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

News News & Technology Physics WORMHOLES are often thought of as tunnels throughspace-time with black holes for entrances, but they couldtheoretically be flat instead, like a door into another location,another universe or the past. These "ring wormholes" would be made up of a string ofexotic matter with negative energy -- a property that ispossible due to quantum effects, but only, as far as we know,in extremely small amounts. [Extracted from the article]