Hundreds of Black Holes Roam Loose In Milky Way

sciencehabit writes "From Science: 'Astronomers suspect that hundreds of medium-sized black holes are roaming loose in the Milky Way. These rogues, according to a new study, are the orphaned central black holes of the many smaller galaxies that the Milky Way has swallowed over its billions of years of existence.'"

Comments

Gamma ray bursts and high energy cosmic rays

[MichaelSmith]

I wonder if black holes could account for either of these things? Gamma rays would be released if a large mass hits a black hole. A cosmic ray could be accelerated if it passes too close to a black hole.

Nah, I call BS

[DavidRawling]

Scenario. The Milky Way swallows a galaxy, and by extension, all the stars around the central black hole. Yet, the same gravity that causes the stars to amalgamate completely misses the biggest mass in that swallowed galaxy? Why would that make sense?

Re:Nah, I call BS

[MichaelSmith]

Scenario. The Milky Way swallows a galaxy, and by extension, all the stars around the central black hole. Yet, the same gravity that causes the stars to amalgamate completely misses the biggest mass in that swallowed galaxy? Why would that make sense?

The only bit which I think is strange is that the black hole from the swallowed galaxy hangs around in our galaxy. It should have enough velocity to pass right through our galaxy and never come back. Most likely the captured stars would die of old age before they passed though our galaxy. Only red dwarfs would keep going because of their long life. Gas clouds in the captured galaxy would interact with our gas clouds. I think that is the only component which would really get captured.

Re:Nah, I call BS

[Brown]

The Milky Way swallows a galaxy, and the swallowed galaxy's stars get added to the milky way, orbiting the galactic centre in the usual way. Presumably the same happens to the black hole - there's no reason why it should be sucked into the middle. Black holes will happily orbit around each other, as long as they're outside each other's event horizons.

Re:Nah, I call BS

[MichaelSmith]

Why would the stars and black hole change their trajectory significantly?

Gravity?

Gravity can change the direction of travel of a black hole or star. It can't significantly change momentum unless the object passes very close to a large mass. Our space probes do that at Jupiter, etc, but that requires guidance or an extreme amount of luck. To be captured by gravitational slingshot a black hole would have to pass very close to our own central black hole. Thats not very likely.

Re:Nah, I call BS

[AlecC]

Because the galaxy is not a point mass. Most ordinary star/planet modelling is based on viewing each object as a sphere, which behaves as a point mass at the centre. But when you penetrate inside another body, as two galaxies do when they collide, this simplification no longer applies. Some of the mass of the "other" galaxy moves behind the penetrating galaxy, slowing it down rather than, as the point mass model would suggest, continuing to accelerate into the centre. In the simplest model, of inter-penetrating spheres, gravity no longer has an inverse square law but an inverse linear law. Of course, galaxies are not uniform spheres, and the modelling is much harder. However, it is widely accepted that when two galaxies collide, they merge and the vast majority of the mass forms a single galaxy - though clusters may be flung out. If the galaxies are of broadly similar masses, the distinctive spiral structure is wiped out and the merged result becomes an elliptical galaxy for a few hundred million years before the spiral structure re-establishes.

Google "andromeda collisions" for simulations of the collision between our galaxy and the Andromeda galaxy in about 3 billion years.

Re:Nah, I call BS

[stonewallred]

What happens when two black holes actually intersect at their event horizons? Inquiring non-astrophysicists would like to know.

Re:Nah, I call BS

[beanyk]

Well for one thing, the "time moving slowly" thing is an observer-dependent effect. If you were the one falling into the hole, you wouldn't notice any real time lag at all [depending on the size of the hole -- and your personal oxygen supply, etc -- you might even survive crossing the horizon].

But to a distant observer, your progress would look more and more gradual. Signals leaving you would also get more and more red-shifted, and eventually pass out of the visible spectrum. So a distant observer would never see you cross the horizon.

Re:Nah, I call BS

[Anonymous Coward]

>So a distant observer would never see you cross
>the horizon.

Right...it seems to me that from our frame of reference here on Earth, we should never be able to observe a black hole that has merged with another black hole at any point in the past. Two black holes approaching each other would seem to move ever more slowly, and never manage to merge.

Re:Nah, I call BS

[Anonymous Coward]

I don't think so. This would be one of those weird "objects can't move faster than c, but these two objects are moving apart at 1.5 c, but that's ok, it's just space expanding" type of things. Think about it this way, as the two black holes approach, their gravitational effects are already combining from the point of view of objects being drawn to them. So, their event horizons change shape... Actually, the more I think about this, the weirder it gets. For an object on the opposite side of one of the black holes from the other black hole, the gravity would increase, so the event horizon would extend outwards... On the other hand, between the two black holes, the gravity would cancel out, there would be a lagrange point between them, and the event horizons would recede. I'm having trouble conceptualizing what that means for an object falling into the even horizon at a point where the horizon recedes. Can it actually escape the event horizon at that point (if it can, it's a pity we'd probably never be able to observe it, it would, after all, be escaping into an inferno of unimaginable energies), or does it simply follow the even horizon instantaneously as it recedes? Presumably, the event horizon between the black holes would be stripped back all the way to... to what? Can we imagine a surface of the black hole? Does it have one, or is it all time dilated unknowable singularity in there? Anyway, as the black holes get close enough, the event horizons around them, twisted into bizarre geometries though they may be, would enlarge and merge into one larger event horizon, so that bit in the middle where the event horizon recedes would be surrounded and cut off from the rest of the universe.
That got me thinking about gravity cancellation and lagrange points. It's generally accepted that forces cancel out. If Bob and Joe are pulling on Tim's arms in opposite directions with the same amount of force, Tim doesn't go anywhere. If Bob and Joe are elephants, Tim becomes TimA and TimB, two probably unequally divided parts of Tim, now moving apart. This doesn't happen with gravitational forces because they're pulling evenly on every particle of Tim. The particles themselves don't pull apart, maybe because the nuclear forces are so strong at close range, or is it because the granularity of gravity acts at the level of the particles, so it doesn't pull unevenly on any part. So what happens when the gravity pulling from both sides is intensely powerful? What level does gravity act on. Do atoms hold together? Yes, no, what about individual baryons? It sounds like it would be a very interesting wall to be a fly on.

Re:First swine flu, now loose-roaming black holes?

[Nephrite]

Looks like everyone has already forgot the LHC...

What About Black Holes from Common Massive Stars?

[Fleetie]

What about black holes NOT from the centre of galaxies? Fairly normal large (massive) stars end up as black holes too, so I'd expect a lot of black holes in this galaxy anyway, even if it hadn't interacted with any other galaxies.

Re:First swine flu, now loose-roaming black holes?

[Opportunist]

At least you left out all the conspiracy crap, like alien invasions, mind control probes and terrorism.

Re:First swine flu, now loose-roaming black holes?

[cpartrid]

I'm curious as to what exactly Partially Infinite means?
Just another way of saying: Not infite?
Or its not quite inifite yet, but it will be?
Or does it mean that some of it is infinte but other bits aren't?

Black holes can certainly roam. They are no different from other interstellar bodies in that respect.

Re:First swine flu, now loose-roaming black holes?

[jc42]

... add up ALL the various possible dangers, and the odds of it happening in a given year...odds of getting hit by lighting, 1:22 million. Odds of being in a car accident 1:50,000, etc, etc...I suspect that once you compiled a comprehensive list, you'd end up being nearly certain you'll die in the next year.

A few years back, I read an interesting article whose authors pretty much did just that, and wrote about the conclusions. One of their more interesting ways of expressing the results of the study was: Suppose that medical science found a way to eliminate aging, so that we all stayed permanently in the state of health of a person in their 20s, and only died of things at the rate that 25-year-olds died of them. How much would this increase our expected lifespan? The answer turned out to be about 15 years.

It put an interesting perspective on a lot of the hype around various medical scare stories. Yes, we do get older and feebler, and it'd be nice if we could fix that. But such medical advances by themselves wouldn't actually increase our lifespan by much. To do that, we have to fix the zillions of (mostly unlikely) things that would keep killing us off at a low rate.

I don't think black holes were on their list of dangers, though. And what we know of the history our planet tells us that asteroid impacts are a more significant danger.