The simplest definition I found of the difference between the particle horizon and the event horizon is here: http://iapetus.phy.umist.ac.uk/Teaching/Cosmology/Metric.html The particle horizon is defined as the largest comoving distance from which light can have reached us - today. The event horizon is defined as the largest comoving distance from which light will ever reach us - at any time in the future. To have an event horizon, a universe needs to expand "very quickly", e.g. in a de Sitter universe, whose scale factor follows a pure (positive) exponential: http://nedwww.ipac.caltech.edu/level5/Watson/Watson6_1.html Apparently the definitions come from Rindler (1956, MNRAS, 116, 662): http://nedwww.ipac.caltech.edu/level5/Seitter/Seitter3_2_2.html but the figure is not clear to me. Boud