Hunting primordial gravitational waves in the polarised glow of the early universe

Astrophysicists working at the Amundsen-Scott South Pole Station are using the South Pole Telescope (SPT) and BICEP experiments to look for a faint polarisation pattern in the cosmic microwave background (CMB), known as the B-mode signal. If detected, it would indicate primordial gravitational waves and give strong support to the theory of cosmic inflation. A feature from the US Department of Energy, republished by Phys.org, describes the science behind the search and the demands of doing it in Antarctica. It does not report a detection.

Why polarisation carries the clue

The CMB is the light released roughly 380,000 years after the universe began, when matter cooled enough to form neutral atoms and radiation could travel freely. Stretched by cosmic expansion, that light now arrives as microwaves, with a temperature just below 3 kelvins in every direction. Its uniformity is one of its defining features, but small ripples in temperature trace quantum fluctuations from the inflationary era.

Polarisation offers a second record. Because of the way photons scattered as the CMB formed, its polarisation is almost perfectly even. Subtle departures from that evenness could reflect gravitational waves produced in the first instants. The feature notes that inflation is the only process expected to amplify such waves enough to be detectable. Kimmy Wu, an assistant professor at the California Institute of Technology, links the search to a broader question: how gravity behaves at extreme scales and early times.

The CMB's polarisation was first detected in 2002 by the Degree Angular Scale Interferometer at the South Pole, led by John Carlstrom, now a professor at the University of Chicago and director of the SPT. That result opened a new line of research at a time when dark energy and dark matter were prompting physicists to test the standard cosmological model more closely.

Why the South Pole

CMB telescopes must measure millimetre-scale wavelengths, which water vapour in the atmosphere readily disturbs. The South Pole's altitude, dryness and thin atmosphere make it one of the best sites on Earth for the task. The site is also designated a radio-quiet zone, which limits interference.

The environment makes the work hard in other ways. Extremely low humidity raises the risk of static discharge that can damage electronics, snow can build up and cause interference, and replacement parts are difficult to obtain. No planes arrive during winter, so small teams of winter-over staff keep the experiments running for months. Amy Bender, a physicist at Argonne National Laboratory, said the planning is comparable to preparing an instrument for space.

Custom instruments

The SPT has a 10-metre mirror, observes mainly at millimetre wavelengths and carries tens of thousands of sensitive detectors. Its mirror size gives it arcminute resolution. It was installed in 2007 and has since completed surveys with the SPT-SZ camera (2011) and SPTpol (2016). It is now on its third major upgrade, using the SPT-3G camera, which the feature describes as well suited to surveying large areas of the sky. The source text is cut off before describing the camera in more detail.

According to Bender, the detectors are designed, built and operated by the research groups themselves. Zeeshan Ahmed of Stanford University and SLAC National Accelerator Laboratory stressed why such care matters, describing his role as the final microsecond of a 13.8-billion-year photon journey.

Separating signals

A central difficulty is that the B-mode signal is expected to be very weak against other electromagnetic emission. The feature says that combining data from the SPT, BICEP and other telescopes helps researchers sort out these competing signals. High-resolution maps from the SPT and wide-area, sensitive polarisation measurements from complementary experiments serve different purposes, though the source gives few details on how each contributes.

What to watch

The immediate prospects depend on data from the current generation of instruments and on how well foreground contamination can be modelled and removed. A confirmed primordial B-mode would be direct evidence of inflation and of gravity operating at quantum scales. A non-detection at greater sensitivity would still narrow the range of inflation models that remain viable.

Source: Untangling the patterns of the cosmic microwave background to better understand the early universe (Phys.org, 7 November 2026).

Image: AI-generated illustration.