Light from the very beginning of the cosmos has been traveling toward us for 13.8 billion years. During that time, its polarization, the direction of its oscillations, might have been twisted, like a compass needle that someone quietly nudged. Physicists call this phenomenon cosmic birefringence.
The authors of the study show that this subtle twist could serve as a test for swampland conjectures — a set of ideas from string theory that predict which laws of nature are even possible. One such conjecture, the Weak Gravity Conjecture, states that gravity is always weaker than other forces. If the rotation is caused by ultra-light scalar fields (invisible entities filling the cosmos), then those fields must be very weakly coupled to ordinary matter. But then they would violate the swampland conjecture — gravity would be too strong compared to their influence. That's a warning bell for string theory.
Fortunately, there's a fallback. Thin domain walls, imagine a film separating regions with different vacuum properties, could cause the same rotation without the need for ultra-light particles. Their thickness is comparable to the size of an atomic nucleus, yet they stretch across billions of light-years.
Future observations will help settle the debate. The POLARBEAR, Simons Observatory, and LiteBIRD telescopes will look for variations in the rotation across different parts of the sky. If the rotation is patchy — domain walls are to blame. If it's smooth — ultra-light fields are responsible, and string theory is in trouble. Thus, ancient light becomes the arbiter in the dispute over the foundation of reality.
The cosmic microwave background has cooled to just 2.7 degrees above absolute zero — colder than the surface of Pluto, but it was once hotter than the core of a star.
In Stephen Baxter's novel 'The Time Ships', characters travel through domain walls where the laws of physics change from layer to layer.