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Cosmic Microwave Background (CMB) polarized light subjected to gravitational lensing effects, in addition to cosmic birefringence. On the far left, the white lines show the polarization pattern of the CMB light generated in the early universe. These rotate due to cosmic birefringence, resulting in the currently observed CMB depicted by the black lines on the right side of the image. However, the path of light is bent by the gravitational distortion of space-time created by the large-scale structure in the middle, and so the white lines showing the polarization pattern on the right side of the image shows what is observed. Credit: Naokawa and Namikawa, 10.1103/PhysRevD.108.063525
Future missions will be able to more accurately detect signs of parity-symmetry violation in the cosmic microwave background polarization, thanks to the efforts of two researchers who considered the impact of gravitational lensing. This advancement is highlighted in a recent study published in
The difference in the cosmic birefringence signal with and without gravitational lensing. The blue dots show the signals when the gravitational lensing effect is ignored, and the red dots are the signals when the gravitational lensing effect is considered. The red error bars show the expected observation errors when the Simons Observatory will be used. The difference with and without gravitational lensing is not negligible. Credit: F. Naokawa & T. Namikawa “Gravitational lensing effect on cosmic birefringence”, Phys. Rev. D 108, 063525, Copyright (2023) the American Physical Society, 10.1103/PhysRevD.108.063525
The Phenomenon of Cosmic Birefringence
In 2020, an interesting new phenomenon called cosmic birefringence was reported from the cosmic microwave background (CMB) polarization data. Polarization describes light waves oscillating perpendicularly to the direction it is traveling. In general, the direction of the polarization plane remains constant but can be rotated under special circumstances. A reanalysis of the CMB data showed the polarization plane of the CMB light may have slightly rotated between the time it was emitted in the early universe and today. This phenomenon violates the parity symmetry and is called the cosmic birefringence.
Because cosmic birefringence is challenging to explain with the well-known physical laws, there is a strong possibility that yet-to-be-discovered physics, such as the axionlike particles (ALPs), lies behind it. A discovery of cosmic birefringence could lead the way to revealing the nature of dark matter and dark energy, and so future missions are focused on making more precise observations of the CMB.
Incorporating Gravitational Lensing into Theoretical Calculations
To do this, it is important to improve the
Simons Observatory in Chile. Credit: Debra Kellner
First, Naokawa and Namikawa derived an analytical equation describing how the gravitational lensing effect changes the cosmic birefringence signal. Based on the equation, the researchers implemented a new program to an existing code to compute the gravitational lensing correction, and then looked at the difference in signals with and without the gravitational lensing correction.
As a result, the researchers found that if gravitational lensing is ignored, the observed cosmic birefringence signal cannot be fitted well by 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 considered, there would be statistically significant systematic biases in the model parameters of ALPs estimated from the observed data, which would not accurately reflect the ALPs model.
The gravitational lensing 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.
Reference: “Gravitational lensing effect on cosmic birefringence” by Fumihiro Naokawa and Toshiya Namikawa, 27 September 2023, Physical Review D.
DOI: 10.1103/PhysRevD.108.063525