Now, maybe I’m just a jaded cynic with a stale physics masters degree, but isn’t there something depressing about this? Like, faced with an interesting anomaly out there in the world, we have to resort to tweaking the model of dark matter that is already an invention to fit observational anomalies. We’ve no way to detect dark matter directly, and so no way to prove or disprove this hypothesis, so is this really progress in physics at all? I mean, I know, I know, modified theories of gravity have a lot of problems, but what are the other possibilities here? Any current physicists care to weigh in?
I actually spent quite a while in grad school thinking about the last parsec problem, and although I'm not in the field anymore I still think about it from time to time. (My thesis was on gravitational dynamics.)
My perception of the field (which is now about a decade out of date, so take it with a grain of salt), is that there is quite a bit of skepticism about invoking exotic physics to solve the last parsec problem. Galaxies are generally pretty messy places, and the centers of galaxies are especially messy, so it's hard to know if you've correctly modeled all the relevant physics. A lot of astronomers aren't convinced that there really is a last parsec problem.
The main "standard" approach to solve the last parsec problem is from scattering stars (which the article mentions). Basically, every now and then stars from the galaxy wander close to the orbit of the black hole binary and then get slingshotted out of the system. This removes energy from the orbit, and causes the black hole binary to shrink. The problem with this approach if you do a naive calculation is that the stars have to come from a particular set of directions, called the "loss cone" in the jargon. And since the orbits of stars in galaxies are probably fairly static, once a star gets kicked out of the loss cone, it doesn't come back. So over time the loss cone empties and the black hole orbit stops shrinking. The question is, does the orbit shrink far enough before the loss cone empties, and the answer to this question has generally been "no."
The way around this is to question how static the orbits of stars in galaxies really are. One of the more important papers on the topic found that if an elliptical galaxy is sufficiently triaxial (that is, sufficiently non-spherical), then interactions between stars in the galaxy can repopulate the loss cone and cause the orbit to keep shrinking. But as I vaguely recall, not everyone was convinced by that result.
I personally have had some ideas that galactic tides might contribute, especially right after the merger before all the orbits have had time to thermally relax. But I'm not in the field anymore and haven't really had time to really model this idea and see if it would work.
> but isn’t there something depressing about this? Like, faced with an interesting anomaly out there in the world, we have to resort to tweaking the model of dark matter that is already an invention to fit observational anomalies
This is unnecessarily negative. Physics progresses in fits and starts, with plenty of blind alleys and red herrings. Maybe we're in a phase akin to the aftermath of the Michelson–Morley experiment. Or maybe it really is that complicated out there.
> We’ve no way to detect dark matter directly, and so no way to prove or disprove this hypothesis
We can and we are already searching for dark matter directly [1] and indirectly [2]. The phase space is being closed every now and then and this is progress. This gives us information about where to look next.
They did mention how they're hoping to find evidence from the pulsar timing array. It's not exactly easy to find the evidence, and it won't be tomorrow, but the idea at least seems falsifiable.
I don't have a physics masters degree, or anything comparable, but when I read "Astrophysicists have a new suggestion: Dark matter could sap angular momentum from the two black holes and nudge them closer" I also thought "Really? Is dark matter now the stand-in explanation for anything unexplainable?"
But who knows, when one day someone will come up with a better model that does away with the "dark matter kludge", it will turn out that these phenomena actually have a common root cause?
OT but, is there any good reason why "parsecs" are used as distance units rather than "light-[time units]" ? The latter enjoys broad familiarity in the general community.
The potential to detect Supermassive Black Hole mergers is one of the reasons I'm really excited about the LISA project [1], and hope it actually gets funded and doesn't delay too much.
Is there an equivalent of Tidal Heating [1] taking place between the two black holes? It would extract kinetic energy and put it into.. heating the black holes.. whatever that may mean. Assuming there is movement and friction in the core of a black hole.
Gravitational waves have a similar effect of sapping the kinetic energy of orbiting black holes. Instead of heating the black holes, the energy is emitted as gravitational radiation. Having said that, now that we have actual observations of the gravitational radiation emmited from merging black holes, we can be pretty confident in our understanding of the magnitude of this energy loss. And that understanding is that it does not become significant until well into the last parsec.
A black hole only has mass, angular momentum, and charge. So you can only deal with those three... not deform and knead a physical mass of planet as in tidal heating.
This is often stated but it literally can't be true. It's true for stationary solutions to the relevant equations, but we're very much not living in a stationary world.
Here's a simple thought experiment disproving your claim. A person hovers just above the origin of a supermassive black hole. They chuck a massively charged object into the black hole. If what you said is true they should observe the charge instantly being transported to the singularity, since a black hole can't have any attributes such as where charge is distributed within the horizon.
Where it gets impossible is that someone very far away around the same supermassive black hole could observe a small charge increment. They in turn could chuck charged stuff into the black hole and now you've got faster than lightspeed communication.
I spent couple of minutes trying to understand your thought experiment and was puzzled why can't I understand it. It seems that it is probably because I don't understand what you mean by "just above the origin of supermassive black hole"?
I feel that you have something interesting to say but not clear.
You can’t actually observe objects going into a black hole. They just become redshifted and fall slower and slower from your perspective towards the event horizon.
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.
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.
Now, maybe I’m just a jaded cynic with a stale physics masters degree, but isn’t there something depressing about this? Like, faced with an interesting anomaly out there in the world, we have to resort to tweaking the model of dark matter that is already an invention to fit observational anomalies. We’ve no way to detect dark matter directly, and so no way to prove or disprove this hypothesis, so is this really progress in physics at all? I mean, I know, I know, modified theories of gravity have a lot of problems, but what are the other possibilities here? Any current physicists care to weigh in?
I actually spent quite a while in grad school thinking about the last parsec problem, and although I'm not in the field anymore I still think about it from time to time. (My thesis was on gravitational dynamics.)
My perception of the field (which is now about a decade out of date, so take it with a grain of salt), is that there is quite a bit of skepticism about invoking exotic physics to solve the last parsec problem. Galaxies are generally pretty messy places, and the centers of galaxies are especially messy, so it's hard to know if you've correctly modeled all the relevant physics. A lot of astronomers aren't convinced that there really is a last parsec problem.
The main "standard" approach to solve the last parsec problem is from scattering stars (which the article mentions). Basically, every now and then stars from the galaxy wander close to the orbit of the black hole binary and then get slingshotted out of the system. This removes energy from the orbit, and causes the black hole binary to shrink. The problem with this approach if you do a naive calculation is that the stars have to come from a particular set of directions, called the "loss cone" in the jargon. And since the orbits of stars in galaxies are probably fairly static, once a star gets kicked out of the loss cone, it doesn't come back. So over time the loss cone empties and the black hole orbit stops shrinking. The question is, does the orbit shrink far enough before the loss cone empties, and the answer to this question has generally been "no."
The way around this is to question how static the orbits of stars in galaxies really are. One of the more important papers on the topic found that if an elliptical galaxy is sufficiently triaxial (that is, sufficiently non-spherical), then interactions between stars in the galaxy can repopulate the loss cone and cause the orbit to keep shrinking. But as I vaguely recall, not everyone was convinced by that result.
I personally have had some ideas that galactic tides might contribute, especially right after the merger before all the orbits have had time to thermally relax. But I'm not in the field anymore and haven't really had time to really model this idea and see if it would work.
> but isn’t there something depressing about this? Like, faced with an interesting anomaly out there in the world, we have to resort to tweaking the model of dark matter that is already an invention to fit observational anomalies
This is unnecessarily negative. Physics progresses in fits and starts, with plenty of blind alleys and red herrings. Maybe we're in a phase akin to the aftermath of the Michelson–Morley experiment. Or maybe it really is that complicated out there.
> Maybe we're in a phase akin to the aftermath of the Michelson–Morley experiment.
Loving this comparison, and I hope this is the case.
> We’ve no way to detect dark matter directly, and so no way to prove or disprove this hypothesis
We can and we are already searching for dark matter directly [1] and indirectly [2]. The phase space is being closed every now and then and this is progress. This gives us information about where to look next.
[1] https://en.wikipedia.org/wiki/Direct_detection_of_dark_matte...
[2] https://en.wikipedia.org/wiki/Indirect_detection_of_dark_mat...
They did mention how they're hoping to find evidence from the pulsar timing array. It's not exactly easy to find the evidence, and it won't be tomorrow, but the idea at least seems falsifiable.
I don't have a physics masters degree, or anything comparable, but when I read "Astrophysicists have a new suggestion: Dark matter could sap angular momentum from the two black holes and nudge them closer" I also thought "Really? Is dark matter now the stand-in explanation for anything unexplainable?"
But who knows, when one day someone will come up with a better model that does away with the "dark matter kludge", it will turn out that these phenomena actually have a common root cause?
link to papers:
- https://arxiv.org/abs/2401.14450
- https://arxiv.org/abs/2311.03412
- https://arxiv.org/abs/2311.18228v1
OT but, is there any good reason why "parsecs" are used as distance units rather than "light-[time units]" ? The latter enjoys broad familiarity in the general community.
The potential to detect Supermassive Black Hole mergers is one of the reasons I'm really excited about the LISA project [1], and hope it actually gets funded and doesn't delay too much.
[1] https://en.wikipedia.org/wiki/Laser_Interferometer_Space_Ant...
Is there an equivalent of Tidal Heating [1] taking place between the two black holes? It would extract kinetic energy and put it into.. heating the black holes.. whatever that may mean. Assuming there is movement and friction in the core of a black hole.
[1] https://en.wikipedia.org/wiki/Tidal_heating
Gravitational waves have a similar effect of sapping the kinetic energy of orbiting black holes. Instead of heating the black holes, the energy is emitted as gravitational radiation. Having said that, now that we have actual observations of the gravitational radiation emmited from merging black holes, we can be pretty confident in our understanding of the magnitude of this energy loss. And that understanding is that it does not become significant until well into the last parsec.
A black hole only has mass, angular momentum, and charge. So you can only deal with those three... not deform and knead a physical mass of planet as in tidal heating.
This is often stated but it literally can't be true. It's true for stationary solutions to the relevant equations, but we're very much not living in a stationary world.
Here's a simple thought experiment disproving your claim. A person hovers just above the origin of a supermassive black hole. They chuck a massively charged object into the black hole. If what you said is true they should observe the charge instantly being transported to the singularity, since a black hole can't have any attributes such as where charge is distributed within the horizon.
Where it gets impossible is that someone very far away around the same supermassive black hole could observe a small charge increment. They in turn could chuck charged stuff into the black hole and now you've got faster than lightspeed communication.
> above the origin of a supermassive black hole
I spent couple of minutes trying to understand your thought experiment and was puzzled why can't I understand it. It seems that it is probably because I don't understand what you mean by "just above the origin of supermassive black hole"?
I feel that you have something interesting to say but not clear.
You can’t actually observe objects going into a black hole. They just become redshifted and fall slower and slower from your perspective towards the event horizon.
> In some scenarios, the lightest of the three holes is ejected
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?
In Neal Stephenson’s Seveneves (read immediately) the moon explodes for an unknown reason.
I always imagined it as being caused by a rogue mini black hole zipping through.
> That's terrifying. Imagine a rogue supermassive black hole floating in intergalactic space.
Why terrifying? It's literally doing nothing, far away from anything. Seems like the safest place for it to be.
I am reminded of a quote from Mass Effect 2. Eventually that black hole could hit something.
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.)
> and you won't get much warning either
A supermassive blackhole floating towards you would have very visible effects, it would be impossible to miss for millenias before it gets to you.
It's the micro black holes which can hit you without a warning.
The article state that small black holes have the size of stars. Micro black holes should be quite big too I guess?
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.
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...)
If you were lucky enough to see the ejection happen, yes. But if one was already on the way?
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.