«BLACK HOLES TRIPTYCH»
EVENT HORIZON #002. The boundary beyond which events cannot ever be observed is an event horizon, and it represents the maximum extent of the particle horizon.The criterion for determining whether a particle horizon for the universe exists is as follows. Define a comoving distance dp as seen on the pictures
In this equation, a is the scale factor, c is the speed of light, and t0 is the age of the Universe. If dp → ∞ (i.e., points arbitrarily as far away as can be observed), then no event horizon exists. If dp ≠ ∞, a horizon is present.