The ACT telescope in Chile's Atacama Desert stared at the sky from 2007 to 2022. Its target: the cosmic microwave background, light that has been traveling toward us for 13.8 billion years. This light is polarized: its waves oscillate in specific planes, like strings stretched in different directions. In a perfectly symmetric universe, these planes would remain unchanged. But ACT's data say otherwise.

The universe is like a hall of mirrors, where each ray, reflecting off invisible mirrors, twists slightly. These mirrors are dark matter and dark energy, which make up 95% of the cosmos. If they break mirror symmetry (parity), then the polarization of light should slowly rotate as it travels through space. Astrophysicists measured this rotation: β = 0.215° ± 0.074°. It seems like a tiny angle, less than half the apparent diameter of the Moon. But in cosmology, such numbers can decide the fate of theories.

Previously, the WMAP and Planck satellites saw a similar signal. But the question remained: could it be an artifact of the instruments themselves? Any telescope might slightly twist polarization due to imperfections in its optics. In the new work, the scientists took a different approach. They used Bayesian analysis (a statistical method that incorporates prior information) and data from ACT's optical model to separate the cosmological rotation from the instrumental one. The result: β is not zero at a significance level of 2.9σ. This is not a definitive discovery, since the threshold in physics is usually 5σ, but it is an important step.

Combining with WMAP and Planck data raises the significance to 4.6σ. That is close to claiming a real signal. If cosmic birefringence is confirmed, physicists will have to revise the Standard Model. Perhaps dark matter or dark energy interacts with light in a way that breaks fundamental symmetries. Then the universe's hall of mirrors would turn out to be not just a metaphor, but an accurate description of reality.