i heard about the phenomenon of rogue black holes which can get ejected when two galaxies collide in certain circumstances

these black holes might carry no stars or planets with them and would therefore be invisible, i guess

which gave me a frightening thought

if one were hurtling through the abyss directly towards us, possibly about to enter our galaxy  near where earth is located

would we know in advance?

Edited October 23, 20205 yr by boo

Possibly. If it passed in front of light sources we could notice that they had been blocked. We might be able to gain distance and velocity information from that blockage and from gravitational lensing. There would also be radiation given off from the accretion disk, as it gobbled up matter.

But if it was "about to enter our galaxy" we would have many years to notice it. We're more than 10,000 LY from the edge radially, and several hundred LY from the top or bottom of our part of the disk.

 

Stellar size BHs ( from collapsed former stars ) tend to follow orbits around the galaxy, just like stars do.
We have an equivalent chance of colliding with another star as we do with a stellar BH.

Galactic core sized ( millions or even billion solar mass ) BHs are another matter.
Although just about every galaxy has one ( or more ), to be ejected and become 'rogue', they need to be involved in a collision.
Just how many galactic collisions are there ( or were ) within 100 million light years ?
Not very many at all.
Chance of one reaching our galaxy within a billion years, virtually nil.

Although in about 4.5 billion years Andromeda will collide with the Milky Way, chances of stellar or BH collisions are very low.
( interstellar pace is HUUUUGE compared to the size of stars )

 

If binary star systems can be torn apart by supermassive black holes at the center of galaxies, a binary of a star and black hole could result in a hypervelocity black hole.  That would be hard to detect if it was heading at us from the worst direction.