Is the Universe's Oldest Light Twisted? A New Planck Test Says the Hint Survives a Key Check
UC San Diego physicists built a way to separate detector errors from a real rotation of the cosmic microwave background's polarization. Applied to Planck data, it recovers a twist of 0.37 degrees, which could point to new physics.
The oldest light in the universe may have picked up a tiny twist on its 13.8-billion-year journey to Earth, and a new study from the University of California San Diego gives physicists a better way to tell whether that twist is real or just a flaw in their telescopes.
The light in question is the cosmic microwave background, the leftover glow released about 380,000 years after the Big Bang, when the universe first became transparent. As that light scattered off free electrons, it became slightly polarized, meaning its waves came to vibrate in preferred directions. Several recent analyses have hinted that this polarization has rotated by a fraction of a degree since then. The effect, called cosmic birefringence, is not predicted by the Standard Model of particle physics. If it is real, it could be a fingerprint of new fields tied to dark matter or dark energy.
The problem is that a telescope whose polarization detectors are misaligned by the same small angle would produce exactly the same signal. From the sky alone, a uniform cosmic rotation and a common instrument error look identical. "The signal we are looking for is incredibly small, so we have to be certain that we are seeing the universe and not our instrument," said Anto I. Lonappan, the postdoctoral fellow who led the work with physics professors Brian Keating and Kam Arnold. The study appears in The Astrophysical Journal Letters.
The team's solution was to compare maps made by different sets of detectors. A genuine rotation of the sky would appear equally in every map, so it cancels out when the maps are compared, leaving only the differences in how each detector set was calibrated. The researchers applied the new estimator to eight polarization maps from the European Space Agency's Planck satellite and compared the calibration pattern with the one produced by the standard approach, known as the Minami-Komatsu analysis. The two agreed, even though they rest on different assumptions.
The method is deliberately blind to any rotation common to all maps, so it cannot on its own measure the cosmic twist. That still requires an independent absolute reference. As a conditional test, the team anchored their result to the common calibration inferred by the existing analysis and recovered a birefringence angle of 0.37 degrees, plus or minus 0.12 degrees, matching earlier estimates. "Before interpreting a tiny rotation as new physics, we want to know that the calibration itself can be trusted," Lonappan said.
The technique matters beyond birefringence. Small calibration errors can convert one kind of polarization pattern, called E modes, into spurious B modes, the curl-shaped patterns that cosmologists hope will reveal primordial gravitational waves from the universe's first instant. A detection of primordial B modes would be strong evidence for cosmic inflation.
That makes cross-checks essential for next-generation experiments such as the Simons Observatory in Chile, where Keating is principal investigator. "The detection of primordial B modes would transform our understanding of the early universe, so the measurement must survive rigorous calibration checks," Keating said. "This is ultimately about knowing when we can trust a measurement."
