Particle horizon

Summary

The particle horizon of an observer bounds the part of the universe from which light could have reached the observer since the beginning. It is a kind of cosmological horizon, and it is the largest distance from which the observer can have received any signal so far. It grows with time. It exists exactly when the integral of $dt/a$ from the beginning converges, as in models with a Big Bang and decelerating early expansion. The size of the particle horizon at early times is at the heart of the horizon problem.