Particle Horizon of The Observable Universe
The particle horizon of the observable universe is the boundary that represents the maximum distance at which events can currently be observed. For events beyond that distance, light has not had time to reach our location, even if it were emitted at the time the universe began. How the particle horizon changes with time depends on the nature of the expansion of the universe. If the expansion has certain characteristics, there are parts of the universe that will never be observable, no matter how long the observer waits for light from those regions to arrive. The boundary past 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 an event horizon for the universe exists is as follows. Define a comoving distance by
In this equation, a is the scale factor, c is the speed of light, and t0 is the age of the universe. If (i.e. points arbitrarily as far away as can be observed), then no event horizon exists. If, a horizon is present.
Examples of cosmological models without an event horizon are universes dominated by matter or by radiation. An example of a cosmological model with an event horizon is a universe dominated by the cosmological constant (a de Sitter universe).
Read more about this topic: Event Horizon
Famous quotes containing the words particle, horizon, observable and/or universe:
“You don’t hold any mystery for me, darling, do you mind? There isn’t a particle of you that I don’t know, remember, and want.”
—Noël Coward (1899–1973)
“Elvis’ disappearing body is like a flashing event horizon at the edge of the black hole that is America today.”
—Arthur Kroker (b. 1945)
“Every living language, like the perspiring bodies of living creatures, is in perpetual motion and alteration; some words go off, and become obsolete; others are taken in, and by degrees grow into common use; or the same word is inverted to a new sense or notion, which in tract of time makes an observable change in the air and features of a language, as age makes in the lines and mien of a face.”
—Richard Bentley (1662–1742)
“Failure or success seem to have been allotted to men by their stars. But they retain the power of wriggling, of fighting with their star or against it, and in the whole universe the only really interesting movement is this wriggle.”
—E.M. (Edward Morgan)