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Particle Horizon is the maximum distance particles can travel.
The particle horizon in physical cosmology is the maximum distance from which particles (of positive or zero mass) can have travelled to the observer in the age of the universe. Particle horizon represents the portion of the universe which we could have conceivably observed by the present day.
In terms of comoving distance the particle horizon is equal to the conformal time?0 that has passed since the Big Bang, times the speed of lightc. The quantity?0 is given by,
wherea(t) is the scale factor of the Friedmann-Robertson-Walker metric, and we have taken the Big Bang to be att = 0.
The particle horizon differs from the Event horizon in that the particle horizon represents the largest comoving distance from which light could have reached us by now, while the event horizon is the largest comoving distance from which light can ever reach the observer at any time in the future.
Source of particle horizon.
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