A strange twist in the universe’s oldest light may be bigger than we thought

The study is the first to quantitatively examine uncertainty in the birefringence angle. This measurement is important because it may provide clues about unknown physical theories that violate the universe's left right symmetry. It could also help scientists better understand dark matter and dark energy.

A Subtle Twist in the Universe's Oldest Light

The cosmic microwave background, which is the faint afterglow left behind by the Big Bang, contains valuable information about the early universe. Recent observations suggest that the polarization of this ancient light may undergo a slight rotation. This effect is known as cosmic birefringence.

Scientists suspect that this subtle rotation could be linked to hypothetical elementary particles called axions. Precisely determining the amount of rotation, known as the birefringence angle, is therefore essential for testing possible new physics. Researchers measure this angle by analyzing the strength of a signal called the CMB EB correlation. Earlier studies estimated the rotation angle to be around 0.3 degrees.

Investigating the Measurement Uncertainty

The research team was led by University of Tokyo Graduate School of Science PhD candidate Fumihiro Naokawa, working with Kavli Institute for the Physics and Mathematics of the Universe (Kavli IPMU, WPI) Project Associate Professor Toshiya Namikawa. Their analysis carefully examined the uncertainties that affect measurements of cosmic birefringence.

Their results suggest that the rotation angle may actually 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.

Solving the Phase Ambiguity Problem

To address this issue, the researchers developed a technique to resolve the ambiguity. They discovered that the detailed shape of the EB correlation signal contains clues about how many times the polarization direction may have rotated.

By analyzing these subtle features within the EB correlation signal, scientists may be able to determine the true rotation angle and eliminate the ambiguity.

Improving Future Cosmology Experiments

The new method provides a tool for analyzing future high precision observations of cosmic birefringence. Upcoming experiments, including the Simons Observatory and LiteBIRD, could use this technique to test new theoretical models of fundamental physics.

The team also discovered that when this phase uncertainty is considered, cosmic birefringence influences another signal in the cosmic microwave background known as the EE correlation. Scientists use the EE correlation to estimate the Universe's "optical depth," an important quantity for studying cosmic reionization. Because of this connection, the new findings may require researchers to revisit previously reported optical depth measurements.

A New Way to Confirm Cosmic Birefringence

In a separate study also published in Physical Review Letters, Naokawa examined ways to reduce errors introduced by telescopes when measuring cosmic birefringence. He proposed a method to confirm the effect by observing particular astronomical sources, including radio galaxies powered by supermassive black holes.

These observations could provide another way to verify cosmic birefringence and may help scientists uncover deeper insights into the nature of dark energy.