Researchers studying a mysterious cosmic phenomenon called cosmic birefringence have developed a new approach to reduce uncertainty in how they measure it. Progress reported in physical review lettercould improve the accuracy of observations that probe fundamental physics.
This study is the first to quantitatively investigate the uncertainty in the birefringence angle. This measurement is important because it may provide clues about unknown physical theories that break the bilateral symmetry of the universe. 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 radiation, the faint afterglow left by the Big Bang, contains valuable information about the early Universe. Recent observations suggest that the polarization of this ancient light may have been slightly rotated. This effect is known as cosmic birefringence.
Scientists think this subtle rotation may be related to hypothetical subatomic particles called axions. Therefore, accurately determining the amount of rotation, known as the birefringence angle, is essential for testing possible new physics. Researchers measure this angle by analyzing the strength of a signal called the CMB-EB correlation. Previous studies estimated the rotation angle to be approximately 0.3 degrees.
Investigation of measurement uncertainty
The research team was led by Fumihiro Naokawa, a doctoral student at the Graduate School of Science, The University of Tokyo, and collaborated with Project Associate Professor Toshiya Namikawa of the Kavli Institute for the Physics and Mathematics of the Universe (Kavli IPMU, WPI). 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, I don’t. To determine the date from the hands of a clock, you need to know how many times the hands have rotated at a specific reference date and time. In scientific terms, a situation like this clock’s hands, where you cannot tell how many times they have rotated in the past by observing only their current state, is described as having a 360-degree phase ambiguity.”
“Just like a clock, the only CMB we can observe is its current state. Therefore, rotation angles such as 0.3 degrees, 180.3 degrees, and 360.3 degrees should be indistinguishable. This means that the birefringence angle has a phase ambiguity of 180 degrees,” Naokawa said.
Solving topological ambiguity problems
To address this issue, researchers have developed ambiguity resolution techniques. They found that the detailed shape of the EB correlation signal contained clues about how many times the polarization direction had rotated.
By analyzing these subtle features within the EB correlation signal, scientists may be able to determine the true rotation angle and eliminate ambiguity.
Improving future cosmology experiments
The new method provides a tool to analyze future high-precision observations of cosmic birefringence. Future experiments, including those at Simons Observatory and LiteBIRD, could use this technique to test new theoretical models of fundamental physics.
The researchers also found that, given this phase uncertainty, cosmic birefringence affects another signal in the cosmic microwave background known as the EE correlation. Scientists use EE correlations to estimate the “optical depth” of the universe, a key quantity for studying cosmic reionization. Because of this relevance, the new findings may require researchers to reconsider previously reported optical depth measurements.
A new way to check cosmic birefringence
In another study published in physical review letterNaokawa considered ways to reduce the errors introduced by telescopes when measuring cosmic birefringence. He proposed a way to confirm the effect by observing specific astronomical sources, such as radio galaxies powered by supermassive black holes.
These observations could provide another way to examine birefringence in the universe, and could help scientists uncover deeper insights into the nature of dark energy.
