It has been suggested that this article or section be merged into Observable universe. (Discuss)
The particle horizon in physical cosmology is the maximum distance from which particles (of positive or zero mass) can have travelled to the observer in the age of the universe. Wikipedia does not have an article with this exact name. ... The observable Universe is a term used in cosmology to describe a ball-shaped region of space surrounding the Earth that is close enough that we might observe objects in it. ... Cosmology, as a branch of astrophysics, is the study of the large-scale structure of the universe and is concerned with fundamental questions about its formation and evolution. ... In particle physics, an elementary particle or fundamental particle is a particle not known to have substructure; that is, it is not made up of smaller particles. ... Mass is a property of a physical object that quantifies the amount of matter and energy it is equivalent to. ... In general, an observer is any system which receives information from an object. ... The deepest visible-light image of the cosmos, the Hubble Ultra Deep Field. ...
It is different from the event horizon: This article or section is in need of attention from an expert on the subject. ...
The particle horizon is defined as the largest comoving distance from which light can have reached the observer — at the present moment. This is also known as conformal time.
Conformal time can also be defined as dt/a(t), where a(t) is the scale factor of the FLRW metric. It is describing a cosmology expanding in scale (and does not have a horizon).
The event horizon is defined as the largest comoving distance from which light can ever reach the observer — at any time in the future.
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The comoving distance or conformal distance of two objects in the universe is the distance divided by a time-varying scale factor representing the expansion of the universe. ... Prism splitting light Light is electromagnetic radiation with a wavelength that is visible to the eye (visible light) or, in a technical or scientific context, electromagnetic radiation of any wavelength. ... The scale factor, parameter of Friedmann-Lemaître-Robertson-Walker model, is a function of time which represents the relative expansion of the universe. ... The Friedmann-Lemaître-Robertson-Walker (FLRW) metric is an exact solution of the Einstein field equations of general relativity and which describes a homogeneous, isotropic expanding/contracting universe. ...
Source
Lars Bergström and Ariel Goobar: Cosmology and Particle Physics, WILEY (1999), page 65. ISBN 0-471-97041-7
The particlehorizon in physical cosmology is the maximum distance from which particles (of positive or zero mass) can have travelled to the observer in the age of the universe.
If a particle is moving at a constant velocity in a non-expanding universe free of gravitational fields, any event that occurs in that universe will eventually be observable by the particle, because the forward light cones from these events intersect the particle's world line.
For the case of a horizon perceived by an occupant of a de Sitter universe, the horizon always appears to be a fixed distance away for a non-accelerating observer.
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