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UltimateRetroModern echoes of theearly universe

Two teams of astronomers have for thefirst time detected the surviving notes of acosmic symphony created just after the BigBang, when the universe was a foggy soupof matter and radiation. The discovererssay that the survival of the acoustic imprintfrom this early epoch, 13.7 billion years ago,provides compelling new evidence that theblueprint for the present distribution ofgalaxies was set at the time of the Big Bangby random subatomic fluctuations.

In 1999, researchers detected a specificpattern of acoustic oscillations in the faint,ancient whisper of radiation—the cosmicmicrowave background—left over from theBig Bang. This week, Shaun Cole of the Uni-versity of Durham in England and his col-leagues announced that they had discernedremnants of that pattern while analyzing

data from the Two-Degree Field RedshiftGravity Survey, a large-scale analysis of220,000 galaxies. The map covers one-twentieth the area of the sky out to a dis-tance of 2 billion light-years from Earth.

Another team, led by Daniel Eisensteinof the University of Arizona in Tucson,examined a subset of 46,000 galaxies fromanother sky map, the Sloan Digital Sky Sur-vey, which covers one-quarter of the sky.

Each team used a different method ofanalysis but found the same acoustic pat-tern. The groups reported their findingsthis week at a meeting of the AmericanAstronomical Society in San Diego.

The signals are so weak that, to detectthem even in large-galaxy surveys, “bothgroups had to work quite hard,” notes cos-mologist David N. Spergel of PrincetonUniversity. “The result is another impor-tant milestone in establishing a standardmodel for cosmology.”

The early universe rang like a bell, notesSpergel. As gravity drew together clumps ofatomic matter, radiation—then tightlybound to that matter—exerted an outwardpressure. The tug-of-war between gravity’spull and radiation’s push generated pres-sure waves, or acoustic oscillations.

About 400,000 years after the Big Bang,the universe had cooled sufficiently for theradiation to break free from matter andtravel unimpeded into space. Now in theform of microwaves, this radiation pervadesthe universe and provides a snapshot of thecosmos at that early time, ripples and all.

The small size of the fluctuations, both inthe microwave background and the galaxydistribution today, provides additional evi-dence that most of the mass of the universeis composed of dark matter—an exotic,invisible, and primordial material that has

never interacted with light and so had nevergenerated sound waves, notes Spergel.

Eisenstein notes that, using the length ofthe sound waves as a cosmic ruler,astronomers can calculate the universe’sexpansion. Both of the new studies agreewith earlier reports that cosmic expansionis speeding up (SN: 5/22/04, p. 330). Theuniverse’s first symphony therefore providesindependent evidence that the cosmos isfilled with dark energy, which causes theacceleration. —R. COWEN

When LazinessPaysMath explains howcooperation and cheating evolve

Researchers have proposed a solution to along-standing evolutionary conundrum:Why do populations of identical organismssometimes split into two strains, coopera-tors and cheaters?

Cooperation and cheating, inescapableparts of human existence, also occur in awide range of other creatures, down to thetiniest. For instance, in a yeast population,some cells produce the enzymes required todigest sugars, while other cells mooch offtheir enzyme-producing fellows.

Since all the yeast cells live in the sameenvironment, why are some cooperatorsand others cheaters? For decades,researchers have struggled to explain howsuch dichotomies arise. Now, a trio of math-ematicians and zoologists has proposed amodel for the split.

The team assumed that the populationstarts out consisting of identical organisms.Each individual is capable of producing, atsome cost to itself, a certain common good—such as an enzyme—that benefits both theindividual and the population. Theresearchers then set up mathematical equa-tions describing how the individuals couldchange their levels of production over time.

In many cases, the researchers found thatin their model, the organisms first evolvetogether to an equilibrium, beyond whichthe production cost to each individualincreases faster than its benefit does. Butthat’s not the end of the story. For some cost-benefit parameters, the population goes onto diversify into cooperators and cheaters.

“For a long time, no one had realized thiscould happen,” said Timothy Killingback ofthe College of William and Mary inWilliamsburg, Va., last week at the JointMathematics Meetings in Atlanta. Killing-back developed the model with MichaelDoebeli of the University of British Colum-bia in Vancouver and Christoph Hauert ofHarvard University.

SCIENCENEWSThis Week

SOUND REASONING Distribution of modern-day galaxies as revealed by the Sloan DigitalSky Survey (top). Lower Images show, left to right, a typical patch of sky in the survey, thetelescope used for the survey, and a depiction of a primordial sound wave detectable today(also presented as red circle in sky survey).SD

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The reason for the split, Killingback says,is that sometimes at equilibrium the pop-ulation is making enough of the desirablegood to support some slackers. If one organ-ism is then lucky enough to mutate in a waythat reduces its production, it will repro-duce in high numbers, thriving at theexpense of others.

As the cheaters become more plentiful,the other organisms have less of the goodfor themselves and compensate by pro-ducing even more. With time, the twostrains grow farther and farther apart.

The researchers have performed computersimulations that confirm their analysis.

The new work is a “really important con-tribution” to the study of cooperation, saysPriyanga Amarasekare, an ecologist at theUniversity of Chicago. However, she cau-tions, the model is more likely to be usefulfor simple organisms such as yeast cells thanfor more-complicated creatures like peo-ple, whose strategies are not determinedpurely by their genes.

Killingback agrees but says that themodel could be formulated to deal withcases in which cooperative and cheatingstrategies are passed on through cultureand education rather than just throughgenes. “It would be fascinating to seewhether the model checks out in humans,”he says. —E. KLARREICH

ReptilianRepastAncient mammals preyedon young dinosaurs

Two nearly complete sets of fossilized ani-mal remains from 130-million-year-oldrocks in China are revealing fresh detailsabout the size and dietary habits of ancientmammals. The newly described findscounter the common presumption thatsuch creatures remained small and ecolog-ically suppressed until dinosaurs becameextinct about 65 million years ago.

The fossils were unearthed from volcanicash laid down by an ancient eruption. Manyof the entombed animals have been pre-served in their original three-dimensionalforms, says Jin Meng, a paleontologist atthe American Museum of Natural Historyin New York.

The larger of the two finds, a 1-meter-long mammal dubbed Repenomamus

giganticus, had a stout build and squat pos-ture like that of modern-day badgers. Thewear pattern on the creature’s teeth hintsthat the mammal was a young adult whenthe lethal eruption occurred, says Meng.

He and his colleagues, who describe thefossils in the Jan. 13 Nature, estimate that R.giganticus weighed between 12 and 14 kilo-grams, which makes the species the largestmammal known to have lived during the170-million-year reign of dinosaurs.

The other set of remains—in some ways,the more astounding one—is a nearly com-plete specimen of Repenomamus robustus,a smaller cousin of R. giganticus. This opos-sum-size species was first described fromfragmentary remains about 5 years ago.

Within the well-preserved rib cage of thenew specimen, the researchers found a com-pact wad of broken bones and teeth. Someof those remains, including a skull, spinalcolumn, and limb bones, still had some jointsintact, suggesting that the bones derive fromprey that had been dismembered and swal-lowed in chunks. Comparisons with knownfossils suggest that those fragments belongto a hand-length hatchling of a Psitta-cosaurus, a common plant-eating dinosaurthat grew to a length of around 2 meters.

The newfound Repenomamus fossils arechallenging the popular notion that themammals that lived side by side withdinosaurs generally were as small as shrewsand ecologically marginalized, says AnneWeil, a paleontologist at Duke University in

Durham, N.C. A few previous finds hadshown signs that some ancient mammalswere substantially larger than shrews andrats, but the fossils were so fragmentarythat the size estimates were imprecise.

The dinosaur-as-prey scenario upsetsanother long-held paleontological con-tention by suggesting that predatory mam-mals may have influenced dinosaur evolu-tion. For example, says Weil, some feathereddinosaur contemporaries of Repenomamusover time became smaller—and thus pre-sumably better equipped to fly or escapepredation. At the same time, some mid-sizedinosaurs were becoming larger in perhapsanother evolutionary response to mam-malian predation. —S. PERKINS

Hands-on MathInsightsTeachers’ mismatchedgestures boost learning

As teachers instruct a child, they typicallyuse their hands as well as their voices, butonly certain gestures pack a powerful edu-cational punch, a new study suggests.Grade-schoolers best learn how to solve aparticular mathematics problem when ateacher’s gestures convey different infor-mation than his or her words do, say

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SCIENCENEWSThis Week

COLD MEAL Two mammals from the genus Repenomamus—badger-size creatures thatlived in Asia about 130 million years ago—dine on a dinosaur hatchling.

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