De onde veio o motor a jato no filme Donnie Darko?

Esta é provavelmente uma das melhores explicações que encontrei, do usuário Bon Gart :

The Jet Engine comes out of the divergent time line involving the events of the movie (and from his mother's plane), and is supposedly sent back in time through the same wormhole that Donnie uses to time travel with. Unfortunately, Donnie was miles away when he actually traveled back in time. The vortex might have been forming over his house, but that's not where he was. It was where he ended up... but the vortex and subsequent wormhole should have followed him... or he should have returned to it at that moment he decided to travel through time. The Jet Engine most certainly traveled through that wormhole... it came from the future to kill him in the past. IE... more proof of the actual Time Travel theory, as opposed to the Future Visions theory. This again points to the paradox and says "See what you made me do?" If Donnie never got out of bed and was killed by the Jet Engine, how could he be killed by a Jet Engine in the first place when it came off a plane hitting the atmospheric turbulence created by the vortex that was there from Donnie corrupting the time line and NOT getting killed by the Jet Engine? Answer? Paradox!

ref: Donnie Darko explicado

CHAPTER : THE ARTIFACT AND THE LIVING
When a Tangent Universe occurs, those living nearest to the Vortex will find themselves at the epicenter of a dangerous new world. Artifacts provide the first sign that a Tangent Universe has occurred. If an Artifact occurs, the Living will retrieve it with great interest and curiosity. Artifacts are formed from metal, such as an Arrowhead from an ancient Mayan civilization, or a Metal Sword from Medieval Europe. Artifacts returned to the Primary Universe are often linked in religious iconography, as their appearance on Earth seems to defy logical explanation. Divine Intervention is deemed to be the only logical conclusion for the appearance of the artifact.

Todo o universo do Tangente está acontecendo por causa de um problema que ocorreu com a 4ª dimensão. Os detalhes disso nunca nos são contados. O propósito do Receptor Vivo é enviar o artefato ao Universo Primário para restaurar o equilíbrio. Donnie não envia o artefato que pousou em sua casa de volta ao Universo Primário. Isso permanece no universo da tangente. Ele envia a cópia duplicada do Universo Tangente de volta ao Universo Primário. Sim, o Receptor Vivo tem o poder de fazer isso no Universo Tangente.

Então, de onde vem o artefato para o Universo Tangente? Bem, esse é o texto mencionado no texto de Roberta Sparrow - Divine Intervention. Não há explicação lógica. Uma cópia do artefato existirá aparentemente do nada. Roberta também não entende como. É causado pelo rasgo no tecido da 4ª dimensão.

No que diz respeito ao Universo Primário, eles sempre continuarão a se perguntar de onde veio esse motor a jato. Eles não receberão resposta. Mas para o Universo Primário, não há Receptor ou Artefato. O espaço-tempo se corrigiu e a vida continua.