B Questions about dark matter/energy

[imsmooth]

Does the astrophysics community really think dark energy/matter is real, or that we just haven't figured out the actual physics of what is happening and this just makes it work out (to our current understanding).

 

[PeroK]

Which would you like to discuss? Dark energy or dark matter?

 

[Orodruin]

The question doesn’t make sense. Apart from the fact that they are completely different things, we do not know exactly what they are, but we know that whatever they are they should behave in a particular way. For example, we don’t know what dark matter is - we need to find out - but whatever it is it will still be dark matter.

 

[phinds]

Does the astrophysics community really think dark energy/matter is real, or that we just haven't figured out the actual physics of what is happening and this just makes it work out (to our current understanding).

To expand on @Orodruin's answer:

"Dark Matter" and "Dark Energy" are place holder names for phenomenon which, without any possible question, exist. We know what they DO, even if we do not know HOW they do it, and whatever name you give the phenomena the eventual understanding of what they are will just change the name (maybe) but what they do is not dependent on the name, so yes, of course they exist.

 

[Ibix]

Does the astrophysics community really think dark energy/matter is real, or that we just haven't figured out the actual physics of what is happening and this just makes it work out (to our current understanding).

Dark matter and dark energy are different things.

Dark matter effects may be explicable as modified gravity or as matter that we can't see for some reason. Neither explanation is wholly satisfactory and the debate continues. "The community" does not have an opinion - it has several. Time will tell which is correct.

Dark energy, as far as I am aware, has no widely accepted physical model behind it so far. It might be the quantum mechanical energy of the vacuum (although the numbers aren't hopeful) or it might be something else. So again, I don't think there's a single opinion.

 

[ohwilleke]

Does the astrophysics community really think dark energy/matter is real, or that we just haven't figured out the actual physics of what is happening and this just makes it work out (to our current understanding).

This is the subject of active academic debate upon which a consensus has not been reached.

Dark Matter

The majority view is that dark matter is real, and a minority view is that it is a gravitational or 5th force effect of some kind, possibly due to misapplying the gravitational laws we already know and possibly due to some modification of Standard Einstein's General Relativity (GR).

The pros and cons of these alternatives are well explored in other threads in this forum.

Dark Energy

In the case of dark energy, as a practical matter, in the LambdaCDM model (which is the paradigm and baseline model used by cosmologists despite its known problems), "dark energy" phenomena are explained with a cosmological constant term in Einstein's GR equations, and this is the "baseline" assumption with respect to which alternatives are evaluated.

One of the most common things to do is for the cosmological constant to be reinterpreted in a physically equivalent way, as a very weak, constant, energy field with a fixed amount of energy per unit volume, that grows in aggregate amount as the light cone of the Big Bang expands. Using this description of the cosmological constant as a starting point, instead of a single number added to the General Relativity equations without a cosmological constant, has some desirable features.

If you use a dark energy field equivalent to the cosmological constant, instead of the cosmological constant itself, it is a lot easier to tinker with in the event that your observational data doesn't perfectly match what it should look like if the cosmological constant hypothesis is correct. The way that you tinker with it, in order to modify GR with a cosmological constant, is by having the strength of the dark energy field change over time and space according to some formula.

For example, some astronomers think that their astronomy observations would be a better fit to a dark energy field that was initially stronger than it is now, a hypothesis called "early dark energy." This is easier to fit if dark energy is actually a real, physical thing.

Thinking of the effects of the cosmological constant as a real world energy field also makes it easier to quantify the total amounts in the universe of ordinary matter other than neutrinos, neutrinos, radiation, dark matter, and dark energy in a well-defined and consistent way that can be measured with precision, and are comparable to each other in a meaningful way. The LambdaCDM model of cosmology takes this approach, with the relative proportions of different kinds of mass-energy in the universe as a whole used as parameters for the model.

Another virtue of thinking of dark energy as a real physical field is that it makes dark energy and another cosmological concept, called cosmological inflation, easier to integrate with each other, because then you just have two real physical energy fields, or perhaps even two different manifestations of a single real physical energy field.

But this isn't the only possible way to explain dark energy phenomena.

There are several quite subtle ways in which it possible to modify Einstein's GR equations that reproduce the observations that motivate and fix the value of the cosmological constant, without actually inserting a cosmological constant in the equation. Some astrophysicists are optimistic that one of these alternative GR equations might be correct because they are "more beautiful and elegant" without a cosmological constant attached. These alternatives to GR don't require energy fields of dark energy to exist.

Also, a cosmological constant can make formulating a quantum gravity analog to general relativity (which there are good suggestive reasons to think should exist) significantly harder to formulate. So, if you can formulate an alternative to GR that replicates dark energy phenomena without a real energy field and without a cosmological constant, this could be a better foundation for a quantum gravity version of that theory, than trying to come up with a quantum gravity theory that works from GR (a task which theoretical physicists have been attempting and failing at for almost a century now).

One of the big problems with coming to any consensus view on the reality of the physical energy fields that are sometimes used to explain the phenomena often attributed to "dark energy" is that our measurements of "dark energy" aren't very precise (low single digit percentage precision and these observations also have a lot of hard to quantify potential sources of systemic errors and theoretical model calculation conceptual errors). This is because, formulated as a dark energy field, this field is extremely weak to the point of being basically undetectable directly with man made detectors. The total amount of hypothetical dark energy is huge, but only because it is absolutely everywhere and there are vast amounts of virtually empty space in the universe and this adds up.

Anyway, presented with any two different possible explanations for dark energy phenomena, our observations frequently aren't good enough to say which theory is right, with a high level of statistical confidence.

Furthermore, a key physical constant closely related to dark energy, called the Hubble constant, has been measured in different ways producing inconsistent values, which suggests that the overall conceptual basis of the cosmological constant model may be flawed. This has made the search for alternatives more urgent.

 

[collinsmark]

Regarding the dark matter part of the original post, here is a lovely video by Dr. Angela Collier, explaining that "Dark Matter" is not a theory in and of itself, but rather is a collection of observations.

Sure, there are individual (and sometimes disparate) theories to explain the dark matter observations, but dark matter, at least at this time, isn't itself a theory: it's a collection of observations.

The video is worth watching at the very least (for the OP) because it introduces the list of these observations.

(And as others have pointed out in this thread, "Dark Matter" is a placeholder for a more detailed explanation/theory, once we know more about it. But for the time being, dark matter is a collection of observations.)



Dark energy, while not discussed directly in the video, is likewise, at this point in time, a collection of observations (albeit a completely different set of observations about different cosmological aspects).

[Edit: As a side note, I love Dr. Angela Collier's videos, as they are chock-full of extremely subtle bits of humor together with serious and well researched, informative sources. The effort she puts into the editing is brilliant. It's both seriously informative and somehow, also comedic gold. Be prepared to pause the video from time to time, so as not to miss the subtle bits of commentary splashed on the screen.]

 

[srb7677]

I am no expert so will likely spend a lot of time having things explained to me by those with more knowledge. I am a bit like someone with a passing interest in cosmology, who watches documentaries, and reads books and magazines on the subject, along with many other interests, like history and politics, trying to mix here with experts in the field. I feel a bit out of my depth, so will read much more than I post.

However, my simple understanding is that dark matter was hypothesized in the first instance to explain the gravitational behaviour of rotating galaxies, which only make sense, according our current understanding of gravity, if there is a lot more matter in the galaxy that we cannot see. In other words, we have inferred the existence of matter we cannot actually see by it's observed gravitational effects. Is this correct?

And I understand that some scientists are proposing a modified form of gravity to explain the observations rather than invoking dark matter. Though insofar as I can tell majority scientific opinion seems to be behind the existence if dark matter, though there is no consensus on what it might actually be yet. Is this correct?

What evidence aside from the observed gravitational effects on galaxies, is there for the existence of dark matter? And what suggestions have scientists thus far come up with as to what it might be?

And is there any evidence to support or refute a modified theory of gravity?

As an amateur in these matters, it does seem to me that the more humanity learns about the deep workings of the universe, the more we come to see how much we still don't know. And I find it awe-inspiring that learned minds can understand as much as we do already using mathematics, and the fact that the universe seems to operate on mathematically understandable principles.

 

[Ibix]

And I understand that some scientists are proposing a modified form of gravity to explain the observations rather than invoking dark matter. Though insofar as I can tell majority scientific opinion seems to be behind the existence if dark matter, though there is no consensus on what it might actually be yet. Is this correct?

Pretty much. "A" modified form of gravity is probably a stretch - there are quite a few versions, I believe.

What evidence aside from the observed gravitational effects on galaxies, is there for the existence of dark matter? And what suggestions have scientists thus far come up with as to what it might be?

As I understand it there is some evidence in the cosmic microwave background for dark matter. The spectrum of the temperature variation has a peak that makes sense if there's some form of matter that only interacts gravitationally. However, various lines of enquiry looking to directly detect dark matter, whether it is a new particle, or a lot of small black holes, and probably other things I'm unaware of, have come up empty. Hence a resurgence of interest in modified gravity.

And is there any evidence to support or refute a modified theory of gravity?

Loads. One that seems to be in the news lately is analysis of wide binary star systems. As far as I can tell, some analyses decisively rule out alternative gravity and some decisively rule out dark matter. So there's at least one thing we don't understand and it's very much not clear what the answer is. And nobody, so far as I am aware, has managed to make a modified gravity theory that leads to a cosmology that looks like what we see at really large scales.

So both dark matter and modified gravity have problems they can't yet solve. I don't think it's possible to say which is right yet.

As an amateur in these matters, it does seem to me that the more humanity learns about the deep workings of the universe, the more we come to see how much we still don't know.

Yes and no. Bear in mind that we've got far fewer assumptions in our theories than we used to have, and the failures we're discussing are in fairly extreme circumstances. Our current theories are extremely accurate for almost everything (that's one of the reasons it's so hard to find flaws).

 

[Ken G]

I think another way to frame the question of "modified gravity vs. dark matter" is instead of asking "which one is what's really happening," ask simply "which one provides the simpler way to parametrize what's happening." Here we find interesting support for modified gravity in some contexts, such as in orbits within a galaxy, where those orbits can be understood using fewer free parameters in a modified gravity picture rather than a dark matter picture. This is in keeping with the standard approach of "Occam's razor," which asks us to adopt whichever model involves the fewest unconstrained assumptions. But note that is much different than the often seen (and incorrect) way to state Occam's razor, that the simpler theory is most likely to be "right." Rightness is a very different concept in science than ease of modeling!

What this means is, there can never be a "winner" in the "rightness" vein between dark matter and modified gravity until either dark matter is actually directly detected (not modeled), or a complete theory of modified gravity is found that explains cosmology (and not just galactic orbits). Until then, we are in no position to even ask "which one is right," we can only look at the contexts that we have interest in, and ask, 'which one allows an accurate treatment with the fewest unconstrained parameters." In some situations, the answer to that is modified gravity, a state of affairs that I feel is too often overlooked.

For doubters, a classic example of what I'm talking about is Newton's laws of motion. It is well known that these laws are not universally applicable, and fail as badly in some contexts as modified gravity fails in cosmology. Yet Newton's laws are used vastly more often than the more accurate and more universal laws we already have. So much for universality vs. practicality.

 

[collinsmark]

Here's a follow-up video to the one in post #7.

While the video below references MOND quite a bit, the thrust of the video is about science communication.

 

[Vanadium 50]

Did not really like that video, but I only watched 10-15 minutes of it. 120 minutes is a heavy lift.

(1) It is true that MOND is far more popular among random people on the internet than professional astronomers. Saying there are only three people working on it is a bit of an exaggeration, and I found that very unconvincing. I recall from my own career a time when only three people (including me) were arguing in favor of a (particular and somewhat esoteric) theory which today is completely accepted.

(2) Her description of MOND is very, very restrictive. If there were a new theory of gravity that had GR at a limiting case, but modified the rotation curves of galaxies, and also cranked up enough PBH production to explain all the other observations, she would say "Nope. Not MOND."

(3) She misstates the MOND predictions of rotation curves. Yes, every rotation curve is different. ΛCDM says, of course they are, and its unpredictable. Galaxies have different amounts of luminousb to dark matter. You wouldn't expect them to look the same, and indeed you can't predict what a rotation curve should look like, because you don't know that ratio. MOND would say, but then why can we predict this with a single parameter, a0?

She is correct that rotation curves are only a part of the story. MOND does the best here, and as you go to larger scales, it struggles more and more. But it is still an empirical relationship that deserves explanation. I personally believe when there is such an explanation, it will tell us nothing about gravity, but something about galaxy formation. Time will tell.

 

[Bandersnatch]

Did not really like that video, but I only watched 10-15 minutes of it. 120 minutes is a heavy lift.

I like how there's a bit in the video where she wonders about people commenting on her video without watching it.

 

[Vanadium 50]

I like how there's a bit in the video where she wonders about people commenting on her video without watching it.

Guilty.

In my defense, if it takes her an hour to make her point, and then a 2nd hour to say "I didn't really make my point in the first hour - let me try again" it's not a very good sales pitch.

Besides, I know the data, and she's overselling. If the final outcome is that GR needs to be extended and that particle dark matter is maybe a quarter of what we think it is (not something I think is correct, but a logical possibility), I don't think "See! I was right all along!" is what I would conclude. She is defining it so narrowly as to make it almost false by definition. That is not so helpful.

One of the strongest MOND proponents is Stacy McGaugh. I have known Stacy for a very, very long time indeed. He can give a stronger lecture in support of Dark Matter than this video. IMHO, because he's not trying to sell people on an idea.

Her frustration with "marketing" misses a critical point - it's easier to explain, because its simpler. Dark Matter could have been simple as well: "Dark matter is the photino, the supersymmetric counterpart of the photon, and it weighs 30 GeV" But it didn't turn out that way. That's significant.

And maybe she got to it later in the video, but MOND and DM are not as different as their respective internet cheerleaders think. The observational problem is that at large scales we observe too much acceleration. Models that try and address this observation will of course make similar predictioins.

 

[collinsmark]

let me try again" it's not a very good sales pitch.

Besides, I know the data, and she's overselling.

I don't see what you mean. She's clearly not pushing any particular theory (she spends an hour saying that in this recent video, and 45 minutes in the last, after all).

All she's saying is that it's an open topic in physics, and that's what makes it exciting. And fun.

 

[Vanadium 50]

In the part I watched, she was lamenting "How can so many people still believe in MOND? Where did I go wrong?" She even had a pie chart. To me, that sounds like she's unhappy that so many people "got the wrong answer".

 

[collinsmark]

In the part I watched, she was lamenting "How can so many people still believe in MOND? Where did I go wrong?" She even had a pie chart. To me, that sounds like she's unhappy that so many people "got the wrong answer".

That wasn't her exact quote, btw. Whatever the case, I think you're taking her presented ideas out of context. If she's lamenting about anything, it's that people believe in a particular dark matter theory (MOND being one of many such theories) when in fact it is still an open topic.

That, and it was surprising to her how wildly over-represented MOND is in the public, compared to what it is in the scientific community.

She elaborates, starting at 15:19, "...and you were like 'MOND is my favorite Theory' I would put that in 'other' [in the pie chart]. Cuz like that's not an incorrect statement. That doesn't mean you got something wrong from the video. Like, you can like MOND. It's totally fine. That's not wrong, right. It's okay to like MOND."

That wasn't lamenting that people got the "wrong" answer (she specifically emphasizes otherwise), rather that the public's understanding of the science is grossly over-represented by MOND, given that the whole thing is in fact still an open question. And that most in scientific community are exploring so many other possible models.

 

[Ken G]

I think she makes a good central point, which is that what she is calling the "dark matter problem" is something that is extremely complicated, and cannot be answered by some simple modification to gravity. Full stop, she really could have honed in on just that point and made a very convincing case. In maybe fifteen minutes, so more people would get to the end.

The problem is, she wanted to do two other things, that I think are a lot less successful. The first is, she wanted to stress how bad of a job she had done to communicate what I just summarized above, because so many people (90% of the comments) still supported the idea that MOND is the best way to solve the dark matter problem, presumably because it is so simple. So OK, the point that nothing as complicated as the dark matter problem is ever going to be solved by something as simple as MOND did not come across, and that was the "failure" of the first video. She is trying to say that the main claim to fame of MOND, its simplicity, is also its dooming characteristic. Good point, well taken. By why be so surprised its dooming characteristic explains its extreme popularity on the internet? Yes, it is easier to market, point taken there as well. No surprise there either, why all the sturm and drung about it? She shouldn't have been so surprised that people will overstress the value of something simple, we want the simple solution and we will tend to gravitate toward it, it's not that big of a surprise really.

The second problematic element is the way she defines "the dark matter problem." She could have called it "the problem that we don't understand gravitational dynamics in the universe" and it would have been fine, but she doesn't seem to recognize that if you call it "the dark matter problem", you are already quite clearly intimating that you think the solution is going to be dark matter. Dark matter is not just any solution to gravitational dynamics that we don't understand, it is a particular solution, one that involves invisible matter (and yes, dark matter should not be called dark, it should be called invisible). So it's like she chooses to use words that seem to imply X, and then is bothered that people thought she meant X when she used those words! I completely agree with her that we should regard the situation as a "problem", one that does not yet have a theory to solve it. But call it the "gravitational dynamics problem", not the "dark matter problem," unless you really think the solution is going to be invisible particles. Otherwise, you will suffer a failure in communication that is exactly the one she is bemoaning.

That said, I do think her core point, that MOND is way overstressed as a solution to all the problems with gravitational dynamics, is a valid one, on grounds that the solution will be complicated. I also commend her for reaching half a million people, in an engaging way! However, I would add that if the solution is expected to be complicated, then we might also expect it to have many elements. Maybe we are wrong to think we are looking for any single solution in the first place. So perhaps the real problem is not the oversimplicity of MOND, but rather, the oversimplicity of thinking we are looking for just one answer, and arguing over which one it should be. Wouldn't it be something if there was a whole list of things we don't currently have right? I mean, when has that ever happened?

 

[Vanadium 50]

For the record, I don't like MOND. I don't like the it isn't relativistic, I don't like its treatment of composite objects, and I don't like how well it (doesn't) work at large scales.

I do think a universal scale a0 is an empirical fact that needs to be explained. A historical analogy is the Law of Dulong and Petit. That took almost a century to sort out.

I also like less the unscientific (IMO) dismissal of MOND, even by people I otherwise respect. "Look at the Bullet Cluster! A smoking gun!" "But what about Abell 520, which is just like the Bullet Cluster but the other way." "Well, everybody knows you can't draw conclusions from a single cluster!" What? What the...?

 

[Ken G]

That's why I agree with her that the really important point to make here is that this is not the time to be arguing over "which is the solution to the dark matter problem", it is to be exploring just what the problem really is, and how do various approaches take a bite out of it. Maybe none of the things we've suggested so far are going to suffice, because we're missing something even deeper, but perhaps they can be stepping stones.

I'll give an example of how sublimely far off our thinking about what we are trying to solve can be. There was a famous debate between Newton and Huygens about whether refraction of light should be understood by thinking of light as a particle or a wave. For Newton's particle picture to work, the particles had to have their momentum increase as they entered the glass, but for Huygen's wave model to work, the wave had to slow down as it entered the glass. Surely both could not hold, you could not have a wave slowing down at the same time some particle was increasing its momentum, so it really looked like a battle between two different world views that was resolved when observations showed that the speed of light slows down in glass (or water).

Of course we now know that to really understand light, you must combine the notions of Newton and Huygens, because light is indeed a particle, just like Newton claimed, but it is ruled by the mathematics of waves such that this particle exhibits a remarkable property that Newton never dreamed of: its (group) velocity can slow down even though its momentum increases in exactly the way Newton claimed it did! That is a property of the dispersion relation of light in glass, a wave concept just like Huygens used, but in thinking that he was contradicting Newton they both had the wrong dispersion relation in mind (one in which an increase in particle momentum should come with an increase in velocity). What a remarkably subtle way to resolve what looked like a simple slam dunk of a debate between two nonoverlapping possibilities!

Physics is full of surprises, that's why it's so great, so we should probably expect the solution to the "dark matter problem" to be full of surprises also. Which I believe is her central point-- we live in interesting times, and the main flaw of MOND, perhaps even crime of MOND, is that it tends to coax us into missing this point by thinking we can make the whole business go away by adding a0 to the acceleration. On the other hand, if the simplest way to understand the gravitational dynamics in some galaxy is via some very simple reparametrization of the gravitational acceleration, then that is probably telling us something we should be listening to.

 

[strangerep]

I like how there's a bit in the video where she wonders about people commenting on her video without watching it.

I did watch the whole video. I think Vanadium's assessment is fair, perhaps even too mild.

I also watched the previous video she refers to. My impression is that, (like Dr Becky, for example), these astrophysics youtubers do not bother to study the MOND-related literature properly, e.g., recent wide binary results. Indeed, they seem unwilling/unable to comprehend the careful detailed rebuttal paper(s) (using nontrivial statistical methods requiring careful study) that have been published in recent months. They also seem blithely unaware of Stacy McGaugh's detailed arguments that he gives in his many blog postings.

 

[Bandersnatch]

Oh my. I guess she really is a failure at communicating. Maybe yet another video will do the trick.

 

[Orodruin]

Oh my. I guess she really is a failure at communicating. Maybe yet another video will do the trick.

The solution is always yet another hour+ video ...

 

[Astronuc]

Does this help?

http://astroweb.case.edu/ssm/mond/LCDMmondtesttable.html

I also like less the unscientific (IMO) dismissal of MOND, even by people I otherwise respect. "Look at the Bullet Cluster! A smoking gun!" "But what about Abell 520, which is just like the Bullet Cluster but the other way." "Well, everybody knows you can't draw conclusions from a single cluster!" What? What the...?

 

[Ken G]

I think what is emerging clearly here is we have to wonder why the great majority of people in the wider public prefer MOND type solutions, and the great majority in the astrophysics community favor the dark matter approach (dark energy is a whole different matter, I don't see any connection with either of them). Are they both just different bandwagons, and what we really need is to learn the lessons and insights that both have to offer, open to the possibility we might either need both, or neither, but something deeper that kind of encompasses them both? I have the sense we are framing the whole "gravitational dynamics problem" wrongly when we frame it as a need to make a choice of one or the other.

 

[Vanadium 50]

I think what is emerging clearly here is we have to wonder why the great majority of people in the wider public prefer MOND type solutions, and the great majority in the astrophysics community favor the dark matter approach

That's really a sociology question.

The "general public" believes all sorts of things. There are people who study this, people not me.

The theory community writes more papers on DM because there is more to say. Maybe its the LSP - but which particle? Maybe its two particles. Maybe its axions. Maybe its PBH's. MOND is sterile. "Here's a new law of gravity, Deal with it." The experimental/observational community hears these talks and responds as you might expect.

Remember too, that most asstronomers do not work on either family of models. If I study, say mass transfer and blue stragglers, my primary information comes from what other people tell me, in seminars and papers. So there is an amplification effect.

At the same time, MOND is incomplete. It's non-relativistic. People don't like that. It's also quite difficult to test in the lab. People really don't like that. \\

Also, Scientists are people too. At least 85% of us anyway. People get emotionally invested in their ideas. I an sure I have written more papers om DM than Stacy has written on MOND - including two moderately influential theory papers. Does that bias me? Maybe. How could I tell?

There is also some wokeness involved. The story is that Dark Matter was discovered by Vera Rubin, not Fritz Zwicky, and as such it is all about rotation curves. That's not true, and it overfocuses (IMHO) attention on that piece of the puzzle. It's a good story, and I understand why the public eats it up, but as usual reality is not so black and white.

What we see is more acceleration than we expect. There are three possibilities:

1. There is more mass than we think
2. Gravity is stronger than we think
3. At small acclerations there is less inertia per given mass.

 

[Ken G]

That's really a sociology question.

Indeed. Neveretheless, it is the fundamental topic of those two videos, and I believe is a valid question to ask. After all, astronomy is a societal issue itself.

The theory community writes more papers on DM because there is more to say. Maybe its the LSP - but which particle? Maybe its two particles. Maybe its axions. Maybe its PBH's. MOND is sterile. "Here's a new law of gravity, Deal with it." The experimental/observational community hears these talks and responds as you might expect.

That's not by itself a valid reason. If all of physics was organized in terms of the number of papers being proportional to the number of possible ways to address the problem, it would be the general equivalent of an army of blind squirrels looking for acorns. For example, much of what you just listed is based on the various types of possible particles in dark matter, but imagine the measure space of possibilities that don't involve dark matter at all. Indeed, MONDlike approaches have a vastly large measure space of possibilities, but only simple versions are considered, based on Occam's Razor. But there's a difference between considering one specific theory based on a single parameter, versus considering all the possible ways to build a theory based on a single parameter. If one considers the latter, the MOND space is equally vast as the dark matter space, but we're back to blind squirrels.

Instead, we tend to try certain focused strategies to look for solutions, and the strategies are not in proportion to the measure space of possibilities. The focused strategies are almost always generated based on philosophical priorities. Hence, we could say that the basic question of those two videos is, why do the two different groups, internet afficionados and the astrophysical community, adopt such different philosophical priorities when choosing their focus strategies?

Remember too, that most asstronomers do not work on either family of models. If I study, say mass transfer and blue stragglers, my primary information comes from what other people tell me, in seminars and papers. So there is an amplification effect.

Yes, and this is where the problem of the "bandwagon effect" comes in, and it applies equally in both groups. Astronomers want to specialize, so when they go outside their area, they don't want a long list of possible solutions, that's only what they want in their own area. But of course, all areas are like that, a long list of possible solutions, so the problem is in wanting all the other areas to be different. This may lead to the "amplification" you mention, the community as a whole gloms onto one approach because that's all the other astronomers want to hear about. But a bandwagon effect is exactly what I am cautioning against, it is the logical fallacy of her point that there are "only 3 people" doing MOND at any given conference on the problem of gravitational dynamics. Internet afficionados have the same problem. So when she says that MOND has better marketing in the internet world, she kind of overlooks how dark matter has better marketing in astronomy conferences.

At the same time, MOND is incomplete. It's non-relativistic. People don't like that. It's also quite difficult to test in the lab. People really don't like that. \\

So these are focusing strategies (given how nonrelativistic the gravitational dynamics problem is, it doesn't seem like a terrible weakness of MOND, it's only that issue of "unifying what is behind the frontier" that comes up there). They are perfectly valid ones, we need focusing strategies, but still it's not just a question of measure space. Dark matter seems easier to test in the lab, but there's little success there yet. So it's hard to know how to apportion one's resources. Lots of money will be spent culling down the possible space for finding dark matter, which is a lot like looking for one's keys in a dark parking lot, not just under the streetlights, but also when one is not even sure one lost one's keys in the parking lot at all. Still, one wants to search the parking lot, that's clear, it's just a question of where else one might wish to also look.

Also, Scientists are people too. At least 85% of us anyway. People get emotionally invested in their ideas. I an sure I have written more papers om DM than Stacy has written on MOND - including two moderately influential theory papers. Does that bias me? Maybe. How could I tell?

My point is that there is no problem with individual bias. We need people looking under all the rocks, and it never matters what an individual believes is the right rock to look under, it only matters that the rock is getting looked under. In other words, it is completely fine for an individual scientist to be utterly biased in what they choose to consider (Einstein was biased that the universal laws should be the same for all observers, for example), their success or failure will eventually dictate the truth regardless of their individual bias. But group bias is another matter altogether, because that dictates what rocks get attention, and what rocks get overlooked. Still, one needs focusing strategies, one does not want blind squirrels everywhere. So the question keeps coming back to, why does the astrophysical community use a focusing strategy firmly dominated by dark matter, and the internet afficionados use a focusing strategy dominated by MOND, and are both just examples of group bias?

There is also some wokeness involved. The story is that Dark Matter was discovered by Vera Rubin, not Fritz Zwicky, and as such it is all about rotation curves. That's not true, and it overfocuses (IMHO) attention on that piece of the puzzle. It's a good story, and I understand why the public eats it up, but as usual reality is not so black and white.

That's quite an ironic point, because that video makes the point that too much focus on solving rotation curves, rather than the larger puzzle, is what leads to MONDlike thinking. So you're agreeing with her that focusing on Vera Rubin's piece is also focusing too much on solving rotation curves, which is where MOND does better (at least according to that "score sheet" presented above, I don't know how controversial that is). But if the astrophysics community is too focused on rotation curves, they should be led to MOND, instead of dark matter. Why don't we say, therefore, that Vera Rubin discovered the need for MOND, rather than the need for dark matter?

What we see is more acceleration than we expect. There are three possibilities:

1. There is more mass than we think
2. Gravity is stronger than we think
3. At small acclerations there is less inertia per given mass.

Or of course, some combination thereof. Normally, we think of combinations as ugly, but that's why I brought up the Newton vs. Huygens debate. That was also resolved by a combination, but not an ugly one at all, it was quantum mechanics. Sublime and shocking yes, ugly no. I wonder if the resolution of the gravitational dynamics problem will be sublime and shocking, and will make a simple "search for dark matter" look like a red herring in hindsight, perhaps akin to the search for the aether. On the other hand, I agree there is no reason why most of the matter in the universe should come replete with low mass charged particles, that would be a classic bias toward things we can see. But the answer might involve generalizing our concept of what mass is and where it comes from, as well as how gravity and inertia work. Hopefully, in a way that is more beautiful and sublime than either new particles or parametric modifications to our laws!

If I'm wrong, we'll be glad we spent so much time and money looking for new types of particles that still behave like we expect particles to behave. If I'm right, MOND is not going to be the answer either, but it could well be a stepping stone to the answer, in the sense that its effectiveness is a clue to something hiding deeper under the surface. There is also a practical utility to MOND: it can provide a low order parametrization of the dynamics we actually see, even if you don't regard it as an explanatory theory in its own right (and I agree it is certainly not that).

 

[ohwilleke]

That's really a sociology question.

The "general public" believes all sorts of things. There are people who study this, people not me.

The theory community writes more papers on DM because there is more to say. Maybe its the LSP - but which particle? Maybe its two particles. Maybe its axions. Maybe its PBH's. MOND is sterile. "Here's a new law of gravity, Deal with it." The experimental/observational community hears these talks and responds as you might expect.

Remember too, that most asstronomers do not work on either family of models. If I study, say mass transfer and blue stragglers, my primary information comes from what other people tell me, in seminars and papers. So there is an amplification effect.

At the same time, MOND is incomplete. It's non-relativistic. People don't like that. It's also quite difficult to test in the lab. People really don't like that. \\

Also, Scientists are people too. At least 85% of us anyway. People get emotionally invested in their ideas. I an sure I have written more papers om DM than Stacy has written on MOND - including two moderately influential theory papers. Does that bias me? Maybe. How could I tell?

There is also some wokeness involved. The story is that Dark Matter was discovered by Vera Rubin, not Fritz Zwicky, and as such it is all about rotation curves. That's not true, and it overfocuses (IMHO) attention on that piece of the puzzle. It's a good story, and I understand why the public eats it up, but as usual reality is not so black and white.

What we see is more acceleration than we expect. There are three possibilities:

1. There is more mass than we think
2. Gravity is stronger than we think
3. At small acclerations there is less inertia per given mass.

I can think of another:

4. There are one or more non-gravitational fifth forces.

5. There could be entropy effects.

6. There could be mixed models.

Gravity might have different strengths that are a functions of the shape of a mass distribution.

The way that I like to describe the original 1983 version of MOND is "toy model MOND" and it has a limited domain of applicability. Basically, the assumption is that a less toy model-ish MOND would be indistinguishable from GR at gravitational field strengths much above a₀ and that MOND gravitational forces would have the qualitative features of GR (e.g., it would bend light, unlike true Newtonian gravity, to the same extent as a GR effect of the same magnitude, and gravity travels at the speed of light not instantaneously). This is realized, for example, in versions of MOND that extend it to the timing of structure formation in the early Universe (addressing the impossible early galaxies problem) and generalizations of MOND that reproduce the CMB. Now, of course, even non-toy-model MOND has to violate the strong equivalence principle due to the external field effect, so it isn't quite GR in the strong field limit.

I also prefer the phase "dark matter phenomena" to Sabine's, but her terminology on that point is good enough.

I do think that the gravitational route is going to be more fruitful than the dark matter particle route, although I see a potential path to convergence. One of the big arguments in favor of the gravitational route is that "dark matter phenomena" can be predicted purely from baryonic matter distributions to a high degree of precision, with fine details of bumps and curves in rotation curves, for example, mirrored in the source baryonic matter distributions.

Love him or hate him, another point which I think Alexandre Deur has made quite convincingly is that the variation in dark matter phenomena among elliptical galaxies, and the cluster scale problems of MOND can both be addressed by considering the geometry of the mass distributions. This solves the stronger MOND effect in clusters (although improved modeling also shows that the MOND apparent DM deficit in clusters isn't as bad as initially believed), and solves systems like the Bullet Clusters which directly rule out MOND. Something is correlating the shape of matter distributions with dark matter phenomena that are observed, and the correlation has to have some cause, even though correlation is not causation.

Back to the point of convergence. Assuming that dark matter phenomena have a gravitational cause and quantum gravity, effectively, gravitons become dark matter particles. And, gravitons gravitate and each one has a mass-energy that is tiny but non-zero.

Ultra-light dark matter candidates such a fuzzy dark matter and very light axion-like particles similarly have extremely low, but non-zero mass-energy per particle, producing wave-like behavior with wave lengths on the same order of magnitude as graviton wave-lengths. See, e.g., Tonatiuh Matos, Luis A. Ureña-López, Jae-Weon Lee, "Short Review of the main achievements of the Scalar Field, Fuzzy, Ultralight, Wave, BEC Dark Matter model" arXiv:2312.00254 (November 30, 2023); Tim Zimmermann, et al., "Dwarf galaxies imply dark matter is heavier than 2.2 × 10^−21eV" arXiv:2405.20374 (May 30, 2024).

The more we get data, the more it favors lower mass, wave-like candidates for dark matter in a dark matter particle paradigm.

For example, simulations of warm dark matter show that it doesn't solve many of the galaxy scale problems of cold dark matter. Neither do self-interacting dark matter models with keV and more massive fermions plus a carrier boson for the self-interaction piece. You need any dark matter particle candidate to have interactions not just with other dark matter but also with baryonic matter to reproduce the tight linkage between dark matter phenomena and baryonic matter distributions.

As Stacy McGaugh has noted repeatedly, feedback doesn't get you there either.

Now, there is data we are missing from a theory discrimination standpoint. For example, the jury is still out, as far as I am concerned, on the wide binary data at this point. It will probably be resolved eventually, but we aren't there yet and this data is pretty important to deciding what a more correct modified gravity theory looks like beyond toy-model MOND.

Another body of data that is important for theory discrimination and hasn't gotten enough attention is the dynamics of stars near spiral galaxies that aren't in the plane of the spiral.

And, there are definitely more than three people doing MOND. There may be half a dozen real "stars" in that research program, but that's not it. And, McGaugh, much as I love him, along with the whole DM particle community, are also ignoring a lot of other work by less famous and less well-cited folks on other gravity based programs to explain DM phenomena that reproduce the core conclusions of MOND in the easy cases of galaxy dynamics, which in spiral galaxies are mostly dynamics of matter in the plane of the spiral.

 

[strangerep]

[...] McGaugh, much as I love him, along with the whole DM particle community, are also ignoring a lot of other work by less famous and less well-cited folks on other gravity based programs to explain DM phenomena that reproduce the core conclusions of MOND in the easy cases of galaxy dynamics, which in spiral galaxies are mostly dynamics of matter in the plane of the spiral.

Which folks? Which theories?

(And you think galaxy dynamics in the plane of the spiral is "easy"??)

 

[ohwilleke]

Which folks? Which theories?

(And you think galaxy dynamics in the plane of the spiral is "easy"??)

This is a good and fair question, but takes a little time to compile a list of, since my notes on this are piecemeal and I've never really compiled them in one place, and it probably deserves a new thread of its own.

Galaxy dynamics in the plane of the spiral that are consistent with the Tully-Fischer relation can be achieved in multiple ways due to the symmetries present. Additional tests of galaxy dynamics (e.g. wide binaries, galaxy clusters, the Bullet cluster, of plane dynamics) are more theory discriminating.

 

[strangerep]

Galaxy dynamics in the plane of the spiral that are consistent with the Tully-Fischer relation can be achieved in multiple ways due to the symmetries present.

Well, I await a well-founded justification of your assertion. (It seems to me to be speculative wishful thinking, but I'm willing to be convinced otherwise, if the math holds up.)

 

[ohwilleke]

Well, I await a well-founded justification of your assertion. (It seems to me to be speculative wishful thinking, but I'm willing to be convinced otherwise, if the math holds up.)

As a quick example to get across the idea, a spiral galaxy's rotation curve is fairly insensitive in a dark matter particle scenario to (1) a halo in the plane of the galaxy, (2) a rugby ball shaped halo (more typical of inferred halo shapes), (3) a halo in a sphere around it (like the NFW which observation shows is very rarely how inferred DM distributions are shaped), or (4) masses distributed in a spinning top-like configuration where mass is on the plane and at the axis in line with a galaxy's ultrafast outflows up and down from the plane at the axis.

MOND is unidirectional, somewhat in analogy to a spherical halo.

Other gravity based approaches, e.g. Deur and those that argue for some increase in gravitomagnetic forces over those found in GR, increase gravitational pull just in the plane of the galaxy.

Observational data from the dynamics of out of plane stars can discriminate between these kinds of different theories.

 

[berkeman]

Thread closed for Moderation...

 

[PeterDonis]

An off topic post has been deleted. Thread reopened.

 

[PeterDonis]

instead of asking "which one is what's really happening," ask simply "which one provides the simpler way to parametrize what's happening."

But in order to parametrize, you need a model with adjustable parameters, and you can't just put them in by hand.

"Dark matter" is the name for the model that uses our standard theory of gravity and takes advantage of the key adjustable parameter in it, which is the distribution of matter (or stress-energy in GR).

"MOND" is the name for a set of models that change the theory of gravity; but what the new adjustable parameter is depends on the model. In the original MOND the adjustable parameter was a threshold acceleration.

But in either case, asking which parametrization is simpler gets things backwards. Nobody picks the simpler parametrization (how would you even do that anyway?) and then says, ok, let's pick the model that has that. Everybody does it the other way: they first pick the model they prefer, and then adjust its parameters to fit the data as best they can.

 

[ohwilleke]

"MOND" is the name for a set of models that change the theory of gravity; but what the new adjustable parameter is depends on the model. In the original MOND the adjustable parameter was a threshold acceleration.

MOND is the name for one model that changes the theory of gravity and some slight variations on it. It is not the name for all models that change the theory of gravity. Essentially all MOND models have the same threshold acceleration adjustable parameter a₀ and are just extensions of the original toy-model MOND.

 

[PeterDonis]

It is not the name for all models that change the theory of gravity.

Is there are more general name for that class of models?

 

[ohwilleke]

Is there are more general name for that class of models?

I usually call models that don't use dark matter particles to explain "dark matter phenomena" either "gravity based models" or "modified gravity based models".

"Modified gravity models" is more commonly used than "gravity based models". See, e.g., Kyu-Hyun Chae, "Distinguishing Dark Matter, Modified Gravity, and Modified Inertia by the Inner and Outer Parts of Galactic Rotation Curves" arXiv:2207.11069 (July 22, 2022). But I prefer "gravity based models" because some of the models seek to develop explanations for dark matter phenomena without purporting to modify GR (whether or not they are successful in doing so is another matter, either way, they are proposals on the table).

Some of the more notable theories that are gravity based models but are not MOND include (in part in response to an earlier request at #29 in which strangerep said: "Which folks? Which theories?", I don't take on the task of identifying the proper MOND research community in this post):

* Moffat's MOG, see, e.g., J.W. Moffat (2015); MOG also explains the cosmic microwave background power spectrum. J.W. Moffat (2001); John W. Moffat, "Wide Binaries and Modified Gravity (MOG)" arXiv:2311.17130 (November 28, 2023), Mahmood Roshan, "Stellar Bar evolution in the absence of dark matter halo" (January 25, 2018);

* Verlinde's entropic/emergent gravity, see e.g., Erik P. Verlinde, "Emergent Gravity and the Dark Universe" arXiv:1611.02269 (November 7, 2016), and also Jungjai Lee, Hyun Seok Yang, "Dark Energy and Dark Matter in Emergent Gravity" arXiv:1709.04914 (September 14, 2017, last revised November 1, 2022) (published at 81(9) Journal of the Korean Physical Society 910-920 (2022)); A. Schlatter, R. E. Kastner, "Gravity from Transactions: A Review of Recent Developments" arXiv:2209.04025 (September 8, 2022);

* Deur's work, see, e.g., A. Deur, "Effect of the field self-interaction of General Relativity on the Cosmic Microwave Background Anisotropies" arXiv:2203.02350 (March 4, 2022)), with a somewhat similar approach from Yuta Ito, "Gravitational Amplification of Test-Mass Potential in the Self-gravitating Isothermal Gaseous Systems" arXiv:2303.02631 (March 5, 2023);

* f(R) gravity, Eddington-inspired-Born-Infeld (EiBI) and general relativity with renormalization group effects (RGGR) (three different modified gravity theories discussed in one paper) see, e.g., Alejandro Hernandez-Arboleda, Davi C. Rodrigues, Aneta Wojnar, "Normalized additional velocity distribution: a fast sample analysis for dark matter or modified gravity models" arXiv:2204.03762 (April 7, 2022);

* f(R) gravity, see, e.g., Vesna Borka Jovanović, Predrag Jovanović, Duško Borka, Salvatore Capozziello, "Fundamental plane of elliptical galaxies in f(R) gravity: the role of luminosity" (December 28, 2018);

* f(Q) gravity, see, e.g., Gaurav N. Gadbail, "Cosmological dynamics of interacting dark energy and dark matter in f(Q) gravity" arXiv:2406.02026 (June 4, 2024);

* f(T) gravity, see, e.g., A. R. P. Moreira, "Geometrically contracted structure in teleparallel f(T) gravity" arXiv:2212.08948 (December 17, 2022);

* scalar-tensor gravity theories, see, e.g., Thomas P. Sotiriou, Valerio Faraoni, "Modified gravity with R-matter couplings and (non-)geodesic motion" arXiv:0805.1249 (September 27, 2008);

* long range quantum gravity, see, e.g., Matteo Tuveri, Mariano Cadoni "Galactic dynamics and long-range quantum gravity" arXiv:1904.11835 (April 26, 2019);

* modified general relativity, see, e.g., Gary Nash, "Modified general relativity" arXiv:1904.10803 (April 22, 2019);

* metric skew tensor gravity, see, e.g., W.M. Stuckey, Timothy McDevitt, A.K. Sten, Michael Silberstein, "The Missing Mass Problem as a Manifestation of GR Contextuality" 27(14) International Journal of Modern Physics D 1847018 (2018). DOI: 10.1142/S0218271818470181;

* curvature models of gravity, see, e.g., Valeri P. Frolov, "Limiting curvature models of gravity" arXiv:2111.14318 (November 29, 2021);

* conformal gravity a.k.a. Weyl Conformal gravity, see, e.g., Philip D. Mannheim, "Is dark matter fact or fantasy? -- clues from the data" (March 27, 2019), James G. O'Brien, et al., "Radial Acceleration and Tully-Fisher Relations in Conformal Gravity" (December 7, 2018), Philip D. Mannheim, "Making the Case for Conformal Gravity" (October 27, 2011), and Leonardo Modesto, Tian Zhou, Qiang Li, "Geometric origin of the galaxies' dark side" arXiv:2112.04116 (December 8, 2021);

* GR with torsion added, see, e.g., S. H. Pereira, et al., "Dark matter from torsion in Friedmann cosmology" arXiv:2202.01807 (February 3, 2022);

* negative mass models, see, e.g., Hector Socas-Navarro, "Can a negative-mass cosmology explain dark matter and dark energy?" arXiv:1902.08287 (February 21, 2019);

* the effort of Naman Kumar to explain the dark energy with an anti-matter mirror universe, see, e.g., Naman Kumar, "On the Accelerated Expansion of the Universe" 30 Gravitation and Cosmology 85-88 (April 4, 2024);

* string theory/brane theory approaches, see, e.g., Naman Kumar, "Variable Brane Tension and Dark Energy" arXiv:2404.17941 (April 27, 2024);

* non-Verlinde efforts based on entropy and Mach's principle, see, e.g., Santanu Das, "Aspects of Machian Gravity (III): Testing Theory against Galaxy Cluster mass" arXiv:2312.06312 (December 11, 2023) and Kimet Jusufi, Ahmad Sheykhi, Salvatore Capozziello, "Apparent dark matter as a non-local manifestation of emergent gravity" arXiv:2303.14127 (March 23, 2023), Rubén Arjona, et al., "A GREAT model comparison against the cosmological constant" arXiv:2111.13083 (November 25, 2021). Report number: IFT-UAM/CSIC-2021-136m, and Andre Maeder "Dynamical Effects of the Scale Invariance of the Empty Space: The Fall of Dark Matter?" 849(2) The Astrophysical Journal 158 (November 10, 2017) (pre-print here);

* general co-variance breaking gravity, see, e.g., Alexander P. Sobolev, "Foundations of a Theory of Gravity with a Constraint and its Canonical Quantization" 52 Foundations of Physics Article number: 3 arXiv:2111.14612 (open access, pre-print November 25, 2021, publication date anticipated 2022) DOI: 10.1007/s10701-021-00521-1;

* Pascoli's K-model, see, e.g., Gianni Pascoli, "A comparative study of MOND and MOG theories versus the κ-model: An application to galaxy clusters" arXiv:2307.01555 (July 4, 2023);

* GEM and other perturbative GR effect approaches, see, e.g., Kostas Glampedakis, David Ian Jones, "Pitfalls in applying gravitomagnetism to galactic rotation curve modelling" arXiv:2303.16679 (March 29, 2023), A. N. Lasenby, M. P. Hobson, W. E. V. Barker, "Gravitomagnetism and galaxy rotation curves: a cautionary tale" arXiv:2303.06115 (March 10, 2023), Yogendra Srivastava, Giorgio Immirzi, John Swain, Orland Panella, Simone Pacetti, "General Relativity versus Dark Matter for rotating galaxies" arXiv:2207.04279 (July 9, 2022), G. O. Ludwig, "Galactic rotation curve and dark matter according to gravitomagnetism" 81 The European Physical Journal C 186 (February 23, 2021) (open access), F.I. Cooperstock, S. Tieu, "Galactic dynamics via general relativity: a compilation and new developments." 22 Int. J. Mod. Phys. A 2293–2325 (2007). arXiv:astro-ph/0610370 See also follow up papers in 2007, in 2011, and 2015; H. Balasin, D. Grumiller, "Non-Newtonian behavior in weak field general relativity for extended rotating sources." 17 Int. J. Mod. Phys. D 475–488 (2008) (arXiv version here); M. Crosta, M. Giammaria, M.G. Lattanzi, E. Poggio, "On testing CDM and geometry-driven Milky Way rotation curve models with Gaia DR2." 496 Mon. Not. R. Astron. Soc. 2107–2122 (2020) (open access); Missing Mass Problem as a Manifestation of GR Contextuality" 27(14) International Journal of Modern Physics D 1847018 (2018). DOI: 10.1142/S0218271818470181, Federico Re, "Fake dark matter from retarded distortions" (May 30, 2020), Felipe J. Llanes-Estrada, "Elongated Gravity Sources as an Analytical Limit for Flat Galaxy Rotation Curves" 7(9) Universe 346 arXiv:2109.08505 (September 16, 2021) DOI: 10.3390/universe7090346, P. Tremblin, et al., "Non-ideal self-gravity and cosmology: the importance of correlations in the dynamics of the large-scale structures of the Universe" arXiv:2109.09087 (September 19, 2021) (submitted to A&A, original version submitted in 2019), Priidik Gallagher, Tomi Koivisto, "The Λ and the CDM as integration constants" arXiv (March 9, 2021), and Johan Hansson, et al., Nonlinear Effects of Gravity in Cosmology arXiv:1805.11043 (2016);

* non-local gravity theories, see, e.g., Ivan Kolář, Tomáš Málek, Anupam Mazumdar, "Exact solutions of non-local gravity in class of almost universal spacetimes" arXiv: 2103.08555; Reza Pirmoradian, Mohammad Reza Tanhayi, "Non-local Probes of Entanglement in the Scale Invariant Gravity" arXiv: 2103.02998, J. R. Nascimento, A. Yu. Petrov, P. J. Porfírio, "On the causality properties in non-local gravity theories" arXiv: 2102.01600, Salvatore Capozziello, Maurizio Capriolo, Shin'ichi Nojiri, "Considerations on gravitational waves in higher-order local and non-local gravity" arXiv: 2009.12777, Jens Boos, "Effects of Non-locality in Gravity and Quantum Theory" arXiv: 2009.10856, Jens Boos, Jose Pinedo Soto, Valeri P. Frolov, "Ultrarelativistic spinning objects in non-local ghost-free gravity" arXiv: 2004.07420;

* massive graviton theories, see, e.g., Kimet Jusufi, Genly Leon, Alfredo D. Millano, "Dark Universe Phenomenology from Yukawa Potential?" arXiv:2304.11492 (May 14, 2024) (Phys. Dark Univ. 42 (2023), 101318), Kimet Jusufi, et al., "Modified gravity/entropic gravity correspondence due to graviton mass" arXiv:2405.05269 (April 25, 2024), and O. Costa de Beauregard, "Massless or massive graviton?" 3 Foundations of Physics Letters 81-85 (1990));

* varying G and running gravitational coupling constant approaches, see, e.g., Hikaru Kawai, Nobuyoshi Ohta, "An Observation on the Beta Functions in Quadratic Gravity" arXiv:2405.05706 (May 9, 2024), Dimitris M. Christodoulou, Demosthenes Kazanas, "Gravitational Potential and Nonrelativistic Lagrangian in Modified Gravity with Varying G" (November 21, 2018);

* fifth force models, see, e.g., Marcus Högås, Edvard Mörtsell, "The Hubble tension and fifth forces: a cosmic screenplay" arXiv:2309.01744 (September 4, 2023); and

* Einstein-Aether theories, see, e.g., Vincenzo F. Cardone, Ninfa Radicella, "Can MONDian vector theories explain the cosmic speed up?" arXiv:0908.0095 (August 1, 2009) ("Generalized Einstein - Aether vector field models have been shown to provide, in the weak field regime, modifications to gravity which can be reconciled with the successful MOND proposal.").

See also McGaugh's tree of gravity based theories:

Another summary from a conference paper illustration is:

 

[Ken G]

But in order to parametrize, you need a model with adjustable parameters, and you can't just put them in by hand.

"Dark matter" is the name for the model that uses our standard theory of gravity and takes advantage of the key adjustable parameter in it, which is the distribution of matter (or stress-energy in GR).

"MOND" is the name for a set of models that change the theory of gravity; but what the new adjustable parameter is depends on the model. In the original MOND the adjustable parameter was a threshold acceleration.

But in either case, asking which parametrization is simpler gets things backwards. Nobody picks the simpler parametrization (how would you even do that anyway?) and then says, ok, let's pick the model that has that. Everybody does it the other way: they first pick the model they prefer, and then adjust its parameters to fit the data as best they can.

I would remind you that when Planck invented his constant, he did it the way you are saying no one does it. He simply chose a one-parameter solution to the ultraviolet catastrophe. He had no model in mind for doing that, he always regarded it as a mathematical trick. The model came later. I actually think that historical sequence is more common than you imagine (consider for example the Rydberg series of energies in the hydrogen atom, discovered 35 years before quantum mechanics.) I'm suggesting that what MOND looks like today might end up like what the Rydberg series was in 1890.

 

[PeterDonis]

when Planck invented his constant, he did it the way you are saying no one does it. He simply chose a one-parameter solution to the ultraviolet catastrophe. He had no model in mind for doing that

Choosing the one-parameter solution is choosing a model. The fact that Planck just considered it a "mathematical trick" does not mean it isn't a model. A model is an algorithm for making predictions. What the person using it thinks of it is irrelevant.

The model came later.

No, a physical interpretation of the model that physicists could accept (and not all of them even then--IIRC Planck never did accept it) came later.

 

[PeterDonis]

consider for example the Rydberg series of energies in the hydrogen atom, discovered 35 years before quantum mechanics

This is an example of a first, very crude model being replaced by a better one later on.

I'm suggesting that what MOND looks like today might end up like what the Rydberg series was in 1890.

Yes, that's possible, but it doesn't mean MOND as it is today is not a model. It is. Possibly a very crude one that will end up being replaced by a better one later on. Or possibly just a mistaken one that will end up being discarded. We'll see.

 

[ohwilleke]

Yes, that's possible, but it doesn't mean MOND as it is today is not a model. It is. Possibly a very crude one that will end up being replaced by a better one later on. Or possibly just a mistaken one that will end up being discarded. We'll see.

Definitely.

Indeed, my preferred description of the original version of MOND proposed in 1983 by Milgrom is "toy-model MOND", because not even its creator believes it to be a final theory that has an unlimited domain of applicability.

MOND is only intended to apply in weak field astrophysics cases where Newtonian gravity is used as an approximation because relativistic effects are believed to be small.

It is also widely known among MOND proponents, and had been known from a very early date, that MOND underestimates dark matter phenomena in galactic cluster phenomena.

But, not a single one of the leading MOND astrophysicists doubts the accuracy of general relativity relative to Newtonian gravity in settings like black holes or the perihelion of Mercury or the impact of gravity on clocks at different altitudes on Earth.

Essentially, scientists who are MOND proponents really believe that in gravitational fields that are stronger than the MOND constant a₀, that general relativity (or something observationally indistinguishable from it in those contexts), and not Newtonian gravity, is correct. It just so happens that in many astrophysical contexts, Newtonian gravity and general relativity are indistinguishable using modern astronomy instruments.

Likewise, all MOND proponents believe that gravitational fields impact massless photons, and not just matter (even in the MOND regime, to the same extent as in GR for gravitational fields of comparable strength) even though this isn't the case in Newtonian gravity. And, almost all of them (except non-local gravity advocates who believe that MOND is a product of non-local gravity effects) believe that gravitational effects propagate at the speed of light, which also is true in general relativity but isn't the case in Newtonian gravity.

I have also never encountered or read anything from a MOND proponent that doubts the validity of special relativity (a.k.a. Lorentz invariance), outside the currently impossible to detect slight variations that might be induced by discreteness in space-time (perhaps at the Planck scale) or some other slight quantum gravity related tweak.

The stunning thing is not that the simple tweak to Newtonian gravity of toy-model MOND doesn't have an unlimited domain of applicability. This was known by the scientist who proposed it before it was even proposed. The stunning thing is that toy-model MOND (including its external field effect) works as well as it does in essentially all galaxy scale contexts that involve galaxies in or near equilibrium. It is predictive far beyond the context of the data sets used to devise it. This was totally unexpected.

There is also one big question mark in the domain of applicability of MOND, which is whether it applies to wide binary stars (i.e. pairs of stars that are in a gravitationally bound system, where the strength of the gravitational field between them if Newtonian gravity were correct in that case would be weaker than a₀ because they are so far apart) in places that are not subject to a full external field effect. This is an area of bleeding edge research. The current analysis of this question from several different research groups is not in agreement at this point, despite the fact that they are all using the same, or at least overlapping, source data from astronomy observations.

 

[strangerep]

There is also one big question mark in the domain of applicability of MOND, which is whether it applies to wide binary stars (i.e. pairs of stars that are in a gravitationally bound system, where the strength of the gravitational field between them if Newtonian gravity were correct in that case would be weaker than a₀ because they are so far apart) in places that are not subject to a full external field effect. This is an area of bleeding edge research.

This is no longer a "question mark" for anyone who has actually studied and understood the most recent papers that use several different statistical analysis methods. This new version (v3) from Chae performs yet a 3rd type of analysis, different from the earlier "acceleration-plane" and "stacked velocity profile" analyses. All 3 analyses agree well with each other.

The current analysis of this question from several different research groups is not in agreement at this point, despite the fact that they are all using the same, or at least overlapping, source data from astronomy observations.

I've noticed that Bannik's paper begins to receive ridicule, not least from the fact that he refuses to engage Chae or Hernandez in constructive debate even after they have carefully pointed out problems in his analysis method (e.g., he uses velocity cell sizes that are incompatible with the error bounds on the data).

Separately, there's also this new paper on Indefinitely Flat Circular Velocities and the BTFR from Weak Lensing by Mistele, McGaugh, Lelli, Schombert and Li, which analyzes weak lensing data from the KiDS survey in a new way (due to Mistele).

Abstract:

We use a new deprojection formula to infer the gravitational potential around isolated galaxies from weak gravitational lensing. The results imply circular velocity curves that remain flat for hundreds of kpc, greatly extending the classic result from 21 cm observations. Indeed, there is no clear hint of a decline out to 1 Mpc, well beyond the expected virial radii of dark matter halos. Binning the data by mass reveals a correlation with the flat circular speed that closely agrees with the Baryonic Tully-Fisher Relation known from kinematic data. These results apply to both early and late type galaxies, indicating a common universal behavior.

McGaugh talks about this on his recent blog entry, titled Rotation Curves Still Flat After A Million Light Years.

 

[pinball1970]

This recently also on phys.org

https://phys.org/news/2024-06-black-holes-dark-explanation.html

"University of Warsaw have announced the results of nearly 20-year-long observations indicating that such massive black holes may comprise at most a few percent of dark matter."

https://www.nature.com/articles/s41586-024-07704-6

 

[ohwilleke]

This is no longer a "question mark" for anyone who has actually studied and understood the most recent papers that use several different statistical analysis methods. This new version (v3) from Chae performs yet a 3rd type of analysis, different from the earlier "acceleration-plane" and "stacked velocity profile" analyses. All 3 analyses agree well with each other.

I've noticed that Bannik's paper begins to receive ridicule, not least from the fact that he refuses to engage Chae or Hernandez in constructive debate even after they have carefully pointed out problems in his analysis method (e.g., he uses velocity cell sizes that are incompatible with the error bounds on the data).

McGaugh has read them and understood them, and he currently takes an inconclusive position on their results. The conclusions of Chae may be consistent, but AFAIK none of the at least three groups looking at wide binaries have reached the same result looking at that data.

I have read and blogged McGaugh's latest paper. The money chart is:

If confirmed, it is huge, because it would more or less generically rule out almost all kinds of dark matter particle theories. But since the methods are novel, it is probably appropriate to give astrophysicists and astronomers a little time to see if they can poke holes in it first.

 

[ohwilleke]

This recently also on phys.org

https://phys.org/news/2024-06-black-holes-dark-explanation.html

"University of Warsaw have announced the results of nearly 20-year-long observations indicating that such massive black holes may comprise at most a few percent of dark matter."

https://www.nature.com/articles/s41586-024-07704-6

This tightens but doesn't completely rule out primordial black hole (PBH) parameter space for some asteroid sized PBHs. The allowed range is discussed at:

Manish Tamta, Nirmal Raj, Prateek Sharma, "Breaking into the window of primordial black hole dark matter with x-ray microlensing" arXiv:2405.20365 (May 30, 2024).

Nonetheless, PBHs have been a dead man walking as a dark matter candidate for a long time and it may be possible to close the remaining window before too long (maybe a few years).

PBHs tempting, because they don't really require any new laws of physics, just some tweaks to the narrative shortly after the Big Bang. But so far there isn't any sign of them (i.e. not a single one has ever been observed) when they should be ubiquitous if they make up most of dark matter.

 

[strangerep]

[...] AFAIK none of the at least three groups looking at wide binaries have reached the same result looking at that [wide binary] data.

Er,... which "three groups" are you referring to?

 

[ohwilleke]

which "three groups" are you referring to?

Hernandez, Chae, and Banik.

Hernandez et al. find γ = 1.0±0.1 for 466 close binaries with 2D separations less than 0.01 pc (about 2000 AU) and γ = 1.5±0.2 for 108 wide binaries with 2D separations greater than 0.01 pc. A purely Newtonian result (γ = 1) is recovered in the high acceleration regime of relatively close binaries where this is expected to be the case. For wider binaries, one finds a boost value consistent with the prediction of MOND and differing from Newton with modest significance (2.6σ).

Chae reports+ γ = 1.49(+0.21/-0.19) for 2,463 “pure” binaries in the low acceleration regime, consistent with his earlier result γ = 1.43±0.06 for 26,615 wide binaries. The larger numbers make the formal error smaller, hence a formally more significant departure from Newton. Many of these binaries are impure in the sense of being triples with one member being itself a close binary as discussed previously, an effect that has to be modeled in large samples. The point of the smaller samples is to select true binaries so that this modeling is unnecessary. For his smaller pure binary sample, Chae finds a smooth transition from γ ≈ 1 at high acceleration (10-8 m/s/s ≈ 100a0) through γ ≈ 1.11 around 7a0 to γ ≈ 1.49 at local Galactic saturation (1.8 a0).

Banik et al. use a slightly different language. Translating, they find γ = 1 at high confidence (16σ)$ from 8,611 wide binaries with separations from 2,000 to 30,000 AU. Newtonian behavior persists at all scales and accelerations; they find no significant deviations from γ = 1 anywhere. Note that despite going out very far, to 30,000 AU, they do not reach especially low accelerations because the EFE of the Galaxy is effectively constant in the solar neighborhood. There is no getting away from the Galaxy’s 1.8 a0. They also do not reach particularly high, purely Newtonian accelerations: 2,000 AU is in the transition regime where MOND effects are perceptible.

Quoting this November 23, 2023 blog post.

 

[Ken G]

Choosing the one-parameter solution is choosing a model. The fact that Planck just considered it a "mathematical trick" does not mean it isn't a model. A model is an algorithm for making predictions. What the person using it thinks of it is irrelevant.

The point was in relation to the claim "Nobody picks the simpler parametrization (how would you even do that anyway?) and then says, ok, let's pick the model that has that." That's exactly what Planck did, he found the simplest possible parametrization, without worrying about anything else. I'm saying that if you call every mathematical expression a model, then the distinction you are drawing (that people don't start with parametrizations they start with a model they like) does not exist, because parametrizations and models are the same thing. But it doesn't matter the semantics of "model," it matters that sometimes we make progress by finding nothing more than the simplest possible parametrization, which is more or less what MOND is.

No, a physical interpretation of the model that physicists could accept (and not all of them even then--IIRC Planck never did accept it) came later.

Yes, that's what I'm saying. The issue is not what we will call a model, that's just definitions. The issue is the way progress is made, which sometimes starts with a simple parametrization and later leads to physical interpretations. Those physical interpretations often involving adding a lot of explanatory scaffolding to the original parametrization, and can also extend the parametrization into a broader context. That might be what eventually happens to MOND, where what now just looks like a parametrization emerges from something akin to how the Rydberg series emerges from the Schroedinger equation, whereby the latter applies in a lot of places the former does not. MOND is waiting for its Schroedinger equation, and if it finds it, it will likely be taken seriously more broadly than it is now.

 

[PeterDonis]

That's exactly what Planck did, he found the simplest possible parametrization

It seems like you are agreeing with me; you just don't like calling this process "picking a model with the simplest parameterization". I can't understand why you object to that phrasing, but in any case, what Planck did is an example of what I was describing.

My point about "models" is that different models (what I'm calling models) have different parametrizations. There might be more than one model that just has one adjustable real parameter, for example--more than one model in the sense of more than one mathematical engine that makes predictions, and the predictions can be different for different models even if they both have just one adjustable real parameter. Even if you just view what is being done as a "mathematical trick", as Planck did, there can still be multiple different "mathematical tricks" you can play with one adjustable real parameter. So you can't just describe what is being done as "pick the simplest possible parametrization, just one adjustable real parameter". You still have to describe what is being done, mathematically, with that parameter: what equation or equations does it get plugged into? What are the resulting predictions? All that is part of what I was calling a "model". If you don't like the word "model" to describe that, well, suggest some other word. But you can't just ignore it.

The issue is the way progress is made, which sometimes starts with a simple parametrization and later leads to physical interpretations. Those physical interpretations often involving adding a lot of explanatory scaffolding to the original parametrization, and can also extend the parametrization into a broader context. That might be what eventually happens to MOND

Again, you are agreeing with me here, since this is what I said might happen with MOND. Or it might not. We'll see.