# Physics:Causal patch

Short description: Special region of spacetime

A causal patch is a region of spacetime connected within the relativistic framework of causality (causal light cones).

## Background

After Leonard Susskind proposed the black hole complementarity conjecture for black holes in quantum gravity, he realized it would also apply to a de Sitter universe with a positive cosmological constant with the cosmological horizon in place of the event horizon. The region within the horizon is the causal patch,[1][2] and it is self-contained. This means we may neglect what happens beyond the cosmological horizon. A consequence of this radical conjecture is that the total number of states of the universe is finite.

## References

1. Dyson, Lisa; Kleban, Matthew; Susskind, Leonard (October 2002). "Disturbing Implications of a Cosmological Constant". Journal of High Energy Physics 2002 (10): 011. doi:10.1088/1126-6708/2002/10/011. Bibcode2002JHEP...10..011D.
2. Goheer, Naureen; Kleban, Matthew; Susskind, Leonard (July 2003). "The Trouble with de Sitter Space". Journal of High Energy Physics 2003 (7): 056. doi:10.1088/1126-6708/2003/07/056. Bibcode2003JHEP...07..056G.