By principles of relativity, the faster something moves through space, the slower its time passes. Theoretically, if the astronauts' ship flew faster than the speed of light, the hands of the astronauts' watches would run backward. Time's direction would reverse. They prefer a universe where cause follows effect, with no chance of effect preceding cause. Einstein and his intellectual heirs were untroubled by the possibility, because the speed of light is the cosmic speed limit.
Nothing can go faster.
However, Einstein also had been untroubled by another theoretical entity, the cosmic string. Well after his lifetime, astrophysicists came up with mathematical and logical evidence for the existence of these thin streams of pure energy. Princeton physicist J. Richard Gott contemplated the effects of cosmic strings on spacetime, and decided they'd be prodigious. The pure energy of a cosmic string translates into a mass equivalence of 40 million billion tons an inch. Ponderous mass puts one whale of a warp in the fabric of spacetime.