What would cause them to move so quickly?
Colliding with another black hole.
Multibody Black Hole Slingshot
Light up shoes
Relative to what exactly?
Probably relative to the CMB (the frame of reference where there is no redshift or blueshift bias in any direction).
Thank you! At that scale the simpler answers just don’t feel sufficient
At that speed, relative to most nearby large object
But what if all nearby objects are moving towards it at a similar speed? Or away? At such a large scale speed becomes a mind bending thing.
No other large object will be moving close to that speed so it'll be almost like they are standing still.
Relative to their point of origin.
I can't even fathom something like this. There's so much energy involved. Can you imagine how bright matter around the black hole is? And there's people who believe reaching relativistic speeds will be possible soon...
Black holes aren’t luminous except when matter is falling into the event horizon. So unless one of these was tearing through a nebula we probably wouldn’t see it.