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In the beginning, there was light. Under the intense condi-tions of the early universe, ionized matter gave off radia-tion that was trapped within it like light in a dense fog. But

as the universe expanded and cooled, electrons and protonscame together to form neutral atoms, and matter lost its abili-ty to ensnare light. Today, some 14 billion years later, the pho-tons from that great release of radiation form the cosmic mi-crowave background (CMB).

Tune a television set between channels, and about 1 percentof the static you see on the screen is from the CMB. When as-tronomers scan the sky for these microwaves, they find that thesignal looks almost identical in every direction. The ubiquityand constancy of the CMB is a sign that it comes from a sim-pler past, long before structures such as planets, stars and galax-ies formed. Because of this simplicity, we can predict the prop-erties of the CMB to exquisite accuracy. And in the past fewyears, cosmologists have been able to compare these predictionswith increasingly precise observations from microwave tele-scopes carried by balloons and spacecraft. This research hasbrought us closer to answering some age-old questions: Whatis the universe made of? How old is it? And where did objectsin the universe, including our planetary home, come from?

Arno Penzias and Robert Wilson of AT&T Bell Laborato-ries detected the CMB radiation in 1965 while trying to find thesource of a mysterious background noise in their radio anten-na. The discovery firmly established the big bang theory, whichstates that the early universe was a hot, dense plasma of chargedparticles and photons. Since that time, the CMB has beencooled by the expansion of the universe, and it is extremely coldtoday—comparable to the radiation released by a body at a

temperature of 2.7 kelvins (that is, 2.7 degrees Celsius aboveabsolute zero). But when the CMB was released, its tempera-ture was nearly 3,000 kelvins (or about 2,727 degrees C).

In 1990 a satellite called COBE (for Cosmic BackgroundExplorer) measured the spectrum of the CMB radiation, show-ing it to have exactly the expected form. Overshadowing thisimpressive achievement, however, was COBE’s detection ofslight variations—at the level of one part in 100,000—in thetemperature of the CMB from place to place in the sky. Ob-servers had been diligently searching for these variations formore than two decades because they hold the key to under-standing the origin of structure in the universe: how the pri-mordial plasma evolved into galaxies, stars and planets.

Since then, scientists have employed ever more sophisti-cated instruments to map the temperature variations of theCMB. The culmination of these efforts was the launch in 2001of the Wilkinson Microwave Anisotropy Probe (WMAP),which travels around the sun in an orbit 1.5 million kilome-ters beyond Earth’s. The results from WMAP reveal that theCMB temperature variations follow a distinctive pattern pre-dicted by cosmological theory: the hot and cold spots in the ra-diation fall into characteristic sizes. What is more, researchershave been able to use these data to precisely estimate the age,composition and geometry of the universe. The process is anal-ogous to determining the construction of a musical instrumentby carefully listening to its notes. But the cosmic symphony isproduced by some very strange players and is accompanied

New observations of the cosmic microwave background radiation show that the early universe resounded with harmonious oscillations

By Wayne Hu and Martin White

SYMPHONY

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SOUND WAVES in the early universe—represented by tuning forks in thisphotograph—reveal the age, composition and geometry of the cosmos.

HE COSMIC

by even stranger coincidences that cry out for explanation.Our basic understanding of the physics behind these obser-

vations dates back to the late 1960s, when P. James E. Peeblesof Princeton University and graduate student Jer Yu realizedthat the early universe would have contained sound waves. (Atalmost the same time, Yakov B. Zel’dovich and Rashid A. Sun-yaev of the Moscow Institute of Applied Mathematics werecoming to very similar conclusions.) When radiation was stilltrapped by matter, the tightly coupled system of photons, elec-trons and protons behaved as a single gas, with photons scat-tering off electrons like ricocheting bullets. As in the air, a smalldisturbance in gas density would have propagated as a soundwave, a train of slight compressions and rarefactions. The com-pressions heated the gas and the rarefactions cooled it, so anydisturbance in the early universe resulted in a shifting patternof temperature fluctuations.

Sounding Out OriginsWHEN DISTANCES in the universe grew to one thousandth oftheir current size—about 380,000 years after the big bang—thetemperature of the gas decreased enough for the protons to cap-ture the electrons and become atoms. This transition, called re-combination, changed the situation dramatically. The photonswere no longer scattered by collisions with charged particles,so for the first time they traveled largely unimpeded throughspace. Photons released from hotter, denser areas were more en-ergetic than photons emitted from rarefied regions, so the pat-tern of hot and cold spots induced by the sound waves wasfrozen into the CMB. At the same time, matter was freed of theradiation pressure that had resisted the contraction of denseclumps. Under the attractive influence of gravity, the denser ar-eas coalesced into stars and galaxies. In fact, the one-in-100,000variations observed in the CMB are of exactly the right ampli-tude to form the large-scale structures we see today [see “Readingthe Blueprints of Creation,” by Michael A. Strauss, on page 54].

Yet what was the prime mover, the source of the initial dis-turbances that triggered the sound waves? The question is trou-bling. Imagine yourself as an observer witnessing the big bang

and the subsequent expansion. At any given point you will seeonly a finite region of the universe that encompasses the distancelight has traveled since the big bang. Cosmologists call the edgeof this region the horizon, the place beyond which you cannotsee. This region continuously grows until it reaches the radiusof the observable universe today. Because information cannotbe conveyed faster than light, the horizon defines the sphere ofinfluence of any physical mechanism. As we go backward intime to search for the origin of structures of a particular physi-cal size, the horizon eventually becomes smaller than the struc-ture [see illustration on opposite page]. Therefore, no physicalprocess that obeys causality can explain the structure’s origin.In cosmology, this dilemma is known as the horizon problem.

Fortunately, the theory of inflation solves the horizon prob-lem and also provides a physical mechanism for triggering theprimordial sound waves and the seeds of all structure in the uni-verse. The theory posits a new form of energy, carried by a fielddubbed the “inflaton,” which caused an accelerated expansionof the universe in the very first moments after the big bang. Asa result, the observable universe we see today is only a smallfraction of the observable universe before inflation. Further-more, quantum fluctuations in the inflaton field, magnified bythe rapid expansion, provide initial disturbances that are ap-proximately equal on all scales—that is, the disturbances tosmall regions have the same magnitude as those affecting largeregions. These disturbances become fluctuations in the energydensity from place to place in the primordial plasma.

Evidence supporting the theory of inflation has now beenfound in the detailed pattern of sound waves in the CMB. Be-cause inflation produced the density disturbances all at once inessentially the first moment of creation, the phases of all thesound waves were synchronized. The result was a sound spec-trum with overtones much like a musical instrument’s. Consid-er blowing into a pipe that is open at both ends. The fundamen-tal frequency of the sound corresponds to a wave (also called amode of vibration) with maximum air displacement at either endand minimum displacement in the middle [see top illustration inbox on page 48]. The wavelength of the fundamental mode istwice the length of the pipe. But the sound also has a series ofovertones corresponding to wavelengths that are integer fractionsof the fundamental wavelength: one half, one third, one fourthand so on. To put it another way, the frequencies of the overtonesare two, three, four or more times as high as the fundamental fre-quency. Overtones are what distinguish a Stradivarius from anordinary violin; they add richness to the sound.

The sound waves in the early universe are similar, exceptnow we must imagine the waves oscillating in time instead ofspace [see bottom illustration in box on page 48]. In this anal-ogy, the length of the pipe represents the finite duration whensound waves traveled through the primordial plasma; the wavesstart at inflation and end at recombination about 380,000 yearslater. Assume that a certain region of space has a maximumpositive displacement—that is, maximum temperature—at in-flation. As the sound waves propagate, the density of the regionwill begin to oscillate, first heading toward average tempera-

46 S C I E N T I F I C A M E R I C A N F E B R U A R Y 2 0 0 4

■ Inflation, the rapid expansion of the universe in the firstmoments after the big bang, triggered sound wavesthat alternately compressed and rarefied regions of theprimordial plasma.

■ After the universe had cooled enough to allow theformation of neutral atoms, the pattern of densityvariations caused by the sound waves was frozen intothe cosmic microwave background (CMB) radiation.

■ By studying the acoustic signals in the CMB,cosmologists have estimated the age, composition andgeometry of the universe. But the results suggest thatthe biggest component of the modern cosmos is amysterious entity called dark energy.

Overview/Cosmic Acoustics

ture (minimum displacement) and then toward minimum tem-perature (maximum negative displacement). The wave thatcauses the region to reach maximum negative displacement ex-actly at recombination is the fundamental wave of the early uni-verse. The overtones have wavelengths that are integer fractionsof the fundamental wavelength. Oscillating two, three or moretimes as quickly as the fundamental wave, these overtones causesmaller regions of space to reach maximum displacement, ei-ther positive or negative, at recombination.

How do cosmologists deduce this pattern from the CMB?They plot the magnitude of the temperature variations againstthe sizes of the hot and cold spots in a graph called a powerspectrum [see box on page 51]. The results show that the re-gions with the greatest variations subtend about one degreeacross the sky, or nearly twice the size of the full moon. (At thetime of recombination, these regions had diameters of aboutone million light-years, but because of the 1,000-fold expan-sion of the universe since then, each region now stretches near-

ly one billion light-years across.) This first and highest peak inthe power spectrum is evidence of the fundamental wave, whichcompressed and rarefied the regions of plasma to the maximumextent at the time of recombination. The subsequent peaks inthe power spectrum represent the temperature variationscaused by the overtones. The series of peaks strongly supportsthe theory that inflation triggered all the sound waves at thesame time. If the perturbations had been continuously gener-ated over time, the power spectrum would not be so harmo-niously ordered. To return to our pipe analogy, consider the ca-cophony that would result from blowing into a pipe that hasholes drilled randomly along its length.

The theory of inflation also predicts that the sound wavesshould have nearly the same amplitude on all scales. The pow-er spectrum, however, shows a sharp drop-off in the magnitudeof temperature variations after the third peak. This discrepan-cy can be explained by the fact that sound waves with shortwavelengths dissipate. Because sound is carried by the collisions

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TIMELINE OF THE UNIVERSEAS INFLATION EXPANDED the universe, the plasma of photonsand charged particles grew far beyond the horizon (the edge ofthe region that a hypothetical viewer after inflation would seeas the universe expands). During the recombination period

about 380,000 years later, the first atoms formed and thecosmic microwave background (CMB) radiation was emitted.After another 300 million years, radiation from the first starsreionized most of the hydrogen and helium.

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COSMIC HARMONICSTHE SOUND SPECTRUM of the early universe had overtones muchlike a musical instrument’s. If you blow into a pipe, the soundcorresponds to a wave with maximum air compression (blue) atthe mouthpiece and maximum rarefaction (red) at the end

piece. But the sound also has a series of overtones with shorterwavelengths that are integer fractions of the fundamentalwavelength. (The wavelengths of the first, second and thirdovertones are one half, one third and one fourth as long.)

SOUND WAVES also oscillated in the plasma of the earlyuniverse. After inflation, the fundamental wave compressedsome regions of plasma and rarefied others, causing thetemperature of the CMB radiation in the regions to reach

maximum (blue) and minimum (red) values by the time ofrecombination. The overtones oscillated two, three or moretimes as quickly, causing smaller regions to reach maximum andminimum CMB temperatures at the time of recombination.

Hot region

Cold region

Maximum compressionMaximum rarefactionSOUND WAVES IN A PIPE

ACOUSTIC OSCILLATIONSIN THE EARLY UNIVERSE

of particles in gas or plasma, a wave cannot propagate if itswavelength is shorter than the typical distance traveled by par-ticles between collisions. In air, this distance is a negligible 10–5

centimeter. But in the primordial plasma just before recombi-nation, a particle would typically travel some 10,000 light-yearsbefore striking another. (The universe at this stage was denseonly in comparison with the modern universe, which is abouta billion times as rarefied.) As measured today, after its 1,000-fold expansion, that scale is about 10 million light-years. There-fore, the amplitudes of the peaks in the power spectrum aredamped below about 10 times this scale.

Just as musicians can distinguish a world-class violin froman ordinary one by the richness of its overtones, cosmologistscan elucidate the shape and composition of the universe by ex-

amining the fundamental frequency of the primordial soundwaves and the strength of the overtones. The CMB reveals theangular size of the most intense temperature variations—howlarge these hot and cold spots appear across the sky—which inturn tells us the frequency of the fundamental sound wave. Cos-mologists can precisely estimate the actual size of this wave atthe time of recombination because they know how quicklysound propagated in the primordial plasma. Likewise, re-searchers can determine the distance CMB photons have trav-eled before reaching Earth—about 45 billion light-years. (Al-though the photons have traveled for only about 14 billionyears, the expansion of the universe has elongated their route.)

So cosmologists have complete information about the tri-angle formed by the wave and can check whether its angles addup to 180 degrees—the classic test of spatial curvature. They doso to high precision, showing that aside from the overall ex-pansion, the universe obeys the laws of Euclidean geometry andmust be very close to spatially flat. And because the geometryof the universe depends on its energy density, this finding im-plies that the average energy density is close to the so-called crit-ical density—about 10–29 gram per cubic centimeter.

The next thing cosmologists would like to know is the ex-act breakdown of the universe’s matter and energy. The am-plitudes of the overtones provide the key. Whereas ordinarysound waves are driven solely by gas pressure, the sound wavesin the early universe were modified by the force of gravity.Gravity compresses the gas in denser regions and, dependingon the phase of the sound wave, can alternately enhance orcounteract sonic compression and rarefaction. Analyzing themodulation of the waves reveals the strength of gravity, whichin turn indicates the matter-energy composition of the medium.

As in today’s universe, matter in the early universe fell intotwo main categories: baryons (protons and neutrons), which

make up the bulk of so-called ordinary matter, and cold darkmatter, which exerts gravity but has never been directly ob-served because it does not interact with ordinary matter or lightin any noticeable way. Both ordinary matter and dark mattersupply mass to the primordial gas and enhance the gravitationalpull, but only ordinary matter undergoes the sonic compres-sions and rarefactions. At recombination, the fundamentalwave is frozen in a phase where gravity enhances its compres-sion of the denser regions of gas [see box on page 52]. But thefirst overtone, which has half the fundamental wavelength, iscaught in the opposite phase—gravity is attempting to compressthe plasma while gas pressure is trying to expand it. As a result,the temperature variations caused by this overtone will be lesspronounced than those caused by the fundamental wave.

This effect explains why the second peak in the power spec-trum is lower than the first. And by comparing the heights of thetwo peaks, cosmologists can gauge the relative strengths of grav-ity and radiation pressure in the early universe. This measure-ment indicates that baryons had about the same energy densityas photons at the time of recombination and hence constituteabout 5 percent of the critical density today. The result is in spec-tacular agreement with the number derived from studies of light-element synthesis by nuclear reactions in the infant universe.

The general theory of relativity, however, tells us that mat-ter and energy gravitate alike. So did the gravity of the photonsin the early universe also enhance the temperature variations?It did, in fact, but another effect counterbalanced it. After re-combination, the CMB photons from denser regions lost moreenergy than photons from less dense areas, because they wereclimbing out of deeper gravitational-potential wells. This pro-cess, called the Sachs-Wolfe effect, reduced the amplitude of thetemperature variations in the CMB, exactly negating the en-hancement caused by the gravity of the photons. For regions ofthe early universe that were too big to undergo acoustic oscil-lations—that is, regions stretching more than one degree acrossthe sky—temperature variations are solely the result of the

w w w . s c i a m . c o m S C I E N T I F I C A M E R I C A N 49

The cosmic symphony is produced by very strange players and is accompanied

by even stranger coincidences.

WAYNE HU and MARTIN WHITE are trying to unveil the history ofthe universe. Hu is associate professor of astronomy and astro-physics at the University of Chicago. He received his Ph.D. in phys-ics from the University of California, Berkeley, in 1995. His re-search pursuits include the investigation of dark energy, darkmatter and the formation of cosmological structure. White, pro-fessor of astronomy and physics at Berkeley, earned his Ph.D. inphysics from Yale University in 1992. In addition to exploring howstructure in the universe came to be, he is interested in the con-nections between astrophysics and fundamental physics.

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Sachs-Wolfe effect. At these scales, paradoxically, hot spots inthe CMB represent less dense regions of the universe.

Finally, cosmologists can use the CMB to measure the pro-portion of dark matter in the universe. The gravity from baryonsalone could not have modulated the temperature variationsmuch beyond the first peak in the power spectrum. An abun-dance of cold dark matter was needed to keep the gravitation-al-potential wells sufficiently deep. By measuring the ratios ofthe heights of the first three peaks, researchers have determinedthat the density of cold dark matter must be roughly five timesthe baryon density. Therefore, dark matter constitutes about25 percent of the critical density today.

Remarkable ConcordUNFORTUNATELY, these calculations of the modern uni-verse’s matter and energy leave about 70 percent of the criticaldensity unspecified. To make up the difference, theorists haveposited a mysterious component called dark energy, whose rel-ative influence has grown as the universe has expanded [see“Out of the Darkness,” by Georgi Dvali, on page 68]. We arethus led by degrees to an improbable conclusion: most of theuniverse today is composed of invisible dark matter and darkenergy. Worse yet, dark matter and dark energy seem to be co-incidentally comparable in energy density today, even thoughthe former vastly outweighed the latter at recombination. Physi-cists dislike coincidences; they prefer to explain the world in

terms of cause and effect rather than dumb luck. What is more,another mysterious component, the inflaton, dominated thevery early universe and seeded cosmic structure. Why shouldwe believe a cosmological model that is based on the seeming-ly fanciful introduction of three enigmatic entities?

One reason is that these three entities explain a wealth ofpreviously known facts. Dark matter was first postulated in the1930s to explain measurements of the local mass density ingalaxy clusters. Albert Einstein introduced the concept of darkenergy in 1917 when he included the so-called cosmologicalconstant in his equations to counteract the influence of gravity.He later disavowed the constant, but it was resurrected in the1990s, when observations of distant supernovae showed thatthe expansion of the universe is accelerating [see “From Slow-down to Speedup,” by Adam G. Riess and Michael S. Turner,on page 62]. The energy densities of dark matter and dark en-ergy, as measured from the CMB, are in striking accord withthese astronomical observations.

Second, the standard cosmological model has predictivepower. In 1968 Joseph Silk (now at the University of Oxford)predicted that the small-scale acoustic peaks in the CMB shouldbe damped in a specific, calculable way. As a result, the corre-sponding radiation should gain a small but precisely known po-larization. (Polarized radiation is oriented in a particular di-rection.) One might assume that the CMB would be unpolar-ized because the scattering of the photons in the primordial

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AFTER THE EMISSION of the cosmicmicrowave background (CMB) radiation,about 380,000 years after the big bang,most of the photons traveled across theobservable universe without scattering.But some photons did scatter offcharged particles, polarizing theradiation across wide swaths of thesky. Observations of this large-anglepolarization by the WMAP spacecraftimply that about 17 percent of the CMBphotons were scattered by a thin fog ofionized gas a few hundred million yearsafter the big bang.

This relatively large fraction isperhaps the biggest surprise from theWMAP data. Cosmologists hadpreviously theorized that most of theuniverse’s hydrogen and helium wouldhave been ionized by the radiation fromthe first stars, which were extremelymassive and bright. (This process iscalled reionization because it returnedthe gases to the plasma state that

existed before the emission of theCMB.) But the theorists estimated thatthis event occurred nearly a billionyears after the big bang, and thereforeonly about 5 percent of the CMB photonswould have been scattered. WMAP’sevidence of a higher fraction indicates amuch earlier reionization and presentsa challenge for the modeling of the firstrounds of star formation. The discoverymay even challenge the theory ofinflation’s prediction that the initial

density fluctuations in the primordialuniverse were nearly the same at allscales. The first stars might haveformed sooner if the small-scalefluctuations had higher amplitudes.

The WMAP data also contain anotherhint of deviation from scale invariancethat was first observed by the COBEsatellite. On the biggest scales,corresponding to regions stretchingmore than 60 degrees across the sky,both WMAP and COBE found a curiouslack of temperature variations in theCMB. This deficit may well be a statisticalfluke: because the sky is only 360degrees around, it may not containenough large-scale regions to make anadequate sample for measuringtemperature variations. But sometheorists have speculated that thedeviation may indicate inadequacies inthe models of inflation, dark energy orthe topology of the universe.

—W.H. and M.W.

NOTES OF DISCORD

FIRST STARS reionized the surrounding gas.

plasma would have randomized their direction. But on thesmall scales where damping occurs, photons can travel with rel-atively few scatterings, so they retain directional informationthat is imprinted as a polarization of the CMB. This acousticpolarization was measured by the Degree Angular Scale Inter-ferometer (an instrument operated at the Amundsen-ScottSouth Pole Station in Antarctica) and later by WMAP; the val-ue was in beautiful agreement with predictions. WMAP alsodetected polarization on larger scales that was caused by scat-tering of CMB photons after recombination [see box on op-posite page].

Furthermore, the existence of dark energy predicts addition-

al phenomena in the CMB that are beginning to be observed. Be-cause dark energy accelerates the expansion of the universe, itweakens the gravitational-potential wells associated with theclustering of galaxies. A photon traveling through such a regiongets a boost in energy as it falls into the potential well, but be-cause the well is shallower by the time the photon climbs backout, it loses less energy than it previously gained. This phenom-enon, called the integrated Sachs-Wolfe effect, causes large-scaletemperature variations in the CMB. Observers have recently seenhints of this correlation by comparing large structures in galaxysurveys with the WMAP data. The amount of dark energy need-ed to produce the large-scale temperature variations is consistent

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THE POWER SPECTRUM

OBSERVATIONS OF THE CMB provide a map of temperaturevariations across the whole sky (a). When researchersanalyze portions of that map (b), they use band filters toshow how the temperature of the radiation varies at differentscales. The variations are barely noticeable at large scalescorresponding to regions that stretch about 30 degreesacross the sky (c) and at small scales corresponding toregions about a tenth of a degree across (e). But thetemperature differences are quite distinct for regions aboutone degree across (d). This first peak in the power spectrum(graph at bottom) reveals the compressions and rarefactionscaused by the fundamental wave of the early universe; thesubsequent peaks show the effects of the overtones.

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GRAVITATIONAL MODULATION

INFLUENCE OF DARK MATTER modulates the acoustic signals inthe CMB. After inflation, denser regions of dark matter thathave the same scale as the fundamental wave (represented astroughs in this potential-energy diagram) pull in baryons andphotons by gravitational attraction. (The troughs are shown in

red because gravity also reduces the temperature of anyescaping photons.) By the time of recombination, about380,000 years later, gravity and sonic motion have workedtogether to raise the radiation temperature in the troughs(blue) and lower the temperature at the peaks (red).

AT SMALLER SCALES, gravity and acoustic pressure sometimesend up at odds. Dark matter clumps corresponding to a second-peak wave maximize radiation temperature in the troughs longbefore recombination. After this midpoint, gas pressure pushes

baryons and photons out of the troughs (blue arrows) whilegravity tries to pull them back in (white arrows). This tug-of-wardecreases the temperature differences, which explains why thesecond peak in the power spectrum is lower than the first.

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FIRST PEAKGravity and sonic motionwork together

SECOND PEAKGravity counteractssonic motion

with the amount inferred from the acoustic peaks and the distantsupernovae. As the data from the galaxy surveys improve andother tracers of the large-scale structure of the universe becomeavailable, the integrated Sachs-Wolfe effect could become an im-portant source of information about dark energy.

No Requiem YetTHE CMB MAY ALSO provide crucial new evidence thatcould explain what happened during the very first moments af-ter the big bang. Few aspects of cosmology are more bizarrethan the period of inflation. Did the universe really inflate, and,if so, what was the nature of the inflaton, the theoretical fieldthat caused the rapid expansion? Current measurements of theCMB have dramatically strengthened the case for the simplestmodels of inflation, which assume that the amplitudes of the

initial density fluctuations were the same at all scales. But ifmore detailed observations of the CMB reveal that the ampli-tudes varied at different scales, the simple inflation modelswould be in trouble. More baroque alternatives would need tobe invoked or altogether different paradigms adopted.

Another exciting possibility is that we could learn about thephysics of inflation by determining the energy scale at which ittook place. For example, physicists believe that the weak nu-clear force and the electromagnetic force were different aspectsof a single electroweak force when the universe was hotter than1015 kelvins. If researchers determine that inflation occurred atthis energy scale, it would strongly imply that the inflaton hadsomething to do with electroweak unification. Alternatively, in-flation could have occurred at the much higher temperatures atwhich the electroweak force merges with the strong nuclearforce. In this case, inflation would most likely be associatedwith the grand unification of the fundamental forces.

A distinctive signature in the CMB could allow researchersto settle this issue. In addition to spawning density perturba-tions, inflation created fluctuations in the fabric of spacetime it-self. These fluctuations are gravitational waves whose wave-lengths can stretch across the observable universe. The ampli-tude of these gravitational waves is proportional to the squareof the energy scale at which inflation took place. If inflation oc-curred at the high energies associated with grand unification,the effects might be visible in the polarization of the CMB.

Last, further observations of the CMB could shed somelight on the physical nature of dark energy. This entity mightbe a form of vacuum energy, as Einstein had hypothesized, butits value would have to be at least 60 and perhaps as much as120 orders of magnitude as small as that predicted from parti-cle physics. And why is dark energy comparable to dark mat-

ter in density now and apparently only now? To answer thesequestions, researchers can take advantage of the fact that CMBphotons illuminate structures across the entire observable uni-verse. By showing the amplitude of density fluctuations at dif-ferent points in cosmic history, the CMB can reveal the tug-of-war between matter and dark energy.

Measurements of two CMB phenomena could be particu-larly useful. The first, called the Sunyaev-Zel’dovich effect, oc-curs when CMB photons are scattered by the hot ionized gas ingalaxy clusters. This effect allows galaxy clusters to be identi-fied during the crucial period, about five billion years ago, whendark energy began to accelerate the expansion of the universe.The number of galaxy clusters, in turn, indicates the amplitudeof density fluctuations during this time. The second phenome-non, gravitational lensing, happens when CMB photons pass

by a particularly massive structure that bends their trajectoriesand hence distorts the pattern of temperature and polarizationvariations. The degree of lensing reveals the amplitude of themass density fluctuations associated with these structures.

To conduct these investigations of inflation and dark ener-gy, however, researchers will need a new generation of CMBtelescopes that can observe the radiation with even greater sen-sitivity and resolution. In 2007 the European Space Agencyplans to launch the Planck spacecraft, a microwave observa-tory that will be placed in the same orbit as WMAP. Planck willbe able to measure CMB temperature differences as small asfive millionths of a kelvin and detect hot and cold spots thatsubtend less than a tenth of a degree across the sky. Such mea-surements will enable scientists to glimpse the full range ofacoustic oscillations in the CMB and thus sharpen their pictureof the inflationary spectrum. A multitude of ground-based ex-periments are also under way to study CMB effects associatedwith structure in the current epoch of accelerated expansion.

Although the standard cosmological model appears to workremarkably well as a phenomenological description of the uni-verse, a deeper understanding of its mysteries awaits the findingsof these experiments. It seems clear that the cosmic symphonywill continue to enchant its listeners for some time to come.

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Wrinkles in Time. George Smoot and Keay Davidson. William Morrow, 1994.

3K: The Cosmic Microwave Background Radiation. R. B. Partridge.Cambridge University Press, 1995.

The Inflationary Universe: The Quest for a New Theory of CosmicOrigins. Alan H. Guth and Alan P. Lightman. Perseus, 1998.

More information about WMAP and the cosmic microwave background can be found at map.gsfc.nasa.gov and background.uchicago.edu

M O R E T O E X P L O R E

We are led by degrees to an improbable conclusion:most of the universe today is composed

of invisible dark matter and dark energy.