Particle horizon
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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.
It is different from the event horizon:
- The particle horizon is defined as the largest comoving distance from which light can have reached the observer — at the present moment. This is also known as conformal time.
- The event horizon is defined as the largest comoving distance from which light can ever reach the observer — at any time in the future.
See also
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