Wormholes come in two main kinds, traversable and comm guage. A stargate is the former type, open to travel by physical objects including, if the wormhole is large enough, entire sophonts or ships of sophonts.
Traversable Wormholes 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 are much smaller but use a much larger amount of negative (exotic) ANEC-violating energy. They are only useful for transmitting electromagnetic beams containing data, but this data transfer is very important to the existence of civilisation in the Terragen Sphere. These are not stargates, except to beings who can pass through them as a data stream. (See also: Communication-Gauge Wormholes).
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.