The particle horizon in cosmology is the distance from which particles (of positive mass or of zero mass) can have travelled to the observer in the age of the Universe.

It is different from the event horizon.

• The particle horizon is defined as the largest comoving distance from which light can have reached us (the observer) - today.
• The event horizon is defined as the largest comoving distance from which light will ever reach us (the observer) - at any time in the future.