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In physical cosmology, a cosmological horizon marks a limit to observability, and marks the boundary of a region that an observer cannot see into directly due to cosmological effects.
The existence, properties, and significance of a cosmological horizon depend on the particular cosmological model being discussed.
In any case, it is interesting to note that the cosmological horizon is a maximal limit of perception and not an actual boundary. Much like an individual who is unable to see the edges of the Pacific Ocean while they are floating in the middle of it; we can only see the light from areas of space within the cosmological horizon.
This is sometimes referred to as the “observable universe”, and it has been said that the observable universe is many orders of magnitude smaller than the greater universe that lies beyond the limits of our perception.
Imagine that the entire cosmological horizon is modeled by a sphere that is the diameter of a quarter. If Alan Guth's inflationary model of early era cosmology is correct, the universe that lies beyond this “quarter-sized” horizon would conservatively be a sphere as large as the Earth globe itself.
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