Gravitational event horizon?

If we cannot see it (i.e. if light from an event has not had sufficent time to reach us on Earth) then we cannot 'feel' its gravitational influence either.

According to General Relativity, gravity travels in waves that propagate at the speed of light.

Expansion of the universe does not restrict our ability to see distant objects, only distance is involved, since gravity propagates at the speed of light we would not be affected by the gravitation field of an object that is so far away light from it has not reached us yet. If a universe had an event horizon, which may be a possibility, we could never observe another universe.

Your question is based on a false assumption: that gravitons only travel at c but effects in space time are instant. This is incorrect, as changes in spacetime curvature travel at c in relativity. Nevertheless, neither theory excludes gravitational effects from beyond horizons.

There are also horizons around black holes, and they gravitate just fine. Forces in quantum mechanics are mediated by *virtual* particles, which are not restricted to travel at c.

This question seems to hinge on object permanence more than physics: does an object exist if we cannot observe it?

If we can measure it, then it exists

If we cannot measure it, then it does not exist

If this it TRUE, then only object in the observable sphere, across an observable multiverse, would exist and would interact with our (limited?) universe. However: because things is our universe behave in a relatively consistent manner, an object at the edge of our observable universe, behaving in a known way, "X", without an observance of the cause of "X" (because it is outside of our measurable universe) could be used to theorize the cause of "X" without direct observation.

This would suggest, assuming our universe behaves regularly, that we can know things outside the measurable universe without actually seeing them.

We would probably have to have a more full understanding of what causes things to interact gravitationally (ie find a graviton, or a higgs particle, or something like it) before we could meaningfully answer that question.

But it's true to say that, logically, they shouldn't suffer from the same restrictions that we experience in our relative location in their relative location.