Cosmological horizon

Event horizon

The particle horizon differs from the cosmic event horizon, in that 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.

In general, the proper distance to the event horizon at time ${\displaystyle t}$ is given by $${\displaystyle d_e(t)=a(t){\int }_t^{t_{\text{max}}}\frac{cd{t}^{\prime }}{a(t^{\prime })}}$$ where ${\displaystyle t_{\text{max}}}$ is the time-coordinate of the end of the universe, which would be infinite in the case of a universe that expands forever.

For our case, assuming that dark energy is due to a cosmological constant Λ, there will be a minimum Hubble parameter H_e and a maximum horizon d_e which is often referred to as the only particle horizon: $${\displaystyle max(d_e)=\frac{c}{H_e}=c\sqrt{\frac{3}{\mathrm{\Lambda }}}=\frac{c}{\sqrt{{\mathrm{\Omega }}_{\mathrm{\Lambda }}}{H}_0}=17.55\text{Gly}.}$$

Future horizon

The current concordance or Lambda-CDM model of cosmology is based on an accelerating universe. Extrapolating the model into the far future predicts a universe consisting solely of our Milky Way. Light from distant galaxies will be redshifted so much as to become invisible. Thus observational evidence for cosmology, including the particle horizon, will be unverifiable.

Other horizons

While not technically "horizons" in the sense of an impossibility for observations due to relativity or cosmological solutions, there are practical horizons which include the optical horizon, set at the surface of last scattering. This is the farthest distance that any photon can freely stream. The Hubble sphere is also called the "photon horizon". Similarly, there is a "neutrino horizon" set for the farthest distance a neutrino can freely stream and a gravitational wave horizon at the farthest distance that gravitational waves can freely stream. The latter is predicted to be a direct probe of the end of cosmic inflation.

History

The nature of cosmological horizons was clarified by Wolfgang Rindler in 1956. He distinguished instantaneous events like a supernova from world lines, a string of events like light from a durable object like galaxy. The behavior of world lines became the basis for splitting the observable and unobservable universe.