Cosmological horizon

A boundary in spacetime separating events that can be observed or causally influence an observer from those that cannot. Its position depends on cosmic expansion, the universe’s age, and the observer’s location and history.

Event horizon: A boundary beyond which events can never send signals to an observer, even given unlimited time. Accelerating expansion can make distant regions permanently unreachable by signals emitted now.
Light cone: The set of spacetime paths that light can take from or toward an event. A horizon marks the limiting edge of which events lie within an observer’s relevant light cones.
Particle horizon: The greatest distance from which light could have reached an observer since the universe began. Unlike an event horizon, it describes the limit of what has already had time to be seen.
Cosmic event horizon: The boundary separating events that can ever send signals to an observer from those that cannot. In an accelerating universe, some presently observable regions will eventually pass beyond this boundary.
Hubble radius: The distance at which cosmic recession speed equals the speed of light in a given cosmological model. It is often mistaken for a horizon, but signals can cross it in some expanding universes.
Causality: The relation between events where one event can influence another through physical signals or interactions. Cosmological horizons are boundaries on causal influence, not merely visibility.
Hubble sphere: The sphere centered on an observer whose radius is the Hubble radius at a chosen cosmic time. It is not generally a causal boundary; light can travel across it as the Hubble radius evolves.
Cosmic variance: The uncertainty in cosmological measurements caused by observing only one finite realization of the universe. A finite observable region limits how much information can be gathered about the universe as a whole.
Comoving distance: A distance measure that factors out cosmic expansion and remains fixed for objects moving with the expansion. It lets cosmological horizons be compared across epochs without confusing expansion with changing coordinates.
Spacetime: The four-dimensional structure combining three dimensions of space with one dimension of time. A horizon is a geometric boundary in spacetime rather than an object located in space.

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