Untangling the Cosmic Microwave Background's Faint Polarization Patterns from the South Pole

In summer at the South Pole, a scientist in a red parka crosses a flat white plain where the snow is so dry it squeaks underfoot. A line of flags and a few large buildings are the only things standing on the landscape. The sun hangs at the horizon and never sets, under a cloudless blue sky. This is where researchers working at the U.S. National Science Foundation's Amundsen-Scott South Pole Station operate two projects, the South Pole Telescope (SPT) and BICEP, built to study the earliest moments of the universe.

The target is heat left over from the universe's infancy. Within its first second, by the standard account, the universe went from smaller than an electron to more than 100 trillion miles (161 trillion kilometers) across. The fastest part of that growth, called inflation, took place before the first trillionth of a trillionth of a second had passed. The universe swelled from a region smaller than a proton to something about the size of a marble, smoothing and flattening space and leaving little but the energy of the field that drove the expansion. That energy then turned into a hot soup of quarks and gluons, which joined into protons and neutrons about 10 millionths of a second later.

About 380,000 years after that first second, the universe turned from opaque to transparent. Hydrogen atoms formed and stopped scattering radiation, and light moved freely. That light is the cosmic microwave background, or CMB. It still holds the vast majority of the universe's photons, and its energy density exceeds all the light stars have ever produced. We cannot see it because the expanding universe has stretched its wavelengths from visible light through infrared into microwaves, so only radio telescopes detect it. It was first detected in the 1960s. Its temperature is just below 3 kelvins nearly everywhere, which means nothing drifting between galaxies can get colder than that.

The tiny departures from that uniformity matter most. Quantum fluctuations during inflation left small temperature variations in the CMB. Scientists also expect inflation to have produced primordial gravitational waves, ripples in space-time, which would leave a subtle imprint on the CMB's polarization, the orientation of its light. Because of how photons scattered when the CMB formed, that polarization is almost perfectly even, and the way it departs from perfect could signal the waves. Inflation is the only process thought capable of amplifying them, so detecting them would be solid evidence for it. "It's one of the ways that we can understand gravity at these very extreme scales," said Kimmy Wu, an assistant professor at Caltech.

Measuring polarization at all was a milestone. John Carlstrom, a University of Chicago professor and director of the SPT, led the Degree Angular Scale Interferometer project at the Pole, which detected the CMB's polarization in 2002. "There were peaks and wiggles, just as predicted," he recalled of that moment at the computer. The result opened new research just as dark energy and dark matter were coming into view. Brad Benson of Fermilab said the universe proved "much weirder than we expected," and researchers wanted better measurements to test whether the model still held together.

Antarctica suits that work. Measuring millimeter wavelengths demands high altitude, dry air and a thin atmosphere, and the South Pole offers all three. It is also designated a radio-quiet zone. Getting there means flying to New Zealand, collecting cold-weather gear, then riding military planes to the continent and ski-equipped turboprops to the Pole. Lindsey Bleem of Argonne, who visited in 2008 and 2014, called it a weird world. The constant sun scrambles body clocks, Carlstrom said, and winter-overs, who stay when no planes fly, face months of isolation. Low humidity makes static shocks a risk to electronics, mittens complicate simple tasks, snow can interfere, and parts are hard to get. "You have to plan like you're putting it in space," said Amy Bender of Argonne.

The instruments are custom-built. Bender said her team designs, makes and operates its own detectors. Zeeshan Ahmed of Stanford and SLAC put the stakes this way: the CMB's photons have traveled 13.8 billion years, and his job is the last microsecond of the journey. The SPT has a 10-meter (33-foot) mirror, resolves arcminute scales, and carries tens of thousands of sensitive detectors. Installed in 2007, it finished surveys in 2011 (SPT-SZ) and 2016 (SPTpol) and now runs the SPT-3G camera in its third major upgrade, well suited to mapping CMB polarization over large areas. It has also found more than 7,000 galaxy clusters, a window into dark matter and dark energy. The next test is whether those polarization maps show the faint signature of inflation.