7 ms·

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

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?

andrewflnr 2y ago
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.
antognini 2y ago
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 My perception of the field (which is now about a decade out of date, so take it with a grain of salt), is that there is quite a bit of skepticism about invoking exotic physics to solve the last parsec problem.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.
- davedx 2y ago
  Considering the three body problem and extrapolating I would expect star orbits in a galaxy to be completely chaotic on long time scales?
  - deleted 2y ago
    [deleted]
  - neutronicus 2y ago
    A galaxy is a pretty degenerate instance of the n-body problem, though. A system of one or two giant bodies orbited by a collection of tiny ones. Each tiny one spends most of its time in a very weakly-perturbed two-body problem, quasi-stably orbiting the giant one(s) with tiny deviations caused by all the other ones. So you have to do some statistics to see how often an assumed distribution of quasi-stable orbits results in the tiny bodies approaching each other closely enough to kick one another into meaningfully different orbits of the giant ones. This gives you a better idea of how long is "long" and how it compares to the age of the universe
  - privong 2y ago
    > Considering the three body problem and extrapolating I would expect star orbits in a galaxy to be completely chaotic on long time scales? That's more or less captured by the loss cone calculation. On a very "long time scale" the loss cone will probably be occasionally replenished by stars coming in from scattering on larger scales, but the timescale for this to become significant for galaxies is substantially longer than the age of galaxies (and also longer than the Hubble time, which is the ~age of the Universe). So, at least from this back of the envelope calculation, that doesn't solve the final parsec problem. See, e.g., https://www.astro.umd.edu/~richard/ASTRO620/Dynamics_Lec3.pdf https://www.astro.umd.edu/~richard/ASTRO620/Dynamics_Lec3.pd...
    - deleted 2y ago
      [deleted]
rob74 2y ago
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?
- red1reaper 2y ago
  I mean... isn't that the point of dark matter and dark energy? To be a stand-in name for stuff we have to account for but can't detect properly? The dark here means dark as in "we are in the dark about that". It does not really matter if those exist or are an artifact of current theories, replacing relativity is not that easy and there are already a ton of physicists working on that anyway. In the meanwhile of either having instruments capable of detecting these or a new theory that demostrates that those are artifact emerge... they have to exist. That's the point of them.
  - at_a_remove 2y ago
    Dark matter does not mean "we are in the dark about this," at all. It's astronomy. It means "anything that is not shining like a star but we can detect it via inference." Light matter is stars. Dark matter can include MACHOS: neutron stars, black holes, brown dwarfs, rogue planets. No new physics required for MACHOs, as opposed to WIMPs.
- dguest 2y ago
  More positive spin: every "kludge" you add puts more constraints on what dark matter could be. If we can verify that the angular momentum transfer under this theory is consistent with other dark matter models, we can build a better framework to test a whole suite of theories with one experiment.
- ykonstant 2y ago
  If it exists and is ubiquitous then it makes sense to contribute to all sorts of disparate phenomena; are we surprised that "mass" pops up in all sorts of places? Besides, epistemologically it is very good to suspect and investigate the effects of DM on all kinds of phenomena. Then, if it does not exist, we will have many more points of reference from which to derive experimental or theoretical contradictions.
elashri 2y ago
> 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_matter https://en.wikipedia.org/wiki/Direct_detection_of_dark_matte... [2] https://en.wikipedia.org/wiki/Indirect_detection_of_dark_matter https://en.wikipedia.org/wiki/Indirect_detection_of_dark_mat...
- empath75 2y ago
  One way to think of non-detection is as evidence that we're looking in the wrong place. Another way to think of non-detection is as evidence that it doesn't exist.
  - mr_mitm 2y ago
    > One way to think of non-detection is as evidence that we're looking in the wrong place We aren't looking in only one place > Another way to think of non-detection is as evidence that it doesn't exist. Absence of evidence is not evidence of absence. Exhausting the parameter space until evidence is found is simply the normal scientific progress.
    - zmgsabst 2y ago
      How many places do I need to look before I start to have evidence my lost wallet isn’t in my house? The normal spot on the shelf? …checking my pockets? …searching the bags I used to go shopping? At each step, another of my theories about where my wallet is gets disproven — but none are direct evidence it’s not in my house. We are playing the same game with dark matter: they keep checking spots and it keeps not being there. At what point does checking wrong theories start to suggest that the entire idea is flawed?
      - mr_mitm 2y ago
        When you exhausted the parameter space (assuming you can with our current technical abilities) or a competing idea with actual evidence comes along. It's not like people aren't trying.
biorach 2y ago
> 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.
- pantulis 2y ago
  > 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.
- horsawlarway 2y ago
  I don't really find it unnecessarily negative. I find dark matter seems to fit into the same pattern as epicycles. We can add additional complexity to the theory to make our models better match observational data, and that's useful but also strongly hints that something more basic about the model is fundamentally incorrect. That's depressing.
  - mr_mitm 2y ago
    Which additional complexity has been added to dark matter? I'm not aware of anything. A WIMP would still explain everything, no? I don't see the comparison to epicycles at all. Then again, almost no one with a formal education in cosmology or partical physics at the PhD level does. Curious how that works.
    - horsawlarway 2y ago
      See - now this... this I find needlessly negative. > Then again, almost no one with a formal education in cosmology or partical physics at the PhD level does. This is also just wrong. I think we're seeing a number of established voices starting to question the underpinnings of dark matter as a theory. As for >Which additional complexity has been added to dark matter? Let's start with the fucking article we're discussing, where we now need to have "self-interacting" dark matter, as a new spin on the theory to account for the forces needed to explain what we're observing...
misja111 2y ago
The article did mention some other possibilities: stars swung out of the surrounding galaxy into the orbiting black holes, our friction due to surrounding gas disks. Both of these events would take out angular energy as well but it's not sure if it would be enough though.
bradley13 2y ago
I don't think you're a cynic at all. I object to the entire "dark matter" hypothesis, because there is zero evidence for it. What we have, are observations that indicate errors in our models. We need to look for those errors, not invent ever more abstruse version of "ether" to explain them away.
marcosdumay 2y ago
When you get a more complex blank to fill, the normal thing to do is to improve all of your blank-filling models to adapt for it. There's nothing wrong with that part. I do have a problem with the single-minded insistence on disqualifying any exploratory study on alternatives to a model that gets more and more partially falsified all the time¹. To the point that only iffy personalities that don't care for their careers decide to work on them. But on a situation like this, those other models would probably be tweaked too. 1 - I've never noticed it before, but that phrase gives me great "Superstring Theory" vibes.
InDubioProRubio 2y ago
It is interesting though.. if you think about the stage of physics before relativity. The made up compensations accumulating, but these made up formulas and compensation strategies had a signature- a sort of kinematic model connection, from which one could have guessed with a unconventional thinker at the encompassing more correct model. Like a formula, whos convex hull you can trace by swinging it wildly.

andrewflnr 2y ago

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.

antognini 2y ago

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.

davedx 2y ago
Considering the three body problem and extrapolating I would expect star orbits in a galaxy to be completely chaotic on long time scales?
- deleted 2y ago
  [deleted]
- neutronicus 2y ago
  A galaxy is a pretty degenerate instance of the n-body problem, though. A system of one or two giant bodies orbited by a collection of tiny ones. Each tiny one spends most of its time in a very weakly-perturbed two-body problem, quasi-stably orbiting the giant one(s) with tiny deviations caused by all the other ones. So you have to do some statistics to see how often an assumed distribution of quasi-stable orbits results in the tiny bodies approaching each other closely enough to kick one another into meaningfully different orbits of the giant ones. This gives you a better idea of how long is "long" and how it compares to the age of the universe
- privong 2y ago
  > Considering the three body problem and extrapolating I would expect star orbits in a galaxy to be completely chaotic on long time scales? That's more or less captured by the loss cone calculation. On a very "long time scale" the loss cone will probably be occasionally replenished by stars coming in from scattering on larger scales, but the timescale for this to become significant for galaxies is substantially longer than the age of galaxies (and also longer than the Hubble time, which is the ~age of the Universe). So, at least from this back of the envelope calculation, that doesn't solve the final parsec problem. See, e.g., https://www.astro.umd.edu/~richard/ASTRO620/Dynamics_Lec3.pdf https://www.astro.umd.edu/~richard/ASTRO620/Dynamics_Lec3.pd...
  - deleted 2y ago
    [deleted]

rob74 2y ago

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?

red1reaper 2y ago
I mean... isn't that the point of dark matter and dark energy? To be a stand-in name for stuff we have to account for but can't detect properly? The dark here means dark as in "we are in the dark about that". It does not really matter if those exist or are an artifact of current theories, replacing relativity is not that easy and there are already a ton of physicists working on that anyway. In the meanwhile of either having instruments capable of detecting these or a new theory that demostrates that those are artifact emerge... they have to exist. That's the point of them.
- at_a_remove 2y ago
  Dark matter does not mean "we are in the dark about this," at all. It's astronomy. It means "anything that is not shining like a star but we can detect it via inference." Light matter is stars. Dark matter can include MACHOS: neutron stars, black holes, brown dwarfs, rogue planets. No new physics required for MACHOs, as opposed to WIMPs.
dguest 2y ago
More positive spin: every "kludge" you add puts more constraints on what dark matter could be. If we can verify that the angular momentum transfer under this theory is consistent with other dark matter models, we can build a better framework to test a whole suite of theories with one experiment.
ykonstant 2y ago
If it exists and is ubiquitous then it makes sense to contribute to all sorts of disparate phenomena; are we surprised that "mass" pops up in all sorts of places? Besides, epistemologically it is very good to suspect and investigate the effects of DM on all kinds of phenomena. Then, if it does not exist, we will have many more points of reference from which to derive experimental or theoretical contradictions.

elashri 2y ago

> 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_matter https://en.wikipedia.org/wiki/Direct_detection_of_dark_matte... [2] https://en.wikipedia.org/wiki/Indirect_detection_of_dark_matter https://en.wikipedia.org/wiki/Indirect_detection_of_dark_mat...

empath75 2y ago
One way to think of non-detection is as evidence that we're looking in the wrong place. Another way to think of non-detection is as evidence that it doesn't exist.
- mr_mitm 2y ago
  > One way to think of non-detection is as evidence that we're looking in the wrong place We aren't looking in only one place > Another way to think of non-detection is as evidence that it doesn't exist. Absence of evidence is not evidence of absence. Exhausting the parameter space until evidence is found is simply the normal scientific progress.
  - zmgsabst 2y ago
    How many places do I need to look before I start to have evidence my lost wallet isn’t in my house? The normal spot on the shelf? …checking my pockets? …searching the bags I used to go shopping? At each step, another of my theories about where my wallet is gets disproven — but none are direct evidence it’s not in my house. We are playing the same game with dark matter: they keep checking spots and it keeps not being there. At what point does checking wrong theories start to suggest that the entire idea is flawed?
    - mr_mitm 2y ago
      When you exhausted the parameter space (assuming you can with our current technical abilities) or a competing idea with actual evidence comes along. It's not like people aren't trying.

biorach 2y ago

> 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.

pantulis 2y ago
> 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.
horsawlarway 2y ago
I don't really find it unnecessarily negative. I find dark matter seems to fit into the same pattern as epicycles. We can add additional complexity to the theory to make our models better match observational data, and that's useful but also strongly hints that something more basic about the model is fundamentally incorrect. That's depressing.
- mr_mitm 2y ago
  Which additional complexity has been added to dark matter? I'm not aware of anything. A WIMP would still explain everything, no? I don't see the comparison to epicycles at all. Then again, almost no one with a formal education in cosmology or partical physics at the PhD level does. Curious how that works.
  - horsawlarway 2y ago
    See - now this... this I find needlessly negative. > Then again, almost no one with a formal education in cosmology or partical physics at the PhD level does. This is also just wrong. I think we're seeing a number of established voices starting to question the underpinnings of dark matter as a theory. As for >Which additional complexity has been added to dark matter? Let's start with the fucking article we're discussing, where we now need to have "self-interacting" dark matter, as a new spin on the theory to account for the forces needed to explain what we're observing...

misja111 2y ago

The article did mention some other possibilities: stars swung out of the surrounding galaxy into the orbiting black holes, our friction due to surrounding gas disks. Both of these events would take out angular energy as well but it's not sure if it would be enough though.

bradley13 2y ago

I don't think you're a cynic at all. I object to the entire "dark matter" hypothesis, because there is zero evidence for it. What we have, are observations that indicate errors in our models. We need to look for those errors, not invent ever more abstruse version of "ether" to explain them away.

marcosdumay 2y ago

When you get a more complex blank to fill, the normal thing to do is to improve all of your blank-filling models to adapt for it. There's nothing wrong with that part. I do have a problem with the single-minded insistence on disqualifying any exploratory study on alternatives to a model that gets more and more partially falsified all the time¹. To the point that only iffy personalities that don't care for their careers decide to work on them. But on a situation like this, those other models would probably be tweaked too. 1 - I've never noticed it before, but that phrase gives me great "Superstring Theory" vibes.

InDubioProRubio 2y ago

It is interesting though.. if you think about the stage of physics before relativity. The made up compensations accumulating, but these made up formulas and compensation strategies had a signature- a sort of kinematic model connection, from which one could have guessed with a unconventional thinker at the encompassing more correct model. Like a formula, whos convex hull you can trace by swinging it wildly.