Portal-like "ring wormholes" could be used as a time machine.

Even if these effects make it impossible for ring wormholes to exist, their relative simplicity compared with regular wormholes means studying them could help us understand these quantum wrinkles better, or why time travel seems to be off-limits in our universe, says Wiseman. WORMHOLES are often thought of as tunnels through space-time with black holes for entrances, but they could theoretically be flat instead, like a door into another location, another universe or the past. [Extracted from the article]