The Imperium is held together by virtue of the fact that Warp-capable vessels may voyage from one world to the next in the span of weeks rather than the many generations such journeys would take by conventional means. Such a feat is only possible because of the Astronomican, a psychic beacon guided by the will of the Emperor Himself and sustained by the souls of those psykers condemned to ritual death by the Adeptus Astra Telepathica.
Having been delivered to Terra aboard the Black Ships, these psykers are made pure, so that when their time comes they may add their very essence to that of many thousands of others, and in their way know a blessed unity with the Emperor that no others will ever experience. As their souls burn within the spirit-cauldron of the Astronomican, they fuel the fire that shines out across two thirds of the entire galaxy. By that brightest of beacons, the Navigators of the Navis Nobilite are able to assay their positions and guide their vessels through the churning Sea of Souls