Sean Carroll in Preposterous Universe:
Welcome to the third annual Mindscape Holiday Message! Just a chance for me to be a little more chatty and informal than usual, although as it turned out this isn’t all that different from a conventional solo episode. With the difference that what I’m talking about — a phenomenon called “cosmic birefringence” — has played a big part in my personal scientific career, so I get to be a bit autobiographical.
Every photon has a direction of polarization, which generally remains fixed as the photon travels through space. Birefringence is an effect by which the polarization rotates rather than staying fixed. It can happen in materials, but generally not in outer space. But there are exotic physics ideas that could cause such a rotation, including the dynamical dark energy candidate known as quintessence. People have put limits on such cosmic birefringence for a while now, but recently there was a claim that there might be a nonzero amount of birefringence visible in the polarization of the cosmic microwave background! Still very tentative, but if this hint turns into real evidence, it would big extremely big news for our understanding of physics and cosmology, possibly helping us pinpoint the nature of dark energy.
More here.