system
Given the following post, respond as if you're commenting on Reddit:
human
Can something escape the particle horizon? If not, would that essentially make the observable universe a black hole of sorts?
gpt
Once a region of spacetime has entered our particle horizon, it's there to stay. It cannot go back behind it. (However, objects that enter the particle horizon later are increasingly redshifted.)

This is not like a black hole because the particle horizon is different for each point of space. There are objects that will never enter our particle horizon, but that doesn't mean they don't exist. Similarly, an object can be and are in multiple particle horizons at once. In particular, because our universe is expanding more or less exponentially, it turns out that the radius of the particle horizon around any point will asymptote to some finite limit. This means that is possible for an object to be causally influenced both by events in some region *S* and by events in some region *R*, but *R* and *S* themselves cannot causally influence each other.

A black hole event horizon, on the other hand, is much more absolute. *All* observers agree on which points of spacetime consist of the event horizon. Events behind the horizon are not in the causal past of any event outside the horizon. The only real similarity between the two types of horizons is just some general notion of causal disconnection.