Does the cosmic event horizon have mass?

When a massive object falls into a black hole it increases its apparent mass.

Does the same thing happen at the cosmic event horizon (at the edge of the observable universe)?

Do the farthest galaxies which fall beyond the cosmic event horizon add to its apparent mass? And if so, could the gravitational effects explain the accelerating expansion of the universe?

Does the same thing happen at the cosmic event horizon (at the edge of the observable universe)?

There is no way to define this. GR doesn't have a general definition of the mass of an extended object. There are definitions of mass such as the Komar mass and the ADM mass, but these only apply to asymptotically flat spacetimes. Cosmological spacetimes aren't asymptotically flat.

When a massive object falls into a black hole it increases its apparent mass.

Here "its" would have to refer to the entire black hole. There is no way to define whether the mass is localized on the horizon. For example, if the ADM mass of a black hole is $m$, that's just telling you a property of the whole thing.

A black hole's event horizon is not really even a place. A place is a timelike surface. The event horizon is a lightlike surface.

• "There is no way to define this. GR doesn't have a general definition of the mass of an extended object." Does it have to be an extended object though? I was thinking more like a "topologically inverted" black hole. Instead of mass being concentrated in a single central point it would be distributed across a spherical surface somewhere "beyond" the spherical surface of the cosmic event horizon. This way the observable universe - instead of expanding because of dark energy - would be falling into a black hole-like thing which is all around it. – kyjo Jan 5 '18 at 15:30

Of course. Some Alternative black hole model hypotheses (Gravastar, Dark energy star)are similar with the cosmic event horizon. For example , when the matter is attracting by Dark energy star's gravitational field，and falling into its event horizon,some or all infalling matter would covert to dark energy , and causing the generation of negative pressure . Th negative pressure counteracts strong gravity(Positive pressure ) inside the event horizon , avoiding a singularity's appearance and halting the collapse process .It also causes the expansion of the event horizon，and following the conservation Principle，all matters still transfer the matter to it, so it has mass like CEH. Is it similar with the expansion of the universe?Despite CEH(Cosmic event horizon ) has no define real event horizon,and I think the CEH has mass like the BLACK HOLE when matter infalling into event horizon , no matter which Black Hole model， Both apparent mass is manifesting for gravitational mass.Those Alternative black hole models are derived for CEH caused by expansion of the universe. The gravity is closely related with expansion of universe for it decided its expansion speed, it explains why universe'expansion faster than Expected value.