Cosmological horizon

Lore & Background

The particle horizon, also called the comoving particle horizon, is the maximum distance from which light from particles could have traveled to an observer in the age of the universe. It represents the boundary between the observable and unobservable regions, and its distance at the present epoch defines the size of the observable universe. When cosmologists say 'horizon' they almost always mean the particle horizon. In an empty, homogeneous, and isotropic universe, the proper distance to the horizon at time t is given by an integral involving the cosmological scale factor. In terms of comoving distance, the particle horizon equals the conformal time that has passed since the Big Bang, times the speed of light.

Reader's Guide

The particle horizon differs from the cosmic event horizon: the particle horizon represents the largest comoving distance from which light could have reached the observer by a specific time, while the cosmic event horizon is the largest comoving distance from which light emitted now can ever reach the observer in the future. The current distance to our cosmic event horizon is about five gigaparsecs (16 billion light-years), well within our observable range given by the particle horizon. Under the Lambda-CDM model, assuming dark energy is due to a cosmological constant, there will be a minimum Hubble parameter and a maximum event horizon of 17.55 gigalight-years. Extrapolating the model into the far future predicts a universe consisting solely of our Milky Way, with light from distant galaxies redshifted so much as to become invisible, making observational evidence for cosmology, including the particle horizon, unverifiable. Other practical horizons include the optical horizon at the surface of last scattering, the Hubble sphere (also called the 'photon horizon'), a neutrino horizon, and a gravitational wave horizon predicted to probe the end of cosmic inflation.

Did You Know?

• The particle horizon is the boundary of the observable universe.
• The event horizon includes all possible future observations; points outside it can never be observed.
• The current distance to our cosmic event horizon is about five gigaparsecs (16 billion light-years).