SEARCH & DISCOVERY

COBE SATELLITE FINDS NO HINT OF EXCESSIN THE COSMIC MICROWAVE SPECTRUM

For several years cosmologists havebeen kept busy puzzling over reportsof a large excess in the cosmic micro-wave background spectrum at wave-lengths shorter than 1 mm. Nowthey've been given a brief respite. Atthe January meeting of the AmericanAstronomical Society in Washington,DC, John Mather of NASA's GoddardSpace Flight Center presented thefirst spectral results1 from COBE—the Cosmic Background Explorer—which had been launched into Earthorbit just eight weeks earlier. Themoment Mather placed the COBEspectrum (reproduced on this page) onthe overhead projector, the packedlecture hall burst into sustained ap-plause. The contrast with all theearlier, fragmentary data was aston-ishing. The spectral measurements,from 1 cm down to 0.5 mm, fitperfectly to a Planck blackbody radi-ation curve for a temperature of2.735 + 0.06 K. At the 1% level onecould see no deviation from an idealblackbody spectrum. And it's not justa question of fitting a shape. Withonly one free parameter—tempera-ture—the normalization also has tocome out right. For a given tempera-ture the absolute brightness of ablackbody spectrum is specified.

These spectacular results comefrom just 9 minutes of observationnear the north Galactic pole by theFar Infrared Absolute Spectrometer.FIRAS is one of the three observinginstruments aboard COBE. Thenorth pole of our Galaxy is relativelyfree of local microwave sources andthese were 9 minutes of particularlygood thermal stability for FIRAS, mak-ing it easy to analyze the data quickly.FIRAS will take data all over the skyfor about a year, until COBE's liquidhelium cryogen runs out. In the endit should be possible to detect depar-tures from a blackbody spectrum assmall as 0.1 % of the peak brightness.

The audience also heard reports offirst results from the Differential

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Cosmic microwave background spectrum measured by the FarInfrared Absolute Spectrometer1 aboard the Cosmic BackgoundExplorer. The COBE satellite was launched in November, and thesedata come from just 9 minutes of early observing near the Galacticnorth pole. The data points are shown here fitted to a Planckblackbody curve. This fit yields a temperature of 2.735 + 0.06 K. Atthe level of 1 % of peak brightness, these new observations show nodeviation from a perfect blackbody spectrum.

Microwave Radiometers and the Dif-fuse Infrared Background Experi-ment, the other two instrument sys-tems aboard COBE. But the DMRand DIRBE results were at even morepreliminary stages than the FIRASdata. George Smoot (LawrenceBerkeley Laboratory) presented thepreliminary sky maps shown on page18, displaying the DMR's first mea-surements of the variation of radi-ation intensity from place to place atthree microwave wavelengths. Thusfar, Smoot told his audience, the skymaps give evidence of no large-scalebackground variation other than thedipole moment attributed to the "pe-culiar" motion of our Galaxy towardthe Virgo cluster. The DMR has arelatively wide angular resolution of7°, designed for all-sky surveys and

measurements of large-angular-scalecosmological variations. Apart fromthe well-known dipole variation, theearly DMR measurements find thatthe microwave background is uniformacross the sky to a part in 104.

The blackbody spectrumMeasuring the cosmic microwavespectrum is difficult. At wavelengthsshorter than 1 cm, ground-based ob-servers have to contend with molecu-lar-absorption bands in the atmo-sphere. Rocket and balloon-borne ob-servations are all too brief. COBE isthe first orbiting observatory de-signed for this part of the spectrum.It cannot observe at wavelengthslonger than 1 cm, simply because itsapertures are too small. Ground-based observations at longer wave-

© 1990 Americon Insnrure of Physics PHYSICS TODAY MARCH 1990 17

lengths suffer only about 1% atmo-spheric attenuation, but the reradia-tion of that little bit of absorbedenergy adds a spurious 3 K to theapparent temperature of the sky.

The most recent reports of a consid-erable short-wavelength excess in thecosmic microwave background spec-trum had come from measurementswith a rocket-borne instrument devel-oped by a collaboration between Na-goya University and the University ofCalifornia, Berkeley. It was lofted bya sounding rocket to an altitude of 300km from a Japanese launch site inFebruary 1987 (see PHYSICS TODAY,August 1988, page 17). Experimentsof this kind are plagued with system-atic uncertainties that are extremelydifficult to sort out with the limiteddiagnostic capabilities of sounding-rocket packages. Paul Richards, lead-er of the Berkeley contingent, hadwarned that "given the history of thefield, confirmation is certainlyrequired."

Nonetheless, the theorists plungedin. In the standard Big Bang scenarioone expects an isotropic cosmic photonbackground with a blackbody tem-perature of about 3 K. The radiationwould have achieved thermal equilib-rium with matter before the profusecreation and absorption of photonscame to an end less than a year afterthe Big Bang. The resulting black-body spectrum would then havecooled, shifting its peak toward everlonger wavelengths as the universecontinued to expand. But if nothingvery bizarre happened, the coolingspectrum would have retained itsblackbody character through the "re-combination time"—some 300 000years later— and right down to thepresent day. "Recombination" refersto the neutralization of the cosmiccharged plasma (mostly protons andelectrons) when the radiation finallycooled down enough to permit thestable existence of neutral atoms. Thecosmos was now for the first timetransparent. One assumes that mostof the microwave photons recorded byCOBE have been traveling undis-turbed ever since recombination.

The cosmic radiation temperatureat recombination would have been4000 K, corresponding to about 2% ofthe binding energy of the hydrogenatom. Because the universe has ex-panded more than a thousandfold inthe 10 or 20 billion years since recom-bination, the blackbody temperatureof the cosmic background radiationshould by now have redshifted down tothe 3 K microwave regime.

ScenariosIn their attempts to explain black-

Maps of the microwave sky from COBE's six DifferentialMicrowave Radiometers, at three frequencies: 90 GHz (top),53 GHz (middle) and 31.5 GHz (bottom). Colors indicatetemperature of the microwave radiation; red is hottest. In theseGalactic coordinates the center of our Galaxy is in the middle andthe Galactic plane is horizontal. Because of the Sun's positionduring these three weeks, there were no data from the Galacticcenter or anticenter. The only large-scale variation to be seen is thedipole anisotropy attributed to our "peculiar" motion, which makesthe upper right about 0.2% hotter than the lower left. Otherwise,apart from obvious foreground sources, these preliminary data showthe cosmic microwave background to be isotropic to within a partin 104. The red points bordering the central hole are statisticalnoise where observing had just begun.

body-spectrum distortions of the kindreported by the Nagoya-Berkeleygroup, with excess radiation at shortwavelengths, the theorists have con-centrated on hypothetical scenariostaking place long after the recombina-tion time. It is conceivable, for exam-ple, that very early superstars orhyperactive protogalaxies released ex-traordinary amounts of radiant ener-gy that was then reradiated in theinfrared by a prodigious density ofcosmic dust. But such models hadgreat difficulty coming up withenough energy and dust to fit theNagoya-Berkeley spectral excess.

Somewhat more plausible were themodels that hypothesized the exis-tence of a hot intergalactic mediumwith a present temperature of 400million kelvin—suggestively similarto the apparent temperature of thediffuse x-ray background we seearound us. This highly ionized medi-um would have been even hotter(about 109 K) when it was formed,supposedly by a large output of energyfrom early quasars.

The hot electrons of this imaginedintergalactic medium would kick thecosmic background photons up tohigher energies by Compton scatter-ing. The dimensionless "Comptoniza-tion parameter" y describing the re-sultant distortion of the blackbodyspectrum is essentially an integral ofthe product of the density and tem-perature of free electrons over thepath of a photon through this medium.A non-negligible y requires that boththe density and temperature of freeelectrons in the intergalactic mediumbe very high. Mather reported thatthe absence of any observed short-wavelength excess in the early FIRASdata has already provided an upperlimit of 0.001 for y. This is smallenough to rule out the idea that a hotintergalactic medium is responsiblefor the diffuse x-ray background. Atthe same session of the AAS meeting,David Helfand and his Columbia col-leagues presented x-ray and radio-telescope data suggesting that high-energy radiation from a large popula-tion of faint radio galaxies is sufficient

1 8 PHYSICS TODAY MARCH 1990

SEARCH 6 DISCOVERYto explain the x-ray background.

As more FIRAS data are compiledand the instrument's calibration isbetter understood, tiny deviations onthe short-wavelength side of the black-body spectrum may eventually be seenat the level of a few tenths of a percent."Such deviations would be a real clueas to what went on in the informationdesert between recombination and thefirst quasars we can see," says COBEtheorist Edward Wright (UCLA). Atthe spring meeting of The AmericanPhysical Society in Washington nextmonth, the FIRAS group will reportnew spectral data all the way down to0.1 mm.

Longer-wavelength distortionsDepartures from the blackbody curveat wavelengths longer than a fewmillimeters would tell us about anearlier era—the time from recombina-tion back to about a year after the BigBang. In that era the density ofionized matter was sufficient to keepphotons from going very far withoutscattering, but insufficient to permitmuch creation or absorption of pho-tons. If during that era there wassome significant reheating of the elec-trons in the expanding universe, theywould have shared this new thermalenergy with the photons. But in theabsence of adequate photon-creatingmechanisms, the reestablished ther-mal equilibrium distribution of pho-tons would no longer be an idealblackbody spectrum. One would see,down to the present day, a Bose-Einstein distribution with a nonvan-ishing "chemical potential"//. That isto say, the exponential expihv/kT1) inthe Planck blackbody formula wouldbe replaced by exp/hv/kT + \x).

The peculiar heat source leading tosuch a chemical potential might havebeen the dissipation of extensive plas-ma turbulence or the decay of someexotic particle species we don't yetknow about. In any case, the earlyFIRAS data show nothing of the kind.The agreement of the long-wavelengthdata with the Planck formula alreadysets an upper limit of 0.009 on //.However, the best place to look for anonvanishing n is at wavelengthslonger than 1 cm, where hv/kT be-comes very small. But that's whereCOBE runs out of room. Ground-baseddata have for the last ten years beensuggesting a cosmic blackbody tem-perature slightly lower than 2.7 K.This might be a spurious consequenceof atmospheric absorption inad-equately considered. On the otherhand, it might be a real indication of apositive chemical potential. "If FIRAScan pin down the spectral brightnessat half a centimeter to within 0.1%,"

Wright told us, "we'll be able to put areally meaningful limit on //."

FIRAS is a Michelson interferometerspectrometer that employs two black-bodies with adjustable temperatures:an internal reference source and anexternal calibrator. The calibrator,which comes within 0.01% of being aperfect blackbody, can be moved tocover the entrance of the instru-ment's input horn, much like a trum-pet mute. The absolute brightnessspectrum of the cosmic microwavebackground is determined with greataccuracy by adjusting the tempera-ture of the internal reference sourceto null out the input from the sky asnearly as possible and then interpos-ing the calibrator in place of the sky.

Meanwhile the ground-based ob-servers have not been idle. A Berke-ley-Milan collaboration that operatesa new microwave spectrometer at theSouth Pole will report its first resultsfor wavelengths longer than 1 cm atthe April APS meeting. This collabor-ation includes Smoot and othermembers of COBE's Lawrence Berke-ley Lab contingent wearing two hats.

Differential radiometersThe preliminary sky maps shown onpage 18 were obtained from threeweeks of DMR observing at31,53and90 GHz, corresponding to wave-lengths of 9.5, 5.7 and 3.3 mm, respec-tively. The maps are shown in Galac-tic coordinates. The empty space inthe middle of each map covers thecenter of our Galaxy, in which direc-tion DMR had not yet taken data.The plane of the Galaxy is horizontal.Because COBE's principal concernsare cosmological, microwave signalsfrom the Galaxy or other nearbysources represent unwanted fore-ground obfuscation. The DMR is infact six separate radiometers, two foreach receiving frequency. These fre-quencies were carefully chosen tooptimize the distinction between localsources and the cosmic background.

The upper right-hand region of eachmap is about 0.1% hotter than themean, and the lower left-hand regionis correspondingly cooler. This issimply the well-known dipole asym-metry attributed to the Doppler shift-ing of the microwave background byour own motion relative to the privi-leged "comoving reference frame" inwhich the cosmic background is pre-sumed to be isotropic. Other hot spotsand mottling appearing on these mapsare attributed to foreground sourcesor statistical noise. True cosmologicalhot spots would show up at all threefrequencies. Even though the statis-tics are still very limited, and parts ofthe sky have not yet been covered,

these maps represent the most exten-sive all-sky microwave survey yetavailable. At this early stage, theDMR data indicated no effective tem-perature variation AT from place toplace in the cosmic microwave back-ground (except for the Doppler dipolemoment) at the level of 10 ~4 in LTIT.

Radiotelescope measurements withmuch finer angular resolution—onthe order of minutes of arc rather thandegrees—have already confirmed2 theuniformity of the microwave back-ground in some directions to a fewparts in 105. But such fine-grainedmeasurements are of necessity limitedto small patches of sky. The search forfluctuations on the arcminute scale isparticularly relevant to the difficultquestion of how the universe managedto evolve from the seemingly wrinkle-free time of recombination to thepresent richly patterned epoch ofgaping voids and superclusters ofgalaxies more than 100 million lightyears across. Even the largest of thesestructures—the 170-megaparsec"Great Wall" of galaxies recentlyreported by Margaret Geller and JohnHuchra:! of the Harvard-SmithsonianCenter for Astrophysics—would havegrown from a seed fluctuation extend-ing over less than one degree of arc inthe microwave background.

Cosmic multipolesWith its 7° angular resolution, theDMR is primarily searching for muchlarger-scale variations across the sky,which one would parametrize in termsof the quadrupole, octopole, hexadeca-pole and higher moments of the angu-lar distribution of cosmic microwavetemperatures. (Any intrinsic cosmo-logical dipole moment would bemasked by the Doppler effect of ourpeculiar motion.) The inflationaryBig Bang model does in fact call forsuch large-scale anisotropies, origi-nating from the large-scale end of themodel's initial spectrum of fluctu-ations. Indeed it predicts definiteratios for the quadrupole, octopole andhexadecapole moments.

Though these predicted variationsare large in angular scale, they arevery small in amplitude. The 10~4

uniformity of the early data is not yetin conflict with the inflationary mod-el. The DMR is expected to continuetaking data for another year after theliquid helium runs out, because unlikethe other COBE instruments, it doesnot require cryogenics. In the end, theDMR will have mapped the entire skyseveral times over with the 10 ~5

sensitivity necessary to address thesepredictions. The DMR achieves itshigh sensitivity to small temperaturedifferences between patches of sky by

PHYSICS TODAY MARCH 1990 1 9

continually switching a single receiv-er between two identical horn anten-nae pointing in different directions.The radiometers sweep the sky as thespacecraft spins on its axis and itsnear-polar orbit stays close to theEarth's twilight zone.

If it turns out that the cosmologicalquadrupole moment is much larger,relative to the higher moments, thanis required by the standard inflation-ary model, one will have to think ofmore exotic explanations: Perhapsthe universe as a whole is spinning, orthe general expansion is not isotropic."There are perfectly good models for anonisotropic Big Bang," Wright toldus. "The only way to discard them iswith the COBE data." At the Wash-ington AAS meeting Smoot pointedout that the early DMR data alreadyput an upper limit on the cosmic

quadrupole moment that translatesinto a reassuring limit on how fast theuniverse might be spinning. "If it isspinning," he said, " it seems to havecompleted less than a thousandth of aturn since the Big Bang."

—BERTRAM SCHWARZSCHILD

References1. J. Mather, E. Cheng, R. Eplee, R. Isaac-

man, S. Meyer, R. Schafer, R. Weiss, E.Wright, C. Bennett, N. Boggess, E.Dwek, S. Gulkis, M. Hauser, M. Jans-sen, T. Kelsall, P. Lubin, S. Moseley, T.Murdock, R. Silverberg, G. Smoot. D.Wilkinson, Astrophys. J. Lett., to bepublished (1990).

2. A. Readhead, C. Lawrence, S. Myers, W.Sargent, H. Hardebeck, A. Moffet,Astrophys. J. 346, 566 (1989).

3. M. Geller, J. Huchra, Science 246, 897(1989).

DO OXIDE SUPERCONDUCTORSBEHAVE AS FERMI LIQUIDS?When a photon knocks an electronout of a material, the freed electronreveals some secrets about the elec-tronic environment it left behind.Such clues are sought by theoristspuzzling over the mechanisms forhigh-temperature superconductivity,but high-resolution data have onlybecome available in the past year.The behavior they reveal resemblesthe familiar patterns of conventionalsuperconductors in the normalstates—but with subtle and complexdeviations that are now the focus ofintense theoretical attention. Thedata have confirmed earlier measure-ments of a superconducting gap andprovided direct evidence for a Fermiedge in momentum space.

Traditionally, photoemission mea-surements, which are very useful forstudying metals, insulators and semi-conductors, have not been applied toconventional superconductors be-cause these methods do not havesufficiently fine energy resolution toprobe the energy region of greatestinterest in a superconductor—thenarrow forbidden-energy gap thatopens as electrons near the Fermisurface pair up at slightly lowerenergy in the superconducting state.According to the BCS theory, whichhas been so successful in describingconventional superconductors, thesize of the superconductivity gap de-pends on the critical temperature.Thus if the same mechanism is re-sponsible for the superconductivity inthe oxide materials, whose criticaltemperatures are about ten timeshigher, the superconductivity gap is

expected to be within the reach ofphotoemission experiments.

Furthermore photoemission offersa unique opportunity to probe thebehavior of the Fermi surface bothabove and below the critical tempera-ture. Not only are such experimentsof renewed interest, but recent im-provements in sample preparationand increases in angular and energyresolution have given these tech-niques the required sensitivity.

Normal-sfQfe behaviorAngle-resolved photoemission dataare uniquely qualified to answer thequestion of whether the oxide materi-als behave as Fermi liquids in theirnormal state. As described by LevLandau, a Fermi liquid is a system ofweakly interacting particles, knownas quasiparticles, whose behavior isqualitatively similar to that of asystem of noninteracting fermions.The Fermi liquid has formed theconceptual framework for under-standing the behavior of the low-temperature superconductors abovetheir critical temperatures. Althoughthere is no a priori reason why theoxide superconductors should act asFermi liquids, many researchers havetended to compare the observed be-havior to this standard. It serves forthem as a starting point from whichthey might extend the theory toexplain high-temperature supercon-ductivity. Others have nearly com-pletely scrapped this viewpoint andhave proposed new mechanisms ofinteraction that depart radically fromthe BCS theory. One such approach is

the resonating-valence-bond theory,which envisions that the departure ofthe electron creates not a hole but twoentities—a chargeless "spinon" and aspinless, charged "holon."

In Fermi liquids the locus of pointsin k space defines a Fermi surface (kis the lattice wavevector for zero-energy single-particle excitations).All electrons in the system must beaccommodated within the volume ofthis surface in momentum space. Ifquasiparticles exist, they should giverise to a peak in the energy spectrum,with that peak growing more narrowas k approaches the Fermi momen-tum. Beyond the Fermi energy thepeak should disappear. By measuringthe energy spectrum for differentphotoelectron emission angles re-searchers can essentially probe differ-ent regions of k space and determinewhether the quasiparticle peak existsand how it behaves as the Fermimomentum is approached.

In photoemission studies, the in-coming photon gives essentially all itsenergy to a single electron, which isassumed not to have appreciable in-teractions as it leaves the crystal.The momentum of the photon isnegligible, so that the electron thatemerges has momentum nearly oppo-site to that of electrons remainingbehind in the sample. The mostenergetic electrons ejected come fromthe Fermi surface. To locate theposition of the Fermi surface theexperimenters first measure the ener-gy spectrum of a normal metal. Then,with the same initial value of photonenergy, they measure the energyspectrum at selected emission angles.Three groups have published angle-resolved photoemission spectra, all onthe compound Bi.2Sr.,CaCu208, forwhich it is easy to prepare high-quality, chemically stable single-crys-talline surfaces.

The figure on page 21 shows theenergy distribution reported by anexperiment' that has simultaneouslyboth high angular and energy resolu-tion (30 meV). It was performed at theSynchrotron Radiation Center of theUniversity of Wisconsin by a collabor-ation consisting of Clifford Olson,Rong Liu, An-Ban Yang and DavidLynch (Iowa State University), ScottList and Al Arko (Los Alamos Nation-al Laboratory) and Boyd Veal, YingChuan Chang, Pei-Zhi Jiang and PaulPaulikas (Argonne National Labora-tory). Similar results2 with betterangular resolution and somewhat low-er energy resolution were obtained bygroup working at HASYLAB in Ham-burg, West Germany. The research-ers there are Recardo Manzke, T.Buslaps and R. Claessen (University

2 0 PHYSICS TODAY MARCH 1990