Particle horizon

The particle horizon marks the greatest present-day distance from which light or other signals could have reached an observer since the universe began. Its size depends on the universe’s expansion history.

Scale factor: A dimensionless measure of how distances between comoving points in the universe change over time. Its history determines how far a signal could travel while space expanded.
Observable universe: The region of the universe from which signals have had time to reach an observer. Its boundary is set by the particle horizon for a given observer and cosmic time.
Hubble sphere: The distance at which the recession speed from cosmic expansion equals the speed of light at a given time. It is not generally a causal boundary and need not match the particle horizon.
Cosmic horizon problem: The puzzle of why widely separated regions of the cosmic microwave background have nearly the same temperature despite limited causal contact. The particle horizon exposes the causal-contact problem that inflation addresses.
Comoving distance: A distance coordinate that factors out cosmic expansion, keeping the separation of freely expanding points constant. The horizon is commonly calculated as a comoving distance.
Light cone: The set of possible paths taken by light signals into or out of an event in spacetime. The particle horizon is the distant boundary of the observer’s past light cone.
Cosmic event horizon: The boundary beyond which events can never send signals that reach an observer, even in the infinite future. It limits future signal reception, whereas the particle horizon records past signal reach.
Cosmic inflation: A hypothesized brief era of accelerated expansion in the early universe. Inflation can place today’s observable region within one earlier causally connected region.
Conformal time: A time coordinate that rescales cosmic time by the expansion factor, making light propagation especially simple. Its accumulated value sets the comoving distance light can travel.
Spacetime: The four-dimensional structure combining three dimensions of space with one dimension of time. Signal reach and cosmic expansion are described within this framework.

Linked from 8 pages