I am reminded of a quote from Mass Effect 2. Eventually that black hole could hit something.
Damn straight! I dare to assume you ignorant jackasses know that space is empty. Once you fire this hunk of metal, it keeps going till it hits something. That can be a ship, or the planet behind that ship. It might go off into deep space and hit somebody else in ten thousand years. If you pull the trigger on this, you are ruining someone's day, somewhere and sometime. That is why you check your damn targets! That is why you wait for the computer to give you a damn firing solution! That is why, Serviceman Chung, we do not "eyeball it!" This is a weapon of mass destruction. You are not a cowboy shooting from the hip!
Not doing anything yet. If galaxies can collide, the rogue black hole can collide with your galaxy, and you won't get much warning either. (I mean, realistically you're right, in the same way that our galaxy colliding with Andromeda is scary but has negligible chance of affecting us. But, imagine.)
That's likely a mistake in the article. "Small" black holes (i.e. smaller than supermassive) have star/stellar mass, but not size.
Micro black holes are only hypothesized so far, but they could get very small - e.g. a black hole with Earth mass would have less than 1 centimeter in diameter.
The size itself is not that important for spotting black holes, though. Even if it's as large as a star, all you see staring at the black hole "object" is nothing. What's important are the gravitational effects on the environment, and there the differences are stark. At a distance of 1000 light years, it will be difficult to spot a stellar-mass black hole floating through empty space, because its pull is strong enough only at stellar distances and won't produce enough disturbance in interstellar space for us to notice. OTOH supermassive blackholes will deform whole surrounding star systems because of its immense mass and gravitational pull. A micro black hole (e.g. Earth mass) passing through the solar systems would likely go undetected unless it collides with something (which is improbable). There could be a measurable disturbance, but it would be one-off and difficult to attribute to a black hole.
> A micro black hole (e.g. Earth mass) passing through the solar systems would likely go undetected unless it collides with something (which is improbable).
Solar wind? Would it generate some interesting effects when coming too close to a black hole? All these protons accelerated to a near light speed, probably hitting each other and running away into a black hole.
An Earth-mass black hole will have a similar gravitational effect on solar winds as Earth. Can we detect these effects on solar winds from distance? Maybe if we watch for them in that particular direction of empty space, but will we? The effects would be localized and short-lived at each particular place on the trajectory.
There are likely several > earth-mass objects in our solar system that are as-yet uncatalogued, and almost certainly many hundreds of dwarf planet-mass objects too. Granted, they are further out, but their influence is almost undetectable even over millions of years. It's pretty unlikely that the presence of an earth-mass black hole for just a few years (since it would likely be moving many times solar escape trajectory) would have much in the way of a measurable impact.
Galaxies are big, including our own. Unless the rogue black hole is traveling near light speed, you will get at least tens of thousands of years of advance warning. (What you could do with that warning is a different matter, though...)
There are lots of easily visible stars on the other side of any supermassive black hole that's nearer to you than other galaxies are. When those stars start lensing in ways visible to the naked eye, you're going to know something weird is going on.
You would absolutely notice the gravitational distortions a very long time in advance. That's a lot of mass. You can't miss the way it's distorting space for a very long way around it.
That's terrifying. Imagine a rogue supermassive black hole floating in intergalactic space.
But I mostly want to know how badly self-interacting dark matter messes up the existing LCDM simulations that most astrophysicists sort of rely on?