Cosmic strings are theoretical objects that predate string theory, but in recent years there’s been some speculation that they may actually be enlarged strings left over from the big bang, or possibly the result of branes colliding. There has also been speculation that they can be used to create a time machine.
Regardless of their origin, if cosmic strings exist, they should have an immense amount of gravitational pull, and this means that they can cause frame dragging. In 1991, J. Richard Gott (who, with William Hiscock, solved Einstein’s field equations for cosmic strings in 1985) realized that two cosmic strings could actually allow time travel.
The way this works is that two cosmic strings cross paths with each other in a certain way, moving at very high speeds. A spaceship traveling along the curves could take a very precise path (several of which were worked out by Curt Cutler in the months after Gott’s publication) and arrive at its starting position, in both space and time, allowing for travel in time.
Like other time machines, the spaceship couldn’t travel further back than when the cosmic strings originally got in position to allow the travel — in essence, the time travel is limited to when the cosmic string time machine was activated.
Gott’s was the second time machine (following Kip Thorne’s) to have been published in a major journal in the early 1990s, and it sparked a wave of work in the area. In May 1991, Gott was featured in Time magazine.
In the summer of 1992, physicists held a conference on time travel at the Aspen Center for Physics (the same place where, nearly a decade earlier, John Schwartz and Michael Green had determined that string theory could be consistent).
When Gott proposed this model, cosmic strings were believed to have nothing to do with string theory. In recent years, physicists have grown to believe that cosmic strings, if they exist, may actually be very closely related to string theory.