Does universe’s oldest light carry hidden twist? Scientists build clever new way to find out

The cosmic microwave background is the oldest light in the universe, leftover radiation from roughly 380,000 years after the Big Bang. Maps of this light offer scientists a picture of the “baby universe” as it looked 13.8 billion years ago.

Researchers have found hints that this ancient light’s polarization may have twisted slightly on its long journey to Earth, a phenomenon known as cosmic birefringence. If confirmed, the effect could point to physics beyond the Standard Model and offer new clues about dark matter and dark energy.

A tricky illusion

There is a catch. Even a tiny error in how a telescope’s polarization detectors are oriented can produce almost exactly the same signal as genuine cosmic birefringence. Looking at the light alone, scientists cannot tell whether they are seeing a real twist in the universe or simply a miscalibrated instrument.

Researchers at the University of California, San Diego built a new method to address that uncertainty. The team, led by postdoctoral fellow Anto I. Lonappan alongside professors Brian Keating and Kam Arnold, published its findings in the Astrophysical Journal Letters.

Comparing the detectors

The approach compares maps built from different groups of detectors within the same telescope. A genuine cosmic rotation would show up identically across every map, so when the maps are compared against each other, that shared rotation cancels out, leaving behind only the differences in how each detector group was calibrated.

“The signal we are looking for is incredibly small,” Lonappan said, adding that the team wanted certainty they were observing the universe rather than an instrument.

The researchers tested their method on eight polarization maps from the European Space Agency’s Planck satellite and compared the results against an existing approach called the Minami-Komatsu analysis. The two methods agreed closely, despite relying on different underlying assumptions.

Still missing a reference point

The new technique cannot determine the absolute cosmic birefringence angle on its own, since it is deliberately insensitive to rotation shared across every map. As a demonstration, the team anchored their results to the existing analysis’s calibration and reproduced a birefringence angle of 0.37 degrees, plus or minus 0.12 degrees, matching prior findings.

The work also carries implications for detecting primordial B-mode polarization, a curl-like pattern in the CMB that would offer strong evidence for gravitational waves generated during the earliest moments of cosmic inflation. Calibration errors of the kind this method detects can mimic those same B-mode signals, making independent checks increasingly important as instruments grow more sensitive.

“This is ultimately about knowing when we can trust a measurement,” Keating said, noting that any future detection of primordial B modes would need to survive exactly this kind of scrutiny.

Source: https://interestingengineering.com/space/cosmic-birefringence-cmb-polarization-calibration-check