A dust storm that has raged for millions of years is not in a desert but between the stars. There, clouds of tenuous gas drift, threaded by magnetic fields. These fields align dust grains like compass needles along field lines. When such dust emits, its light becomes polarized: it oscillates in a single plane, like a rope shaken only up and down.
Astrophysicists ran a simulation of turbulent magnetized gas on the Frontier supercomputer (one of the fastest in the world). This gave them polarization maps — essentially snapshots of magnetic glow. They then decomposed the maps into two components, akin to two types of ripples on water: the E-mode (curl-free, like expanding circles from a dropped stone) and the B-mode (divergence-free, with swirls like a whirlpool). It turned out the curl-free ripples are fewer and behave differently. That's what creates the asymmetry previously seen in Planck telescope data but left unexplained.
The cosmic microwave background, faint light left over from the Universe's hot beginning, is also polarized. But its signal is swamped by foregrounds from our own Galaxy. Understanding exactly how dust creates the magnetic pattern in the sky lets us subtract that foreground. And that means we can reach the signal from the first moments after the Big Bang. The new simulation showed that the curl-free ripples arise mainly from gas compression by shock waves, while the divergence-free ones come from stretching of magnetic fields.
So the computational experiment turned a murky blotch on Planck maps into a clear physical picture. The magnetic ripples in dust turned out not to be random noise but a crisp imprint of turbulence. Now it can be decoded. And cosmologists have a tool to clean the ancient light from the modern dusty veil.
Interstellar dust makes up only about 1% of the mass of galactic gas, yet it accounts for half of all the radiation from our Galaxy.