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How would one detect a black hole when there are no more light sources for it to refract?

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The universe as we know it has ended. The concept of "natural" illumination ended when the last red dwarves and quasars died out. Now, only brown dwarves and blackholes dot the empty void that was once the universe.

Life, still exists! Though, only in big self-sustaining space stations that are running out of fusion fuel. These space stations drift through the void, blindly grasping through the darkness, collecting every atom of hydrogen they could find.

Every now and then, these stations send out manned spaceships capable of FTL. Their mission is to find the few pockets of hydrogen to power their space stations.

Their FTL, isn't really FTL. It's more like moving at "normal" speeds (Normal as in .20c). The way it works, is it "folds" the local space-time so the distance they have to travel is much shorter, giving the effect of moving faster than light.

The remaining bits of hydrogen is usually found near blackholes, their gravity-well actually collects them over vast distances. The problem is; There's literally no point of reference. Since there's no light, there's also no way of knowing that a blackhole is there.

Another species however, the Humans, need a blackhole for another reason. These plucky little humans, they made up a competition of who can survive the longest. They went and built a station and filled it up with the best of humanity. Once complete, they want to toss it to within the event horizon of a supermassive black hole. (What they don't know however is that a new universe is coming, they just need to last long enough, so yay!)

How can I find a blackhole without any points of references or what's to stop me from colliding into one at maximum speed in this lightless universe?

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This post was sourced from https://worldbuilding.stackexchange.com/q/173192. It is licensed under CC BY-SA 4.0.

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Try microlensing other evaporating black holes

Folks have suggested Hawking radiation; I don't think that's a particularly good idea. If you run the numbers, a black hole would need to have a mass of $M\sim10^3\text{ kg}$ before it reached a luminosity of $1L_{\odot}$, and would have only about 10 nanoseconds before it finally evaporated. If you're okay with a dimmer source - say, $0.0001L_{\odot}$ - then you've got about 9 milliseconds, which still isn't much. Any black hole luminous enough for you to effectively detect from interstellar distances won't live long enough for you to manipulate it.

However, you can still use a twist on the idea microlensing. This is a technique used to find dim, compact objects and exoplanets (though it's increasingly found more use when it comes to exoplanets than when it comes to black hole detection!). If you have a dim object in front of you, and there's a light source behind it, the foreground object will bend the light from the background source. This should be detectable, provided the foreground object is within the galaxy and provided you get the timing just right. In your universe, the lensing object can be one of these stellar-mass black holes wandering around. They can act as lenses while still having plenty of time to go before that evaporate.

As for the light source - well, try an evaporating black hole! You'd need to time things just right, and you'd need the background black hole to be fairly close - after all, the black hole would emit enormous amounts of energy in the final fraction of a fraction of a second before it's gone, but that emission would only last a fraction of a fraction of a second. So get the timing just right!

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