A team of researchers studying the uncertainties associated with a phenomenon known as cosmic birefringence has developed a method to reduce uncertainties in its observational measurements, according to a new study published in Physical Review Letters on January 27.

This study is the first to quantitatively address the uncertainty surrounding the birefringence angle, which is a crucial observational quantity that could provide clues to unknown physical theories breaking the universe's left-right symmetry, and to understanding dark matter and dark energy.

The cosmic microwave background, the afterglow of the Big Bang, carries crucial information about the universe's earliest moments. Recent observations have found hints of a subtle rotation in the polarization of this ancient light - a phenomenon known as cosmic birefringence. This rotation is thought to conceal unknown elementary particles called axions. Accurately measuring the rotation angle of cosmic birefringence (the birefringence angle) is important for unraveling the underlying unknown physical theory. The rotation angle can be investigated by measuring the amplitude of a signal called the CMB EB correlation (Figure 2). Previous measurements have reported it to be approximately 0.3 degrees.

A research team led by the University of Tokyo Graduate School of Science PhD candidate Fumihiro Naokawa, together with Kavli Institute for the Physics and Mathematics of the Universe (Kavli IPMU, WPI) Project Associate Professor Toshiya Namikawa, conducted a detailed investigation into the uncertainties associated with cosmic birefringence. The study found that the rotation angle may be larger than the previously reported value of about 0.3 degrees.

“Can you tell what day it is, just by looking at a clock? No, you cannot. To determine the date from the clock hands, you need to know how many times the hands have rotated since a specific reference date and time. In scientific terms, a situation like this clock's hands—where observing only the current state does not reveal how many rotations occurred in the past—is described as having 360-degree phase ambiguity.

“Like a clock, the CMB we can observe is only in its current state. Therefore, rotation angles such as 0.3 degrees, 180.3 degrees, and 360.3 degrees should be indistinguishable. This means the birefringence angle has a phase ambiguity of 180 degrees,” said Naokawa.

The researchers developed a way to resolve this ambiguity, after finding that the shape of the EB correlation signal can encode information about how many rotations the polarization direction underwent. Looking at the details of the EB correlation could resolve this ambiguity.

The uncertainty reduction method developed by the researchers will provide a technique for future observations of cosmic birefringence using high-precision data and for verifying the underlying physical theories such as those from the Simons Observatory and LiteBIRD.

Furthermore, the team has newly discovered that when phase uncertainty is taken into account, cosmic birefringence also affects another type of CMB signal known as the EE correlation. The EE correlation is a key observable used to determine the Universe’s “optical depth” that can be used to explore cosmic reionization. As a result, this finding may require revisions to previously reported measurements of the optical depth.

In a separate paper also published in Physical Review Letters on January 27, author Naokawa studied methods to overcome errors generated by a telescope itself when observing cosmic birefringence. He uncovered a new method to confirm the phenomenon using specific types of celestial objects, including radio galaxies powered by supermassive black holes, which may help future researchers uncover the nature of dark energy.