Astronomy
Cosmological Horizons: Particle and Event Horizons
Quick fact
The particle horizon is not the same as the event horizon: the particle horizon defines the farthest light we can have seen from the past, while the event horizon defines the farthest light we will ever see in the future—and in an accelerating universe, the event horizon is closer than you might think.
Why this is interesting
Have you ever wondered if there's a limit to how far we can see into the universe? It turns out there are two different limits—one looking back to the past, and one looking toward the future.