A quantum theory of the alignment and polarization of very small dust grains
In one sentence
The smallest interstellar dust grains are too quantum-mechanical to be described by the classical alignment theory we have used for decades; here I build the quantum theory and use it to predict when their microwave emission and starlight absorption become polarized.
What’s the question?
Tiny rapidly spinning dust grains in the interstellar medium are thought to be responsible for a microwave glow known as anomalous microwave emission (AME). AME sits in a frustrating part of the spectrum: it is right where cosmologists hunt for the faint polarization patterns imprinted on the cosmic microwave background by primordial gravitational waves. If spinning dust is polarized at all, it is a foreground that contaminates that search. We need a sharp answer to: can it be polarized, and how much?
The standard theory of grain alignment treats grains classically. That works for grains that contain millions of atoms. For the tiniest grains, which dominate AME, that classical picture starts to fail.
What did I do?
I built a quantum-mechanical theory of how very small grains align under the same physical processes the classical theory describes (radiative torques, magnetic relaxation, gas collisions), but with the grain’s rotation treated as a quantum object — a symmetric-top molecule, essentially — using density-matrix techniques. I then computed the polarization signal that this theory predicts, both in microwave AME emission and in UV/optical/IR absorption.
The main results: anisotropic illumination can polarize spinning-dust emission at the percent level in favourable conditions; the same grains should produce appreciable polarized absorption in the UV/optical/IR around strong illumination sources; and infrared vibrational emission from the same grains is negligibly polarized.
Why does it matter?
This gives a clean, testable link between the same population of grains seen in three very different observational windows: microwave (AME), starlight extinction, and IR. Joint measurements directly test alignment mechanisms — and put quantitative constraints on polarized microwave foregrounds for cosmology.
My role
Sole author. The full preprint is on arXiv — comments and questions are very welcome.