Event Horizon - Particle Horizon of The Observable Universe

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:

    Love is life. All, everything that I understand, I understand only because I love. Everything is, everything exists, only because I love. Everything is united by it alone. Love is God, and to die means that I, a particle of love, shall return to the general and eternal source.
    Leo Tolstoy (1828–1910)

    “Dark times” is what they call it in Norway when the sun remains below the horizon all day long: the temperature falls slowly but surely at such times.—A nice metaphor for all those thinkers for whom the sun of mankind’s future has temporarily disappeared.
    Friedrich Nietzsche (1844–1900)

    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)

    How weak and little is the light,
    All the universe of sight,
    Love and delight,
    Before the might,
    If you love it not, of night.
    Edward Thomas (1878–1917)