Justin Khoury, Burt Ovrut, Paul Steinhardt, and Neil Turok devise the Ekpyrotic Universe, a cyclic universe that exploits string theory. It is actually two parallel three-dimensional universes (brane worlds) that periodically move apart and back together until they violently impact, exploiting one of the additional dimensions, which has remained very small, perhaps because it was unaffected by the initial inflation. This could provide a clue to explaining why gravity is so weak compared to the other two forces. Indeed, gravity could act in all dimensions of space, including the parallel brane world, while the other three forces do not. This would also explain why there is much less luminous matter than gravitational matter. The parallel brane world would be gravitationally very close (less than a millimeter apart) but optically incredibly distant.
