INNER WORKINGS INNER WORKINGS Inner Workings: Probing cosmic mysteries in a remote desert Amber Dance Science Writer ever, deciphering that signal is not always straightforward. In 2014, the researchers on the Background Imaging of Cosmic Extra- Amidthevolcanicrangeofthewindy,oth- proof remains elusive. “All of a sudden, galactic Polarization (BICEP) experiment erworldly Atacama Desert, a telescope collects whoosh, inflation occurred, an exponen- thought they had evidence for inflation; ancient light from the cloudless sky. The tial expansion of space,” says Adrian Lee later physicists determined the signal likely international team of cosmologists manning of the University of California, Berkeley, prin- came from dust in the Milky Way (1, 2). ’ the instrument hopes it will illuminate the cipal investigator on the US portion of the Lee s project got the moniker POLAR- conditions of the universe just after its dawn project. “What we’re looking for is really a BEARaswordplayforwhatithopesto 13.8 billion years ago. signal from the beginning of time.” detect: “POLARization of the Background Most physicists believe that the universe The signal, if it exists, should show up in Radiation.” Swirling patterns in the direction inflated rapidly, just a fraction of a nanosecond the primordial light waves called cosmic mi- of polarization should not only offer up a after it came into being, although definitive crowave background radiation (CMB). How- clincher 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 courtesy of Adrian Lee.

www.pnas.org/cgi/doi/10.1073/pnas.1509007112 PNAS | July 14, 2015 | vol. 112 | no. 28 | 8513–8514 Downloaded by guest on September 29, 2021 except the occasional galaxy or other matter that deflected the light, just slightly. That de- flection, called gravitational lensing, will make swirls on a smaller scale than inflation, and allow the cosmologists to map how the matter in the universe has settled into structures. Fast-moving neutrinos have been slower to settle down compared with other matter. Physicists know how much matter there should be, so any mass “missing” from their map must be mostly neutrinos. They also know how many neutrinos there are, so they will be able to work out the neutrino mass. The POLARBEAR researchers are not alone in the Atacama, nor in their collection of CMB. They are in good-natured competition with the astrophysicists next door at the Atacama Cosmology Telescope (ACT) and at the South Pole Telescope in Antarctica, another high site with little water vapor. In addition, the Cosmology Large-Angular Scale Aided by supplemental oxygen, researchers work on telescope instruments some 5,200 Surveyor, soon to join the Atacama set, and meters above sea level. Image courtesy of Adrian Lee. the BICEP experiment, located in Antarctica, should be able to detect the inflationary signature. But the latter two won’t be able to help physicists deduce the mass of elementary onto silicon. These bolometers, six millimeters determine the neutrino mass. particles called neutrinos thought to be key to across, are cooled to 0.25 Kelvin so the incom- “It’s a really terrific situation where you unlocking multiple astronomical mysteries (3). ing CMB will warm them up noticeably. Each have several experiments of comparable capa- Cosmic microwaves travel through space has antennae pointing in a different direction, ’ bilities,” says cosmologist David Spergel of nearly unimpeded. But when they hit Earth s to pick up different angles of polarization (4, 5). “ atmosphere, they get absorbed by water The researchers map the directions of Princeton, who works on ACT. It will be re- ally good to have independent experiments molecules. To circumvent this problem, polarization for each point in the sky, seeking ” the POLARBEAR researchers set up their the telltale sign of inflation. “That really with the same results. The POLARBEAR de- telescope in Chile’s very dry Atacama, at violent event leaves ripples in space−time tectors have the potential to be among the 5,200 meters above sea level, in 2011. itself, and those ripples affect the polariza- most sensitive of the bunch, he says, although The Atacama makes for an unusual work- tion of the light,” says Kam Arnold, the their smallish telescope means lower resolu- site. “It’s sort of like a Martian landscape, all POLARBEAR project manager at University tion. It is not able to resolve galaxy clusters ’ dust and no plants,” says Darcy Barron, a of California, San Diego. A swirling pattern but is sufficient for the team s goals of finding postdoc with Lee at University of California, would verify inflation; if it does not appear, the inflation signature and neutrino mass. Berkeley. Sulfur dust from an old mine perme- the POLARBEAR cosmologists can neither By 2016, the POLARBEAR researchers ates the scientists’ possessions. There’snotonly confirm nor disprove the theory. will have two new telescopes. Arnold expects minimal water vapor but little oxygen as well. They are confident they’ll get one result: to have the neutrino measurement within The researchers tote portable oxygen tanks. the mass of neutrinos. Researchers know 5 years. Beyond that and inflation, Barron Without supplemental oxygen, “you can from previous research that it should be says, they might detect other interesting get pretty stupid pretty quick,” says Zigmund tiny—between 58 millielectron volts and 150 features of the hot, high-energy newborn Kermish, who earned his PhD with Lee and millielectron volts, Lee says—but not the universe. Perhaps, she speculates, they’ll see continues to work on POLARBEAR as a post- precise number (6). (An electron has a evidence for a primordial magnetic field, doc at Princeton University in New Jersey. mass of 511 kilo electronvolts.) which might explain why galaxies today The scientists make their home base in the That answer should be embedded in have magnetic fields. town of San Pedro de Atacama, about an POLARBEAR’s polarization map (7, 8). This “We’re opening a window onto the early hour’s drive away and 2,400 meters above sea is because the CMB waves traveled through universe,” says Lee. “There could be new level. Moisture and life start to appear on the space with nearly nothing in their path— things that no one has predicted yet.” road down: grass, hot springs, and flamingo- filled lagoons. The nearest department store is a couple of hours away, and suppliers of 1 Ade PAR, et al.; (BICEP2 Collaboration) (2014) Detection of B-mode 5 Kermish ZD, et al. (2012) The POLARBEAR experiment. Proc SPIE scientific equipment still farther. When polarization at degree angular scales by BICEP2. Phys Rev Lett 8452:84521C. something breaks, the researchers have to 112(24):241101. 6 Planck Collaboration (2015) Planck 2015 results. XIII. “ ’ ” 2 Ade PAR, et al.; (BICEP2/Keck and Planck Collaborations) (2015) Cosmological parameters. arXiv:1502.01589. make do. There slotsofducttapeinvolved, Joint analysis of BICEP2/Keck Array and Planck data. Phys Rev Lett 114(10): 7 Ade PAR, et al.; POLARBEAR Collaboration (2014) Measurement of the Kermish jokes. 101301. cosmic microwave background polarization lensing power spectrum with To detect the CMB flowing into their tele- 3 McKee M (2014) News feature: Seeing the ghostly universe. Proc the POLARBEAR experiment. Phys Rev Lett 113(2):021301. Natl Acad Sci USA 111(24):8699–8701. 8 The Polarbear Collaboration (2014) A measurement of the cosmic scope, they use an array of 1,274 detectors, 4 Keating B, et al. (2011) Ultra high energy cosmology with microwave background B-mode polarization power spectrum at sub- called bolometers, made of metal layered POLARBEAR. arXiv:1110.2101. degree scales with POLARBEAR. ApJ 794(2):171.

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