Like haute couture, cosmology has its own fads, fashions and fallacies. Gone are the heydays of galaxy surveys and quasar discoveries; now searches for the universe's first stars and for the nature of dark energy are all the rage. But like miniskirts and bell-bottoms, some castoffs experience a resurgence. In particular, cosmic strings, which fell out of favor in the late 1990s, are making a comeback thanks to observations that may have actually detected them.
Cosmic strings are hypothetical one-dimensional defects in the fabric of space-time. Cosmologists once thought these kinky concoctions, weighing as much as one Earth mass per meter, could have caused galaxy clusters to clump. But recent measurements of the cosmic microwave background radiation, the remnant glow of the big bang, convincingly ruled out this scenario. Cosmic strings became old-fashioned almost overnight.
A U.S.-Ukrainian team of astronomers led by Rudolph E. Schild of the Harvard-Smithsonian Center for Astrophysics now claims that the mysterious behavior of a double quasar near the Big Dipper can best be explained by an intervening loop of cosmic string. The image of the quasar is being split in two by the gravity of a massive galaxy in the line of sight. Light from image B takes longer to reach Earth than that from image A, so brightness variations in B lag 417 days behind the same fluctuations in A.
In the mid-1990s, however, the two images winked synchronously--the brightnesses of both rose and fell together several times over a year. In the August 2004 Astronomy and Astrophysics, Schild and his colleagues show that all possible explanations for this behavior fail, except for gravitational lensing by a small loop of cosmic string close to our own Milky Way galaxy. The moving cosmic string would have acted as an additional gravitational lens, affecting both quasar images simultaneously. "It is difficult to propose a less exotic model," they conclude.
Meanwhile a Russian-Italian team found a much stronger cosmic string candidate in the constellation Corvus: a galaxy split into two uncannily similar and completely undistorted images. A normal gravitational lens produces images with different brightnesses and shapes. Intriguingly, many other gravitational lens candidates extend across the surrounding area, as if a piece of cosmic string is stretched out in the foreground. "The more we work on it, the more we think we've found a genuine cosmic string," says team member Giuseppe Longo of Federico II University in Naples.
Britain's Astronomer Royal Martin J. Rees of the University of Cambridge warns that "extraordinary claims demand extraordinary evidence, and we certainly don't yet have this from the current observations." That may change soon. Longo and his colleagues have applied for observing time on large ground telescopes and on the Hubble Space Telescope to study their mystery object in Corvus in much more detail. "If we find a very sharp edge in the image right between the two galaxies," Longo remarks, "that would constitute definitive proof."