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:
“The way to learn German, is, to read the same dozen pages over and over a hundred times, till you know every word and particle in them, and can pronounce and repeat them by heart.”
—Ralph Waldo Emerson (1803–1882)
“Elvis’ disappearing body is like a flashing event horizon at the edge of the black hole that is America today.”
—Arthur Kroker (b. 1945)
“To develop an empiricist account of science is to depict it as involving a search for truth only about the empirical world, about what is actual and observable.... It must involve throughout a resolute rejection of the demand for an explanation of the regularities in the observable course of nature, by means of truths concerning a reality beyond what is actual and observable, as a demand which plays no role in the scientific enterprise.”
—Bas Van Fraassen (b. 1941)
“It is a delicious thing to write, whether well or badly M to be no longer yourself but to move in an entire universe of your own creating.”
—Gustave Flaubert (1821–1880)