Gravitational Lensing Impacts Cosmic Birefringence, Study Finds

Future missions will be able to more accurately find signs of parity symmetry violations in the cosmic microwave background polarization after a few researchers managed to take the gravitational lensing effect into account, reports a new study in Physical Review D, selected as an Editor’s Suggestion.

How far does the universe extend? When and how did the universe begin? Cosmology has made progress in answering these questions by providing observational evidence for theoretical models of the universe based on fundamental physics. The Standard Model of cosmology is widely accepted by researchers today. However, it still cannot explain fundamental questions in cosmology, including dark matter and dark energy.

In 2020, an interesting new phenomenon called cosmic birefringence was reported based on the cosmic microwave background polarization (CMB) data. Polarization describes light waves that oscillate perpendicular to the direction in which they travel. In general, the direction of the polarization plane remains constant, but it can be rotated under special conditions. A reanalysis of the CMB data showed that the polarization plane of the CMB light may have rotated slightly between the time it was emitted in the early universe and today. This phenomenon violates parity symmetry and is called cosmic birefringence.

Because cosmic birefringence is difficult to explain using the known laws of physics, there is a strong possibility that yet-to-be-discovered physics, such as axion-like particles (ALPs), are behind it. A discovery of cosmic birefringence could point the way to revealing the nature of dark matter and dark energy, which is why future missions are aimed at making more precise observations of the CMB.

To do this it is important to improve the accuracy of current theoretical calculations, but these calculations have not been accurate enough so far because they do not take gravitational lensing into account.

A new study by a pair of researchers, led by University of Tokyo Department of Physics and Research Center for Early Universe PhD candidate Fumihiro Naokawa, and Center for Data-Driven Discovery and Kavli Institute for the Physics and Mathematics of the Universe (Kavli IPMU) Project Assistant Professor Toshiya Namikawa has established a theoretical calculation of cosmic birefringence that incorporates gravitational lensing effects, and worked on developing a numerical code for cosmic birefringence that includes gravitational lensing effects, which will be indispensable for future analyses.

First, Naokawa and Namikawa derived an analytical equation that describes how the gravitational lensing effect changes the cosmic birefringence signal. Based on the comparison, the researchers implemented a new program on an existing code to calculate the gravitational lens correction, and then looked at the difference in signals with and without the gravitational lens correction.

As a result, the researchers found that if gravitational lensing is ignored, the observed cosmic birefringence signal cannot be properly fitted to the theoretical prediction, which would statistically reject the true theory.

In addition, the pair created simulated observational data that will be obtained in future observations to see the effect of gravitational lensing in the search for ALPs. They found that if the gravitational lensing effect is not taken into account, there would be statistically significant systematic biases in the model parameters of ALPs estimated from the observed data, which would not accurately represent the ALP model.

The gravitational lens correction tool developed in this study is already being used in observational studies today, and Naokawa and Namikawa will continue to use it to analyze data for future missions.

Details of their research were published September 27 in Physical Review D as an editor’s suggestion.