IS THE UNIVERSE BIREFRINGENT? That is, does the universe behave like a crystal in which light moving in one direction acts differently from light going in another direction? Radio waves from distant galaxies must pass through the vast reaches of an intergalactic medium filled with stray magnetic fields and a tenuous plasma of ions and electrons. Through a well-known phenomenon called the Faraday effect, these ions and fields in the cosmic prairie subtly rotate the polarization of the radio waves (the orientation of their electric fields) on their way toward Earth. This is a very slight effect but it has been measured in the case of light coming from many galaxies; the effect is proportional to the magnetic field strengths and ion densities, as well as the square of the light's wavelength. (Typically about 5- 8% of the light from a galaxy is plane polarized, most of this in the form of synchrotron radiation.) Now two researchers, Borge Nodland at the University of Rochester (bnod@lle.rochester.edu; 716-275-5772) and John Ralston at the University of Kansas (ralston@kuphsx.phsx.ukans.edu; 913-864-4020), have studied polarization rotation data for 160 galaxies and have perceived that in addition to the Faraday effect, there seems to be an extra mysterious angular dependency at work. Indeed, the rotation varies consistently with the angle across the sky, as if the universe had an axis. That is, the amount of polarization rotation depends on the distance to a galaxy as well as on the cosine of the angle between the incoming radio waves and an axis that apparently lies in the direction of the constellation Sextans. This anomaly would seem to challenge some important physics concepts, such as the notion that there is no preferred direction in space and the notion that space itself is isotropic (the same in all directions) or homogeneous (the same in all places). One possible explanation might be the existence of "domain walls" between different realms of the cosmos, as prescribed in certain particle physics theories. The soundness of their study depends, among other things, on the quality and amount of polarization observations, and Nodland and Ralston therefore look forward to acquiring additional data. (To appear in Physical Review Letters, 21 April 1997; see figures at Physics News Graphics. Reminder---science journalists can obtain a copy of PRL articles by contacting AIP Public Information at physnews@aip.org)

SPRINGTIME FOR COMET HALE-BOPP. Now past its prime in the dusk sky, Hale- Bopp was first spotted two years ago as far away as seven astronomical units, allowing astronomers to observe the thawing process at an earlier stage than is usual for comet watches. This in turn permitted the detection of trace species not before seen on comets, such as SO₂ and H₂CS (Science News, 21 April). What else do we know? First of all, the size of the comet nucleus is estimated to be 27-42 km, at least three times bigger than that of Comet Halley. Of the cometary products vaporized on the inward trip toward the sun, the chief gases are H₂O, CO, and CO₂, which seem to be the main constituents of interstellar ice as well. Dust jets are rich in crystalline olivine, and dust production in general was more than 100 times stronger than with Halley at comparable distances. Variations in the vented jet activity will be used to determine Hale-Bopp's rotation rate. Chemical composition suggests that the comet comes from the Oort Cloud rather than the Kuiper Belt. (Several articles in Science, 28 March.)