Gravitational lensing has a significant effect on cosmic birefringence

Cosmic birefringence is an intriguing new phenomenon discovered in 2020 using polarization data from the cosmic microwave background (CMB). The oscillation of light waves perpendicular to their direction of propagation is referred to as polarization.

Although it can spin in some situations, the polarization plane’s direction usually stays unchanged. The polarization plane of the CMB light may have slightly rotated between when it was released in the early cosmos and now, according to a reanalysis of the CMB data. This phenomenon, known as cosmic birefringence, violates parity symmetry.

Given the difficulty of explaining cosmic birefringence within the framework of established physical principles, there is a strong possibility that yet-to-be-discovered physics, such as the axionlike particles (ALPs), lies behind it.

In a recent study, scientists at Kavli IPMU established a theoretical calculation of cosmic birefringence that takes gravitational lensing effects into account. They also worked on developing a numerical code for cosmic birefringence that incorporates gravitational lensing effects.

An analytical formula explaining how the gravitational lensing effect modifies the cosmic birefringence signal was developed by scientists. The scientists computed the gravitational lensing correction using an existing code and a new program they created based on the equation. They then compared the signals with and without the gravitational lensing modification.

Consequently, the scientists discovered that the theoretical prediction cannot correctly fit the observed cosmic birefringence signal if gravitational lensing is disregarded, which would statistically reject the true explanation.

Furthermore, the two generated observational data simulations will be used in subsequent observations to examine the impact of gravitational lensing on the hunt for ALPs. They discovered that the model parameters of ALPs calculated from the observed data would exhibit statistically significant systematic biases if the gravitational lensing effect is ignored, leading to an inaccurate representation of the ALP model.

In addition to being utilized in current observational research, the gravitational lensing correction tool created in this work will be employed by scientists in the analysis of data for upcoming missions.

Journal Reference:

Fumihiro Naokawa and Toshiya Namikawa. Gravitational lensing effect on cosmic birefringence. Physical Review D. DOI: 10.1103/PhysRevD.108.063525

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