatedyoung scientists, and a growing technology base offers an unprecedentedopportunity for further progress. The United States should continueto be a leader in this fast-moving, exciting area of science.
II. THE COSMIC MICROWAVE BACKGROUND RADIATION
What Is the Cosmic Microwave Background Radiation?
The cosmic microwave background radiation (CMBR), discovered in 1964,is a telltale remnant of the early universe. Its very existence iscompelling evidence that the universe has evolved from an extraordinarilyhot, compact beginning. To have produced radiation with the characteristicsof the CMBR, the universe must at one time have been entirely differentfrom what astronomers see today. No galaxies, stars, or planets existed:the universe was filled with elementary particles and radiation atextremely high energies.
The universe is between 8 billion and 15 billion years old. For allof that time, it has been expanding and the CMBR has been cooling.Currently, the radiation temperature is 2.73 K, which means thatmost of the CMBR exists now as radio energy in the microwave band.Man-made microwaves of the same sort link communication satellitesto stations on Earth. But there are two major differences betweensatellite microwaves and the CMBR: First, the CMBR comes from alldirections rather than from only one spot in the sky. Second, theCMBR has its power distributed over a wide range of microwave frequenciesrather than concentrated at a single frequency, as is the case fora radio transmitter. To get accurate information about the earlyuniverse, cosmologists must measure the CMBR over a wide range offrequencies and across most of the sky.
From such measurements, cosmologists believe that the CMBR has beenlargely unchanged, except for cooling down, during the entire historyof the universe. The complex evolution of matter in the universe—such as the formation of stars, galaxies, and large-scale structure—did not affect the CMBR. This radiation is a pristine cosmic remnant.It gives us a wonderful opportunity to look far back in time to studyeven fine details of the early universe. As cosmologists try to understandthe origin and evolution of structure in the universe today, it isessential to know about physical conditions that existed long ago.
What Do We Learn by Measuring the Properties of the CMBR?
Since the discovery of the CMBR, cosmologists have made measurementsof its intensity at different wavelengths—its spectrum. The Big Bangtheory predicts that the remnant radiation will have a special kindof spectrum, a thermal spectrum. The thermal spectrum has a characteristicshape, and the wavelength corresponding to the “peak” depends on the temperatureof the emitting body. The CMBR (at a temperature of 2.73 K) peaksat 2-mm wavelength; the Sun's thermal spectrum (6,000 K) peaks ata visible wavelength. Years of ground-based and balloon-based observationstraced out a crude spectrum that tended to support the Big Bang theory.However, it became clear in the mid-1970s that truly decisive measurementsof the CMBR needed to be done from space, above Earth's obscuringand bright (at these wavelengths) atmosphere. NASA's Cosmic BackgroundExplorer (COBE) satellite, which was launched in November 1989, wasspecifically designed to make accurate measurements of the CMBR.The first scientific result from the COBE satellite was an exquisitelyaccurate measurement of the CMBR spectrum. The spectrum matched thethermal shape, just as the Big Bang theory had predicted. The dataand the prediction are shown in Figure 1 (p. 1). This result provides strong support for the Big Bang theory.
The shape of the spectrum seen in Figure 1 has a distinguished history in physics for reasons not related tocosmology. Early in this century, Max Planck and others reluctantlyintroduced quantum physics to explain this same spectrum, emittedby all cavities at uniform temperature, regardless of the kind ofmaterial used to make the cavity. This same thermal spectrum nowturns out to match the