9 ms·

Rotation curves: still flat after a million light-years

103 points by xqcgrek2 2y ago

deleted 2y ago
[deleted]
Maro 2y ago
Quick explanation: A galaxy (elliptic, or spiral) is made out of billions of stars, like our sun. These stars rotate around the center of the galaxy (very slowly, think millions of years for 1 rotation). A rotation curve measures the velocity of stars as a function of distance from the center of the galaxy. Newtonian physics (or Einstein's GR) says that the rotation curve should decay with distance, ie. with greater distance the stars' velocity should go down --- assuming the matter in the galaxy is the visible matter that we see, ie. the stars (which shine light). The problem is, there is a rich set of observational data, from many different experiments, telescopes, and methodologies that show that the rotational curve is in fact flat, it does not decay. There are 2 big competing theories to explain this discrepancy: 1. Assume that there is a lot of unseen, non-shining, ie. Dark Matter (DM) in the galaxies (also ours). If you put the appropriate amount of dark matter in there, with the right distribution, you can reproduce the observed rotational curve. There are also other places is astrophysics/cosmology where having dark matter (specifically Cold Dark Matter, CDM, where cold just means "slowly moving") is useful. The biggest example is to explain the history of the Universe and the observed Hubble-constant. In fact the standard model of cosmology is called λCDM, CDM for Cold Dark Matter (λ for the cosmological constant, currently modeled as Dark Energy, not relevant for this discussion). 2. Assume that Newton was wrong and gravity is not exactly 1/r^2 --- this is called MOND, Modified Newtonian Dynamics. This way you can also reproduce the observed rotation curves. This is much less popular, because: (i) physicsts don't want to give up the beautiful and geometric simplicity of 1/r^2 (ii) Dark Matter is also useful for solving other discrepancies in astrophysics/cosmology. What this article is saying is that, even in the first Dark Matter model, per the model DM distributions inside galaxies that also work with all the other places where DM is used to explain something (eg. in cosmology), at some distance from the center, the dark matter bubble has an edge and stops --- and then the velocities should finally break down. However, these latest observations are showing that the velocities remain constant even beyond the modeled/assumed DM bubbles. This is an additional ε argument in favor of MOND, and science proceeds.
- merek 2y ago
  Fantastic explanation, thank you. For a more detailed description of the Milky Way's rotation curve, this is a brief segment from David Butler's How Far Away Is It video series (which I highly recommend): https://youtu.be/uVxrsJ5lZlQ?si=ZwpBDpAvTV8AALke&t=1890 https://youtu.be/uVxrsJ5lZlQ?si=ZwpBDpAvTV8AALke&t=1890
- xeonmc 2y ago
  What falloff function will create an exactly flat curve? G ~ 1/r instead of 1/r^2 ? What if gravity has non-scalar components?
  - canjobear 2y ago
    https://en.m.wikipedia.org/wiki/Tensor%E2%80%93vector%E2%80%93scalar_gravity https://en.m.wikipedia.org/wiki/Tensor%E2%80%93vector%E2%80%... The extra components would have energy and function as Dark Matter.
- jprete 2y ago
  I'm not a physicist but 1/r^2 strikes me as conceptually very important, because it's the relative contribution of any fixed area of spherical surface to the total area of that surface. So the total strength of gravitational field emanating from a particular object, at a given distance from that object, is a constant. It's somewhat weird to think of the total gravitational "force field" _increasing_ in magnitude with distance. Decreasing, sure. Increasing? That makes no sense. Certainly not at a large enough function of distance for the rotational curve to be _flat_. That's got to be some kind of wonky power term over distance which implies potential energy from the field goes up with distance as well. As above, I'm not a physicist, but a linear rotational curve breaks every intuition I've ever gleaned from physics about the nature of what's really going on with relativity, particle mediation of forces, or even the concept of a field. Maybe it means spacetime curvature is way higher than we think.
  - throwawaymaths 2y ago
    > spacetime curvature is way higher than we think. You mean way weirder. Remember 1/r^2 does not work quite right for, e.g. mercury.
deleted 2y ago
[deleted]
burnte 2y ago
"MOND predictions keep being corroborated, yet the community persists in ignoring its implications, even in terms of dark matter. It’s gotta be telling us something." No, it doesn't. MOND falls apart in every single theory they put forth. The fact that this happens without fail should lead one to understand the answer probably lies elsewhere than MOND.
- kurthr 2y ago
  It's not like MOND (or even dark matter) are even single theories with a fixed set of predictions. They are groups of competing ever evolving theories. I don't understand the need to believe one over the other as a predictive tool without some conclusive evidence. If you're not the astronomer who makes that measurement (or explains it), it just feels like galactic sports betting.
- MattPalmer1086 2y ago
  MOND successfully predicts many things we observe that DM fails to, or that requires special tuning for DM to fit observations (which is not prediction). DM actually has quite a poor predictive record, even if it can be made to fit observation. MOND as a theory is probably not correct. It's not relativistic for a start, although I believe there are other modified gravity theories that are.
- throwawaymaths 2y ago
  MOND explains (discovered before Mond, but the Mond equations unexpectedly fit): Tully fisher relation Renzos rule MOND predicted (these were observed after mond predicted it): External field effect Keplerian descent in milky way Early galaxies after the big bang Consistently "No dark matter found" in elliptical and lenticular galaxies
- shiandow 2y ago
  I'm getting interested now, the comments have alleged several predictions MOND made. On the other hand I'm pretty sure the evidence for dark matter is pretty strong as well (I believe there are several ways to calculate the amount of dark matter, all of which agree). Can you give an example of MOND falling apart? One that requires so much fine-tuning that it cannot be adequately explained, or one requiring a violation of one of the more fundamental laws of physics?
xanzi2048 2y ago
Why does MOND keep showing up on HN? Pretty odd considering it's not particularly popular elsewhere and fails to explain a lot of observations of dark matter (bullet cluster, CMB). It's worth noting these rotation curves are not all the same curve, and we've discovered galaxies with varying quantities of implied dark matter (eg https://www.aanda.org/articles/aa/full_html/2023/07/aa46291-23/aa46291-23.html https://www.aanda.org/articles/aa/full_html/2023/07/aa46291-...) - so is MOND different for every galaxy?
- deleted 2y ago
  [deleted]
- MattPalmer1086 2y ago
  No. MOND is the same everywhere, with basically no free parameters.
  - xanzi2048 2y ago
    but... how is that compatible with the article I linked here? Some galaxies look like they have very little dark matter, a MOND with no free parameters can't explain why different galaxies have different inconsistencies in their rotation curves.
    - throwawaymaths 2y ago
      You actually have to do the math, it's not just a "no dark matter hurr durr" thing. MOND predicts "no dark matter" if the acceleration regime is high. Most galaxies that havd "very little dark matter" are in the high acceleration regime. Most of the others seem to be susceptible to observational error (e.g. ultra diffuses). Galaxy rotation curves measurements are highly sensitive to orientation of the rotational axis and distance to us (you're gonna have a hard time measuring it if the galaxy is face on)
    - MattPalmer1086 2y ago
      Correct. MOND cannot explain why some galaxies don't fit it's predicted rotation curve. However, the vast majority do, and the linked paper says that the ones they are describing are extremely rare.
thriftwy 2y ago
Are these rotation flats variable in magnitude? Would one galaxy have a different curve height than another one? What does it depend on? Update: https://en.m.wikipedia.org/wiki/Tully–Fisher_relation https://en.m.wikipedia.org/wiki/Tully–Fisher_relation But the linear regression looks unconvincing
mikewarot 2y ago
Nobody wants to give up 1/r^2, including me. However, what if it holds, but if inertia is quantized, then you get less gravitational effect at 90 degrees to the path of motion at astronomic distances as it recedes into the quantum noise. Which fits observations with inventing dark matter, or tweaking gravity.
_nalply 2y ago
I just watched Sabine Hossenfelder's video (Gravity without Mass, https://youtu.be/Q0fwRMvNkoA https://youtu.be/Q0fwRMvNkoA) where she talks about hollow spheres. Inspired by this video I have this hypothesis: There's quantum fluctuation. Particles appear out of nowhere and disappear again. Hossenfelder talked about negative mass, so allow me to do this as well: What if a pair of two particles, one of negative and one of positive mass can very rarely appear? They disappear immediately again but for a short moment we have acceleration (more about that in Hossenfelder's video). Could this be enough to explain "dark matter"?
Razengan 2y ago
As told in certain science fiction, I like to imagine that dark matter/energy is the "pollution" or other side effects caused by some alien civilizations [over]use of faster-than-light travel. Which is actually a terrifying existential threat to think about: The more an earlier civilization uses FTL, the more space will expand, and eventually it will become all but impossible for younger civilizations to traverse space without becoming dependent on the elder races.
hwc 2y ago
that is very surprising.
jameskilton 2y ago
For those who aren't sure what Rotation curves are: it's the observation that the orbits of stars in galaxies around said galaxy's center do not slow down the further away from the center they are. In fact some even rotate faster the further out you get! https://en.wikipedia.org/wiki/Galaxy_rotation_curve https://en.wikipedia.org/wiki/Galaxy_rotation_curve
- DonnyV 2y ago
  Thank you for this! I was confused what they meant by Rotation curve.
- ricksunny 2y ago
  I would settle for achieving intuition or even credible sources'acknowledgment in popular press on just what point in space is being orbited when the satellite is millions of LY (or 8 light minutes :) ) out from the massive object (which is translating at a measurable fraction of c ) it is orbiting.
  - tony69 2y ago
    If it’s translating at constant velocity, the satellite is orbiting the point in space where the object actually is. There is no “lag”, gravity points to where the center is now, not where it was 8 minutes ago.
  - jerf 2y ago
    I can't search it up now, but I have seen a derivation that if you work all the way through the math, it turns out your intuition is wrong and in the case of something like a planet orbiting a star, if the star is moving at a reasonably constant speed (ignoring all the relativity details around how to define that for now) it turns out that the planet actually will effectively orbit around the "current" position of the star, even though gravity only travels at the speed of light [1]. Our naive expectation that it would orbit only where it sees the star in the sky right now fails to account for some additional correction terms that show up when you take the dynamics of the situation into account, and it happens to come out in what you may think of as a coincidence to "correcting" the point actually being orbited to what is also the current position of the star. (Technically I think it is still off, but galactic orbits are very, very slow, so the errors introduced by them are also very very small.) You may recall in physics class how the difficulty of understanding situations amped up once you move from statics to dynamics, especially if you took a real calculus-based version of it. In much the same way that even if you use Newtonian gravity, simply knowing the formula may still leave you surprised at quite a lot of what can happen in orbital mechanics. If the star suddenly disappeared or zoomed off in another direction, it would be a light-speed delay before the planet "noticed" anything, but that generally does not happen, obviously. I expect the derivation I saw would not be valid for orbits involving speeds close to c, but I would expect the general observation that the effective center of the orbit is in fact not the time-delayed location would still hold. [1]: Expected for a long time, but fairly recently observationally proved: https://www.youtube.com/watch?v=Pa_hLtPIE1s https://www.youtube.com/watch?v=Pa_hLtPIE1s
MattPalmer1086 2y ago
A really interesting result. On it's own, it doesn't destroy dark matter as a theory, but it's putting quite a large dent in it. Obviously will need additional work and review, it's only one paper. Maybe there are mistakes or factors not fully considered. There have been a number of other papers recently on measuring wide binaries. Different papers claimed different results on these. Still, it's certainly something that merits a lot more attention. We may be looking at needing some new theory of gravity (maybe not MOND, but something other than dark matter).
- l33tman 2y ago
  Consider also that the author of the linked blog post seems to be a (pretty strong) advocate for MOND, it did read as somewhat biased
  - MattPalmer1086 2y ago
    He is a bit more nuanced than that, I've been reading the blog for quite some time. He has pointed out areas where MOND falls short in other posts, but it is true that he thinks that LCDM has some serious flaws, and that MOND like theories have a better predictive record.
    - l33tman 2y ago
      It's just that from a more complete point-of-view, there are many other areas than rotation curves that depend on dark matter and has been modelled to really require the specific properties of DM that the lambda-CDM model has, and it's just a bit worrisome every time a blog post talks exclusively about rotation curves. Sounds good that he writes about more of the issues elsewhere! The real picture might require a new blend of both DM and MOND of course..
- naasking 2y ago
  > A really interesting result. On it's own, it doesn't destroy dark matter as a theory It refutes LCDM though. No DM halos could be responsible for this behaviour this far out. And this isn't the first time LCDM has been refuted by evidence before they tweaked it with yet more parameters/epicycles to make it fit. It's been clear for awhile now that neither particle dark matter nor MOND are adequate explanations for observations. MOND clearly matches some data better and with fewer parameters (like this), and DM others, like cluster-scale lensing. New thinking is needed, and hopefully this paper will surprise people into taking MOND-like approaches a little more seriously.

103 points by xqcgrek2 2y ago

deleted 2y ago

[deleted]

Maro 2y ago

Quick explanation: A galaxy (elliptic, or spiral) is made out of billions of stars, like our sun. These stars rotate around the center of the galaxy (very slowly, think millions of years for 1 rotation). A rotation curve measures the velocity of stars as a function of distance from the center of the galaxy. Newtonian physics (or Einstein's GR) says that the rotation curve should decay with distance, ie. with greater distance the stars' velocity should go down --- assuming the matter in the galaxy is the visible matter that we see, ie. the stars (which shine light). The problem is, there is a rich set of observational data, from many different experiments, telescopes, and methodologies that show that the rotational curve is in fact flat, it does not decay. There are 2 big competing theories to explain this discrepancy: 1. Assume that there is a lot of unseen, non-shining, ie. Dark Matter (DM) in the galaxies (also ours). If you put the appropriate amount of dark matter in there, with the right distribution, you can reproduce the observed rotational curve. There are also other places is astrophysics/cosmology where having dark matter (specifically Cold Dark Matter, CDM, where cold just means "slowly moving") is useful. The biggest example is to explain the history of the Universe and the observed Hubble-constant. In fact the standard model of cosmology is called λCDM, CDM for Cold Dark Matter (λ for the cosmological constant, currently modeled as Dark Energy, not relevant for this discussion). 2. Assume that Newton was wrong and gravity is not exactly 1/r^2 --- this is called MOND, Modified Newtonian Dynamics. This way you can also reproduce the observed rotation curves. This is much less popular, because: (i) physicsts don't want to give up the beautiful and geometric simplicity of 1/r^2 (ii) Dark Matter is also useful for solving other discrepancies in astrophysics/cosmology. What this article is saying is that, even in the first Dark Matter model, per the model DM distributions inside galaxies that also work with all the other places where DM is used to explain something (eg. in cosmology), at some distance from the center, the dark matter bubble has an edge and stops --- and then the velocities should finally break down. However, these latest observations are showing that the velocities remain constant even beyond the modeled/assumed DM bubbles. This is an additional ε argument in favor of MOND, and science proceeds.

merek 2y ago
Fantastic explanation, thank you. For a more detailed description of the Milky Way's rotation curve, this is a brief segment from David Butler's How Far Away Is It video series (which I highly recommend): https://youtu.be/uVxrsJ5lZlQ?si=ZwpBDpAvTV8AALke&t=1890 https://youtu.be/uVxrsJ5lZlQ?si=ZwpBDpAvTV8AALke&t=1890
xeonmc 2y ago
What falloff function will create an exactly flat curve? G ~ 1/r instead of 1/r^2 ? What if gravity has non-scalar components?
- canjobear 2y ago
  https://en.m.wikipedia.org/wiki/Tensor%E2%80%93vector%E2%80%93scalar_gravity https://en.m.wikipedia.org/wiki/Tensor%E2%80%93vector%E2%80%... The extra components would have energy and function as Dark Matter.
jprete 2y ago
I'm not a physicist but 1/r^2 strikes me as conceptually very important, because it's the relative contribution of any fixed area of spherical surface to the total area of that surface. So the total strength of gravitational field emanating from a particular object, at a given distance from that object, is a constant. It's somewhat weird to think of the total gravitational "force field" _increasing_ in magnitude with distance. Decreasing, sure. Increasing? That makes no sense. Certainly not at a large enough function of distance for the rotational curve to be _flat_. That's got to be some kind of wonky power term over distance which implies potential energy from the field goes up with distance as well. As above, I'm not a physicist, but a linear rotational curve breaks every intuition I've ever gleaned from physics about the nature of what's really going on with relativity, particle mediation of forces, or even the concept of a field. Maybe it means spacetime curvature is way higher than we think.
- throwawaymaths 2y ago
  > spacetime curvature is way higher than we think. You mean way weirder. Remember 1/r^2 does not work quite right for, e.g. mercury.

deleted 2y ago

[deleted]

burnte 2y ago

"MOND predictions keep being corroborated, yet the community persists in ignoring its implications, even in terms of dark matter. It’s gotta be telling us something." No, it doesn't. MOND falls apart in every single theory they put forth. The fact that this happens without fail should lead one to understand the answer probably lies elsewhere than MOND.

kurthr 2y ago
It's not like MOND (or even dark matter) are even single theories with a fixed set of predictions. They are groups of competing ever evolving theories. I don't understand the need to believe one over the other as a predictive tool without some conclusive evidence. If you're not the astronomer who makes that measurement (or explains it), it just feels like galactic sports betting.
MattPalmer1086 2y ago
MOND successfully predicts many things we observe that DM fails to, or that requires special tuning for DM to fit observations (which is not prediction). DM actually has quite a poor predictive record, even if it can be made to fit observation. MOND as a theory is probably not correct. It's not relativistic for a start, although I believe there are other modified gravity theories that are.
throwawaymaths 2y ago
MOND explains (discovered before Mond, but the Mond equations unexpectedly fit): Tully fisher relation Renzos rule MOND predicted (these were observed after mond predicted it): External field effect Keplerian descent in milky way Early galaxies after the big bang Consistently "No dark matter found" in elliptical and lenticular galaxies
shiandow 2y ago
I'm getting interested now, the comments have alleged several predictions MOND made. On the other hand I'm pretty sure the evidence for dark matter is pretty strong as well (I believe there are several ways to calculate the amount of dark matter, all of which agree). Can you give an example of MOND falling apart? One that requires so much fine-tuning that it cannot be adequately explained, or one requiring a violation of one of the more fundamental laws of physics?

xanzi2048 2y ago

Why does MOND keep showing up on HN? Pretty odd considering it's not particularly popular elsewhere and fails to explain a lot of observations of dark matter (bullet cluster, CMB). It's worth noting these rotation curves are not all the same curve, and we've discovered galaxies with varying quantities of implied dark matter (eg https://www.aanda.org/articles/aa/full_html/2023/07/aa46291-23/aa46291-23.html https://www.aanda.org/articles/aa/full_html/2023/07/aa46291-...) - so is MOND different for every galaxy?

deleted 2y ago
[deleted]
MattPalmer1086 2y ago
No. MOND is the same everywhere, with basically no free parameters.
- xanzi2048 2y ago
  but... how is that compatible with the article I linked here? Some galaxies look like they have very little dark matter, a MOND with no free parameters can't explain why different galaxies have different inconsistencies in their rotation curves.
  - throwawaymaths 2y ago
    You actually have to do the math, it's not just a "no dark matter hurr durr" thing. MOND predicts "no dark matter" if the acceleration regime is high. Most galaxies that havd "very little dark matter" are in the high acceleration regime. Most of the others seem to be susceptible to observational error (e.g. ultra diffuses). Galaxy rotation curves measurements are highly sensitive to orientation of the rotational axis and distance to us (you're gonna have a hard time measuring it if the galaxy is face on)
  - MattPalmer1086 2y ago
    Correct. MOND cannot explain why some galaxies don't fit it's predicted rotation curve. However, the vast majority do, and the linked paper says that the ones they are describing are extremely rare.

thriftwy 2y ago

Are these rotation flats variable in magnitude? Would one galaxy have a different curve height than another one? What does it depend on? Update: https://en.m.wikipedia.org/wiki/Tully–Fisher_relation https://en.m.wikipedia.org/wiki/Tully–Fisher_relation But the linear regression looks unconvincing

mikewarot 2y ago

Nobody wants to give up 1/r^2, including me. However, what if it holds, but if inertia is quantized, then you get less gravitational effect at 90 degrees to the path of motion at astronomic distances as it recedes into the quantum noise. Which fits observations with inventing dark matter, or tweaking gravity.

_nalply 2y ago

I just watched Sabine Hossenfelder's video (Gravity without Mass, https://youtu.be/Q0fwRMvNkoA https://youtu.be/Q0fwRMvNkoA) where she talks about hollow spheres. Inspired by this video I have this hypothesis: There's quantum fluctuation. Particles appear out of nowhere and disappear again. Hossenfelder talked about negative mass, so allow me to do this as well: What if a pair of two particles, one of negative and one of positive mass can very rarely appear? They disappear immediately again but for a short moment we have acceleration (more about that in Hossenfelder's video). Could this be enough to explain "dark matter"?

Razengan 2y ago

As told in certain science fiction, I like to imagine that dark matter/energy is the "pollution" or other side effects caused by some alien civilizations [over]use of faster-than-light travel. Which is actually a terrifying existential threat to think about: The more an earlier civilization uses FTL, the more space will expand, and eventually it will become all but impossible for younger civilizations to traverse space without becoming dependent on the elder races.

hwc 2y ago

that is very surprising.

jameskilton 2y ago

For those who aren't sure what Rotation curves are: it's the observation that the orbits of stars in galaxies around said galaxy's center do not slow down the further away from the center they are. In fact some even rotate faster the further out you get! https://en.wikipedia.org/wiki/Galaxy_rotation_curve https://en.wikipedia.org/wiki/Galaxy_rotation_curve

DonnyV 2y ago
Thank you for this! I was confused what they meant by Rotation curve.
ricksunny 2y ago
I would settle for achieving intuition or even credible sources'acknowledgment in popular press on just what point in space is being orbited when the satellite is millions of LY (or 8 light minutes :) ) out from the massive object (which is translating at a measurable fraction of c ) it is orbiting.
- tony69 2y ago
  If it’s translating at constant velocity, the satellite is orbiting the point in space where the object actually is. There is no “lag”, gravity points to where the center is now, not where it was 8 minutes ago.
- jerf 2y ago
  I can't search it up now, but I have seen a derivation that if you work all the way through the math, it turns out your intuition is wrong and in the case of something like a planet orbiting a star, if the star is moving at a reasonably constant speed (ignoring all the relativity details around how to define that for now) it turns out that the planet actually will effectively orbit around the "current" position of the star, even though gravity only travels at the speed of light [1]. Our naive expectation that it would orbit only where it sees the star in the sky right now fails to account for some additional correction terms that show up when you take the dynamics of the situation into account, and it happens to come out in what you may think of as a coincidence to "correcting" the point actually being orbited to what is also the current position of the star. (Technically I think it is still off, but galactic orbits are very, very slow, so the errors introduced by them are also very very small.) You may recall in physics class how the difficulty of understanding situations amped up once you move from statics to dynamics, especially if you took a real calculus-based version of it. In much the same way that even if you use Newtonian gravity, simply knowing the formula may still leave you surprised at quite a lot of what can happen in orbital mechanics. If the star suddenly disappeared or zoomed off in another direction, it would be a light-speed delay before the planet "noticed" anything, but that generally does not happen, obviously. I expect the derivation I saw would not be valid for orbits involving speeds close to c, but I would expect the general observation that the effective center of the orbit is in fact not the time-delayed location would still hold. [1]: Expected for a long time, but fairly recently observationally proved: https://www.youtube.com/watch?v=Pa_hLtPIE1s https://www.youtube.com/watch?v=Pa_hLtPIE1s

MattPalmer1086 2y ago

A really interesting result. On it's own, it doesn't destroy dark matter as a theory, but it's putting quite a large dent in it. Obviously will need additional work and review, it's only one paper. Maybe there are mistakes or factors not fully considered. There have been a number of other papers recently on measuring wide binaries. Different papers claimed different results on these. Still, it's certainly something that merits a lot more attention. We may be looking at needing some new theory of gravity (maybe not MOND, but something other than dark matter).

l33tman 2y ago
Consider also that the author of the linked blog post seems to be a (pretty strong) advocate for MOND, it did read as somewhat biased
- MattPalmer1086 2y ago
  He is a bit more nuanced than that, I've been reading the blog for quite some time. He has pointed out areas where MOND falls short in other posts, but it is true that he thinks that LCDM has some serious flaws, and that MOND like theories have a better predictive record.
  - l33tman 2y ago
    It's just that from a more complete point-of-view, there are many other areas than rotation curves that depend on dark matter and has been modelled to really require the specific properties of DM that the lambda-CDM model has, and it's just a bit worrisome every time a blog post talks exclusively about rotation curves. Sounds good that he writes about more of the issues elsewhere! The real picture might require a new blend of both DM and MOND of course..
naasking 2y ago
> A really interesting result. On it's own, it doesn't destroy dark matter as a theory It refutes LCDM though. No DM halos could be responsible for this behaviour this far out. And this isn't the first time LCDM has been refuted by evidence before they tweaked it with yet more parameters/epicycles to make it fit. It's been clear for awhile now that neither particle dark matter nor MOND are adequate explanations for observations. MOND clearly matches some data better and with fewer parameters (like this), and DM others, like cluster-scale lensing. New thinking is needed, and hopefully this paper will surprise people into taking MOND-like approaches a little more seriously.