EXPANDING COSMOS: This snapshot from a computer simulation models the formation of large-scale structures in the universe. Physicists say there's a chance our universe is not only expanding, but is cracked, with topological deformations called cosmic strings riddling the cosmos. Image: Klaus Dolag and equipment VIMOS-VLT Deep Survey
If the prospect of an ever-expanding universe that eventually stretches into a vast emptiness isn’t depressing enough, there’s this: the universe may have cracks in it.
Cracks, called cosmic strings, are topological defects in spacetime that might have formed when the universe was young. Experiments haven’t found any proof that cosmic strings are even out there, but that hasn’t stopped physicists from calculating how many strings we might expect there to be if, in fact, they turn out to exist after all. And it’s a lot: cosmic strings would produce at least a billion loops throughout the visible universe today, researchers report. “Cosmic strings are these filaments which wind and sneak throughout the universe,” says study co-author Benjamin Shlaer of Tufts University. They are essentially one-dimensional fault lines in space, made not of mass but pure energy. Some could be infinitely long, and all are almost impossibly thin, much narrower than a proton.
Cosmic strings may have formed as the universe cooled after the big bang. Just as water undergoes a “phase transition” from liquid to solid when it freezes, the universe changed radically when it went through a transition from being extremely hot and dense in the instants just after the big bang to slightly cooler and more rarefied just a few fractions of a second later. According to a popular theory, in the hot and dense universe three of the four forces of nature (weak, strong and electromagnetic) were unified but in the cooler universe they separated. When this symmetry among the forces broke, it might have created topological defects in the form of strings, so named because they would be long, thin fissures in space. (Despite the similar names, cosmic strings may or may not be related to the strings predicted to make up fundamental particles in string theory .)
These strings would have started off tangled and wrinkly when the universe was in its hot, dense state but would have stretched out over time as space itself expanded. This movement would cause some strings to cross others. “When they wind back on themselves they break so that the wrinkles snap off as closed loops, like little rubber bands.” The loops are what astronomers might be able to detect because they would oscillate, producing measurable ripples in spacetime called gravitational waves.