Shlaer and his colleagues created a numerical simulation of cosmic string loop formation and ran it on a supercomputer cluster at the university. The results told them how big loops are likely to be when they form, and by extrapolating, the researchers calculated the number and size of the loops that might exist in the universe at any given time. The results depend on how taut the strings are—a property determined by the temperature of the universe when they were formed. For a likely range of tensions, the scientists calculated that billions of cosmic string loops could exist today. “The Tufts group has done a heroic job with the string simulations, and they pin down important features of the loop distribution critical for predicting gravitational-wave emission and their effects on millisecond pulsar timing,” says Tanmay Vachaspati, a physicist at Arizona State University in Tempe who wasn’t involved in the research.
The new study gives observers a better idea of what to look for in the quest to find evidence of cosmic strings. The strings would create gravitational waves that could be detectable by elaborate wave-detecting facilities such as LIGO (the Laser Interferometer Gravitational-Wave Observatory) in the U.S. or by studies of rapidly rotating stars called pulsars, which emit beacons of light with clockwork precision. 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. Whether or not any cosmic strings exist is still an open question.
If they are out there, now we know they would be abundant.