A pendulum swings in thick syrup. In the first fractions of a second, it moves quickly, but the syrup slows it down, and then suddenly the syrup becomes thinner, and the pendulum speeds up again. Something similar might have happened to our universe. Shortly after the Big Bang, when the world was a hot plasma, dark energy briefly switched on — a mysterious force that pushes space apart. It added "extra" expansion, which is why today we see galaxies receding slightly faster than expected.
This same energy could have affected light traveling through space. Just as a sugar crystal rotates the plane of oscillation of a passing light ray, early dark energy could twist the polarization of the cosmic microwave background — the ancient light left over from the universe's hot youth. Astronomers call this twist cosmic birefringence, and they look for its traces in data from telescopes like Planck and the ground-based ACT facility. The authors of the new study tested how the shape of the energy "pendulum"—mathematically, a potential with varying steepness — affects both effects: the extra expansion and the light twist.
They tried three variants: shallow, moderate, and very steep. The shallow potential failed: to reproduce the light twist, it required an unnaturally strong coupling to photons, and predictions disagreed with observations on different angular scales. The steep potential also didn't work. But the moderate one worked perfectly. It gave the best fit to the data and stable, consistent coupling values for both sets of observations, as if the same key fit two different locks.
Thus, the moderate potential not only alleviates the Hubble tension, the dispute over the universe's expansion rate, but also naturally explains the mysterious polarization twist. This strengthens the case for early dark energy as a bridge between two seemingly different problems. The next step is to test the model with new data being collected by observatories like Euclid and future sky-polarization projects. Perhaps the early universe's pendulum really does swing according to a moderate law.
A one-degree rotation of the cosmic microwave background polarization corresponds to light that, after traveling 13 billion years, "remembered" an interaction with a field that switched on for less than a billionth of a second.