The ship moves
Yet another variation on the future of Warhammer 40K created by the Anons of /tg/.
In the not too distant future of 40K, the Imperium's borders have shrunk, devastated by wars, disease and famine. For a brief, shining moment, the Emperor awakes upon His Golden Throne and orders the construction of a massive ship, its keel 1AU in length. What remains of the Imperium is stripped to build The Ship, with Holy Terra itself hollowed out and fitted with massive cogitators and command system, the Imperial Palace itself becoming the bridge. All that is left of humanity is brought aboard this ark of civilization. Here humankind lives and dies in the bowels of its massive holds, whole world's worth of people living in single compartments. The combined psychic presence of humanity lies under the aegis of the God-Emperor's power, thwarting Chaos. Only the God-Emperor, now the God-Captain, knows where The Ship is going.
In other words, it's just battlestar galactica reworked for the Warhammer 40k setting.
The Ship is the Imperium. The Imperium is the Ship. The Ship is the Emperor. The Emperor is the Ship. All is the Ship. The Ship is All. The Ship Moves.
Some Motherfucking Math
In the original idea, that ship is scaled up in dimensions linearly from the Emperor class battleship, which has a length to width ratio of 6.25:1. So, at 1 AU long (1.50 x 10^8 km) it would also be approximately 2.4 x 10^7 km wide. If it were a single, flat plate it would have a surface area of 3.58 x 10^15 km, which is just a little over 7 million times the entire surface area of the Earth. With multiple floors; the ship, in short, would have at least 1-2 orders of magnitude greater surface area than the entire Old Imperium combined.