The cosmic microwave background corresponds to the first light emitted after the Big Bang. It still fills the entire Universe and acts like a photograph of its beginnings. By analyzing it, researchers can go back in time and understand how the cosmos formed.
This light has a property called polarization, which can be compared to an orientation. In theory, this orientation could slightly rotate during its journey through the Universe. This phenomenon is called cosmic birefringence.
Diagram illustrating the ambiguity of polarization rotations.
Credit: Naokawa, Namikawa, higgstan.com
Why is this rotation important? Because it could be caused by still unknown particles, related for example to dark matter. Clearly detecting this rotation would therefore be a valuable clue about the composition of the Universe.
But measuring this rotation is extremely delicate. Until now, scientists were faced with an ambiguity. It's like looking at a clock without knowing how many full turns the hands have already made. We see the final position, but not the total number of rotations.
Previous measurements suggested a small rotation of about 0.3 degrees. However, they could not rule out that the light had actually rotated several full times before reaching us. This introduced a large uncertainty.
To solve this problem, researchers studied in detail a particular signal called "EB". This signal contains fine information about the light's polarization. By analyzing it more precisely, they managed to lift this ambiguity. They can now determine not only the final rotation angle, but also the number of complete rotations performed.
This improvement is not limited to birefringence. It also changes the interpretation of other cosmological signals, such as the "EE" correlation. This is linked to important stages in the Universe's history, notably the period when the first stars transformed the surrounding matter.
By refining these measurements, scientists obtain a more accurate picture of these major phases of cosmic evolution. This allows a better understanding of how the Universe went from a simple state to the complex structure we observe today.
Future instruments, such as the Simons Observatory or the LiteBIRD satellite, should fully benefit from this method. They will be able to measure polarization with unprecedented accuracy.
In parallel, other work seeks to correct errors related to the instruments themselves, using celestial objects as references. All this could open a window onto still unknown aspects of physics, such as the nature of dark matter or dark energy.