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.
If astronomers on Earth notice a change in the arrival time of light from pulsars, it could mean a gravitational wave has hit our planet. The fact that no evidence for gravitational waves has yet been found already eliminates the possibility of cosmic strings with a given range of tensions.