eli5: when a star leaves the observable universe due to expansion, does it get fainter over time from our perspective, or it just appears to turn off, like a flashlight?
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As I’m sure you know the ‘observable universe’ is the area of the universe which is moving away from us slower than the speed of light, which gives photons from that area a chance to reach us. Any further and the relative space between us and those points expands faster than the speed of light and the photons will be forever on our way to us, but space is being stretched out in front of them faster than then can travel through it.
Now for the stars in right on the edge of the observable universe, it helps to think of the light that is emitted in a one-year time span, which will span over the distance of one lightyear when it is expelled from the star. Over the course of time when the light travels to us, this one lightyear span of space will have stretched out, effectively distributing the original amount of photons over a longer span of space.
Take this to the extreme, right on the edge of the observable universe, and the one lightyear span of space will stretch out into an infinitely long distance. Since the hypothetical star will slowly move over the edge of the observable universe, we can say that the density of photons we can record from the star will asymptotically reduce to 0.
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