Bulk

For a more scientific viewpoint, see brane cosmology on Wikipedia.

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In Solar Storms, the bulk represents the hyperspacial space-time that the component universes of the Multiverse rest in. Its properties constrain the universes within it, though certain factors, such as the speed of light, can be cheated using its medium - sometimes called the aether, instead of normal space. Relativity still exists, after a fashion, though the speed of the aether is still far, far faster than that of light.

Tachyons are the name given to the telltale disturbances in the Universe that propogation through this medium creates. Their speed is presumed to be that of the aether, and though it is not infinite, it has yet to be properly measured.