Traversable wormholes, which use a modified Morris-Thorne-Kuhfittig metric. These are very large: traversable wormholes are consistently 327 Astronomical Units (A.U.s) in radius from the surrounding region of effectively flat space to the wormhole mouth, so must be located far from any star or planet.
This kind of wormhole uses a relatively small quantity of negative (exotic) ANEC-violating energy, and is the kind of wormhole often referred to as a stargate.
Hayward(Comm-gauge)Wormholes which are much smaller, but use a much larger amount of negative (exotic) ANEC-violating energy. They are only useful for transmitting em-beams containing data, but this data transfer is very important to the existence of civilisation in the Terragen Sphere.
Chronology Protection - Text by Anders Sandberg The universe seems to counteract attempts to produce time machines, for example due to Visser decay and related vacuum phenomena undermining other forms of time communication. Although the strict Chronology Protection Conjecture by the information age physicist Hawking was subsequently disproved, the chronology protection theorems of Lang, Picard and Joel-5 showed that this tendency can be explained from the consistency theorems, information physics and (eventually) vacuum physics.