> Space-time is usually understood to describe space existing in three > dimensions, with time playing the role of a fourth dimension and all > four coming together to form a continuum, or a state in which the > four elements can't be distinguished from each other.
> Flat space-time and negative space-time describe an environment in > which the Universe is non-compact, with space extending infinitely, > forever in time, in any direction. The gravitational forces, such as > the ones produced by a star, are best described by flat-space time. > Negatively curved space-time describes a Universe filled with > negative vacuum energy. The mathematics of holography is best > understood for negatively curved space-times.