INNER WORKINGS

Inner Workings: Probing cosmic mysteries ina remote desertAmber DanceScience Writer

Amid the volcanic range of the windy, oth-erworldly Atacama Desert, a telescope collectsancient light from the cloudless sky. Theinternational team of cosmologists manningthe instrument hopes it will illuminate theconditions of the universe just after its dawn13.8 billion years ago.Most physicists believe that the universe

inflated rapidly, just a fraction of a nanosecondafter it came into being, although definitive

proof remains elusive. “All of a sudden,whoosh, inflation occurred, an exponen-tial expansion of space,” says Adrian Leeof the University of California, Berkeley, prin-cipal investigator on the US portion of theproject. “What we’re looking for is really asignal from the beginning of time.”The signal, if it exists, should show up in

the primordial light waves called cosmic mi-crowave background radiation (CMB). How-

ever, deciphering that signal is not alwaysstraightforward. In 2014, the researchers onthe Background Imaging of Cosmic Extra-galactic Polarization (BICEP) experimentthought they had evidence for inflation;later physicists determined the signal likelycame from dust in the Milky Way (1, 2).Lee’s project got the moniker POLAR-

BEAR as wordplay for what it hopes todetect: “POLARization of the BackgroundRadiation.” Swirling patterns in the directionof polarization should not only offer up aclincher for inflation, if it occurred, but also

The POLARBEAR telescope in Chile’s Atacama Desert aims to reveal the nature of neutrinos and the origins of the universe. Image courtesyof Adrian Lee.

www.pnas.org/cgi/doi/10.1073/pnas.1509007112 PNAS | July 14, 2015 | vol. 112 | no. 28 | 8513–8514

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help physicists deduce the mass of elementaryparticles called neutrinos thought to be key tounlocking multiple astronomical mysteries (3).Cosmic microwaves travel through space

nearly unimpeded. But when they hit Earth’satmosphere, they get absorbed by watermolecules. To circumvent this problem,the POLARBEAR researchers set up theirtelescope in Chile’s very dry Atacama, at5,200 meters above sea level, in 2011.The Atacama makes for an unusual work-

site. “It’s sort of like a Martian landscape, alldust and no plants,” says Darcy Barron, apostdoc with Lee at University of California,Berkeley. Sulfur dust from an old mine perme-ates the scientists’ possessions. There’s not onlyminimal water vapor but little oxygen as well.The researchers tote portable oxygen tanks.Without supplemental oxygen, “you canget pretty stupid pretty quick,” says ZigmundKermish, who earned his PhD with Lee andcontinues to work on POLARBEAR as a post-doc at Princeton University in New Jersey.The scientists make their home base in the

town of San Pedro de Atacama, about anhour’s drive away and 2,400 meters above sealevel. Moisture and life start to appear on theroad down: grass, hot springs, and flamingo-filled lagoons. The nearest department storeis a couple of hours away, and suppliers ofscientific equipment still farther. Whensomething breaks, the researchers have tomake do. “There’s lots of duct tape involved,”Kermish jokes.To detect the CMB flowing into their tele-

scope, they use an array of 1,274 detectors,called bolometers, made of metal layered

onto silicon. These bolometers, six millimetersacross, are cooled to 0.25 Kelvin so the incom-ing CMB will warm them up noticeably. Eachhas antennae pointing in a different direction,to pick up different angles of polarization (4, 5).The researchers map the directions of

polarization for each point in the sky, seekingthe telltale sign of inflation. “That reallyviolent event leaves ripples in space−timeitself, and those ripples affect the polariza-tion of the light,” says Kam Arnold, thePOLARBEAR project manager at Universityof California, San Diego. A swirling patternwould verify inflation; if it does not appear,the POLARBEAR cosmologists can neitherconfirm nor disprove the theory.They are confident they’ll get one result:

the mass of neutrinos. Researchers knowfrom previous research that it should betiny—between 58 millielectron volts and 150millielectron volts, Lee says—but not theprecise number (6). (An electron has amass of 511 kilo electronvolts.)That answer should be embedded in

POLARBEAR’s polarization map (7, 8). Thisis because the CMB waves traveled throughspace with nearly nothing in their path—

except the occasional galaxy or other matterthat deflected the light, just slightly. That de-flection, called gravitational lensing, will makeswirls on a smaller scale than inflation, andallow the cosmologists to map how the matterin the universe has settled into structures.Fast-moving neutrinos have been slower

to settle down compared with other matter.Physicists know how much matter thereshould be, so any mass “missing” from theirmap must be mostly neutrinos. They alsoknow how many neutrinos there are, so theywill be able to work out the neutrino mass.The POLARBEAR researchers are not alone

in the Atacama, nor in their collection ofCMB. They are in good-natured competitionwith the astrophysicists next door at theAtacama Cosmology Telescope (ACT) andat the South Pole Telescope in Antarctica,another high site with little water vapor. Inaddition, the Cosmology Large-Angular ScaleSurveyor, soon to join the Atacama set, andthe BICEP experiment, located in Antarctica,should be able to detect the inflationarysignature. But the latter two won’t be able todetermine the neutrino mass.“It’s a really terrific situation where you

have several experiments of comparable capa-bilities,” says cosmologist David Spergel ofPrinceton, who works on ACT. “It will be re-ally good to have independent experimentswith the same results.” The POLARBEAR de-tectors have the potential to be among themost sensitive of the bunch, he says, althoughtheir smallish telescope means lower resolu-tion. It is not able to resolve galaxy clustersbut is sufficient for the team’s goals of findingthe inflation signature and neutrino mass.By 2016, the POLARBEAR researchers

will have two new telescopes. Arnold expectsto have the neutrino measurement within5 years. Beyond that and inflation, Barronsays, they might detect other interestingfeatures of the hot, high-energy newbornuniverse. Perhaps, she speculates, they’ll seeevidence for a primordial magnetic field,which might explain why galaxies todayhave magnetic fields.“We’re opening a window onto the early

universe,” says Lee. “There could be newthings that no one has predicted yet.”

1 Ade PAR, et al.; (BICEP2 Collaboration) (2014) Detection of B-modepolarization at degree angular scales by BICEP2. Phys Rev Lett112(24):241101.2 Ade PAR, et al.; (BICEP2/Keck and Planck Collaborations) (2015)Joint analysis of BICEP2/Keck Array and Planck data. Phys Rev Lett 114(10):101301.3 McKee M (2014) News feature: Seeing the ghostly universe. ProcNatl Acad Sci USA 111(24):8699–8701.4 Keating B, et al. (2011) Ultra high energy cosmology withPOLARBEAR. arXiv:1110.2101.

5 Kermish ZD, et al. (2012) The POLARBEAR experiment. Proc SPIE8452:84521C.6 Planck Collaboration (2015) Planck 2015 results. XIII.Cosmological parameters. arXiv:1502.01589.7 Ade PAR, et al.; POLARBEAR Collaboration (2014) Measurement of thecosmic microwave background polarization lensing power spectrum withthe POLARBEAR experiment. Phys Rev Lett 113(2):021301.8 The Polarbear Collaboration (2014) A measurement of the cosmicmicrowave background B-mode polarization power spectrum at sub-degree scales with POLARBEAR. ApJ 794(2):171.

Aided by supplemental oxygen, researchers work on telescope instruments some 5,200meters above sea level. Image courtesy of Adrian Lee.

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