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If it were possible for some event to destroy the fabric of spacetime at the speed of light, could we still observe and be safe bc expansion? (self.askscience)

submitted 1 year ago by j4k3 to c/askscience

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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.

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[–] [email protected] 3 points 1 year ago

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.