If you think about it, time travel is actually very logical. Reading through the grimoir cards of the vex, the "Axis Minds" and "Gatelords" apparently "Hold the secrets to an infinite amount of universes and galaxies, at each point in time" and the Gatelords supposedly hold the key to moving in time and space. [spoiler]Don't tell anyone, but this is just an estimated "guess"[/spoiler]