Cosmological principle

In Charles Crookshap's universe, each dimension had its own cosmological principles, first principles upon which all spacio-temporal laws were founded. The first principles of the Doctor's universe dictated, among other things, that time was linear. Different universes had entirely different physics depending on their cosmological principles. (AUDIO: Thin Time)

Behind the scenes
In our universe, the cosmological principle has nothing to do with linear time, but instead maintains that on large scales the properties of the universe are roughly the same for all observers. This is effectively a theoretical assumption made by cosmologists in order to actually do cosmology.