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.
We would not see it until (if) it hit us.
Observation cannot travel faster than the speed of light. No matter what it is you’re using to observe: Photons (light and radiation), measuring gravity, heat, anything. No matter if the phenomenon’s expansion were traveling at the speed of light, the changes to the universe being made as well as our ability to observe them are also traveling at the speed of light.
If the phenomenon were very far away, we would not be able to observe anything it was causing until its leading edge caught up to us. Then we would be destroyed at exactly the same time. This is because in your example it is expanding at exactly the same rate as the universal speed-of-light constraint allows us to receive any indication of its presence. Any evidence of, e.g. a far away star being destroyed would take X amount of time to reach us by its light no longer arriving. However, in that time the edge of the space-destroying phenomenon will also hit us, because it will also take exactly X amount of time to reach us, at the speed of light, from the point where the star was when it was destroyed. The distance is the same, the speed is the same. We would continue to receive light from that star in the meantime, as we already do. (The light from the stars you see in the sky now is already tens/hundreds/thousands/millions/etc. years old depending on the distance to the star in question.)
If the phenomenon were so far away that it is outside of our observable field of the universe, it will never reach us and we will never have any indication of its presence. That’s what “observable universe” means. Anything can happen anywhere outside of the observable universe and it is objectively meaningless to us, because we will never ever be able to reach it, record it, have it influence us in any way. This is, however, predicated on the theory of the perpetually expanding universe being true (which it probably is).
If you want to actually see the stars in your sky winking out over the millennia, I suggest building your universal destruction bomb such that its shockwave travels at, say, half the speed of light or some other suitable fraction.
Thanks, I was having trouble intuitively on that tipping point of expansion moving objects faster than the speed of light and how that is observed without more than lunch napkin level thought. Makes sense. We would never know about or see “the bubble” if it stopped short due to expansion.
The best we can achieve in this thought experiment is to see through a telescope some faraway alien set up a bomb with a countdown timer that will surely blow up at a specific time in the future and destroy the universe, but which we’ll never see count down to zero or explode. If we saw it reach zero it would of course kill us in the same instant as we see it, because by the rules of the thought experiment the explosion travels at the speed of light. But if the alien is far away and the countdown is long enough, the accelerating expansion of the universe due to dark energy will carry it outside of our cosmic event horizon before it explodes.
Using the cosmic comoving distance definition and the cosmology calculator, the last scattering surface of the Cosmic Microwave Background for example is 45.5 GLy away. Its light was emitted 13.7 GY ago (400kY after the Big Bang) at redshift 1100z. I was told that due to accelerating expansion, we will never see galaxies further than 63 GLy away (we don’t see them yet, the matter that we’ll see form them is beyond the CMB sphere for us at present), and if we hopped onto a lightspeed spaceship right now, we can never reach galaxies beyond 17 GLy comoving distance.
So for example if we looked at a galaxy at redshift 3z which is 21 GLy away, and whose light took 11.5 GY to reach us, and saw the alien set up the bomb timer to 11.49 GY, we know that the bomb must have surely exploded by now, but also know that we are safe because it’s far enough away and we’ll never see it explode, even in the infinite future.
Similarly, we can relish the tiny shred of joy in the knowledge that if we did fuck up something really major, like creating a false vacuum bubble in the LHC or whatever, we can never destroy more of the universe than the 17 GLy bubble around us.
I think if it’s outside the observable universe, then the answer is probably we would never observe it.
If the sphere of destruction is propagating at the speed of light, then any observable effect reaches you at the same time as the sphere itself. Either you don’t observe it because you’re far enough away to be safe, or you don’t observe it because you’re dead the instant it becomes observable.
Incidentally, you might be interested in looking up the idea of false vacuum decay - although if you tend to get anxious about end-of-the-world hypotheticals you might prefer to give it a miss.
“Some say that this has already happened”"
(anything outside of your lightcone can be ignored - there is no such thing as seeing it or knowing about it)
Read somewhere that if the sun blew up we wouldn’t see it before the gravity affected us.
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.
Fun idea. You mean like the expansion of the universe is going 0.5 light speed at the edge of the void, so the spreading void is basically pulled back right? And then any photons that reached us from just before this void are traveling a tiny bit faster I guess? Being just outside the void?
