Cosmic event horizon Event horizon

in cosmology, event horizon of observable universe largest comoving distance light emitted can ever reach observer in future. differs concept of particle horizon, represents largest comoving distance light emitted in past have reached observer @ given time. events beyond distance, light has not had time reach our location, if emitted @ time universe began. how particle horizon changes time depends on nature of expansion of universe. if expansion has characteristics, there parts of universe never observable, no matter how long observer waits light regions arrive. boundary past events cannot ever observed event horizon, , represents maximum extent of particle horizon.

the criterion determining whether particle horizon universe exists follows. define comoving distance




d

p




{\displaystyle d_{p}}

by

d

p


=

∫

0



t

0






c

a
(
t
)



d
t
 
.


{\displaystyle d_{p}=\int _{0}^{t_{0}}{\frac {c}{a(t)}}dt\ .}

in equation, scale factor, c speed of light, , t0 age of universe. if




d

p


→
∞


{\displaystyle d_{p}\rightarrow \infty }

(i.e., points arbitrarily far away can observed), no event horizon exists. if




d

p


≠
∞


{\displaystyle d_{p}\neq \infty }

, horizon present.

examples of cosmological models without event horizon universes dominated matter or radiation. example of cosmological model event horizon universe dominated cosmological constant (a de sitter universe).

a calculation of speeds of cosmological event , particle horizons given in paper on flrw cosmological model, approximating universe composed of non-interacting constituents, each 1 being perfect fluid.