Just a thought, if an event happened well beyond the observable universe that caused entire galaxies to be destroyed radiating from a point source event, what would it look like from our perspective and how close could it get on our observable horizon while still being unable to reach us due to expansion of the universe?
Obviously, the ability to “observe” in this context is extremely relative to the time scale, and far longer than even our species is likely to remain in recognizable form. I want to conjure a mental picture an era where a substantial portion of the distant night sky is a marching black void, and no one is entirely sure if it will halt from expansion or end everything in a flash one day.
Gravity and the light are traveling at the same speed, so we should see it and feel it at the same time.
An explosion wouldn’t change the gravity situation though. Gravitational waves are not relevant here. The danger of an explosion would be the physical matter stripping the atmosphere, and radiation. I think it would take quite a bit longer before Earths gravity is affected significantly based on the drag from traveling through the debris. A gravity well is about the total mass in the center. So wouldn’t a significant amount of material need to make it past the orbit of earth before the orbit is directly altered? The expansion would impact the rotation of matter from the stellar body, but that is not coupled to an orbiting body in a vacuum.
You are right! People often say “what if the sun blew up” in the context of gravity speed vs. light speed thought experiments, but what they really mean is shorthand for what if the entire sun was somehow deleted in a single instant with no trace. But in reality, “blowing up” the sun is much different than “deleting” it and leaves its entire mass behind, just spread around more.
There is even a theorem in general relativity that proves that massenergy cannot be deleted, invalidating a whole swath of such thought experiments. Forgot what it’s called though.