march utc an event horizon is any surface from beyond which no information can ever reach the observer of course that includes objects of all kinds in fact anything moving at or below the speed of light so in a sense you're right an important thing to note though is that the event horizon in the strictest sense belongs to the observer rather than the thing not being observed in the case of a black hole the event horizon is the same for all outside observers which i guess is what you mean by a 'physical boundary' the event horizon article is biased towards this the most commonly encountered form of event horizon the cosmological event horizon is different since every point in the universe is receding from every other point with a speed proportional to distance the event horizon is always a sphere centred on a particular observer so there is no single 'event horizon of the universe' although of course we tend to give a special place to our own event horizon so neither article is wrong although the introduction to event horizon gives a misleadingly narrow interpretation of the term which i'll try to fix the cosmological use is however mentioned in event horizon event horizon of the observable universe there are more details in cosmological horizon and particle horizon