Commentary on Wide Binaries

Last time, I commented on the developing situation with binary stars as a test of MOND. I neglected to enable comments for that post, so have done so now.

Indranil Banik has shared his perspective on wide binaries in a talk on the subject that is available on Youtube, included below.

Indranil and his collaborators are not seeing a MOND effect in wide binaries. Others have, as I discussed in the previous post. After the video posted above, Indranil comments on the work of Kyu-Hyun Chae:

Regarding the article by Chae (, equation 7 of MNRAS 506, 2269–2295 (2021) shows that the relative velocity is limited such that the v_tilde parameter (ratio of relative velocity within the sky plane to the Newtonian circular velocity at the projected separation) is at most 1 for 5 M_Sun binaries and in general is sqrt(5 M_Sun/M) for a binary of total mass M. This means v_tilde only goes up to 2 for M = 1.25 M_Sun, but more generally it goes up to a higher value at lower mass. Since the main signal in MOND is a broader v_tilde distribution at lower acceleration and a lower mass reduces the acceleration, this can lead to an artificial signal whereby lower mass systems have a larger rms v_tilde. Now a simple rms statistic is not exactly what Chae did, but this does highlight the kind of problem that can arise. Indeed, the v_tilde distribution prepared by Chae for the article in its figure 25 does show a rather sharp decline in the v_tilde distribution – there is not much of an extended tail, even less than in the model! This is obviously not due to measurement errors and contaminating effects like chance alignments, which would broaden the tail further. Rather, it is due to the upper limit to v_tilde imposed from the sample selection. This just means the underlying sample used is not well suited to the wide binary test, since it was quite clear a priori that the main signal for MOND would be in the region of v_tilde = 1-1.5 or so. One possibility is to try and restrict the analysis to a narrower range of binary total mass to try and alleviate the above concern, in which case the upper limit to v_tilde would be perhaps above 2 for the full sample used. There is however another issue in that lower accelerations generally correspond to higher separations and thus lower orbital velocities, so the fractional uncertainty in the velocity is likely to be larger. Thus, the v_tilde distribution is likely to be broader at low accelerations. This can be counteracted by having low errors across the board, but then the key quantity is the uncertainty on v_tilde. This aspect is not handled very rigorously – it is assumed that if the proper motions are accurate to better than 1%, then v_tilde will be sufficiently well known. But if the tangential velocity is about 20 km/s, a 1% error means an error of 200 m/s on the velocity of each star, so the relative velocity has an uncertainty of about 280 m/s. This is quite large compared to typical wide binary relative velocities, which are generally a few hundred m/s. Without doing a more detailed analysis, perhaps one thing to do would be to change this 1% requirement to 0.5% or 1.5% and see what happens. I am therefore not convinced that the MOND signal claimed by Chae is genuine.

I. Banik

Kyu-Hyun Chae responded to that, but apparently many people are not able to see his response on Youtube. I cannot. So I asked him about it, and he shares it here:

Since Indranil sent this concern to me in person, I’m replying here. No cut on v_tilde is used in my analysis because it is a gravity test. I did not use equation 7 of El-Badry et al. (MNRAS 506, 2269–2295 (2021)) to cut out high v_tilde data, so there are some (though relatively small number of) data points above equation (7). I removed chance alignment cases by requiring R < 0.01 (El-Badry et al. convincingly show that R can be used to effectively remove chance alignment cases). This is the main reason why there is no high velocity tail. I have already considered varying proper motion (PM) relative errors: there are three cases PM rel error < 0.01 (nominal case), <0.003 (smaller case), and <0.2 (larger case). The conclusion on gravity anomaly (MOND signal) is the same in all three cases although the fitted f_multi (multiplicity fraction) varies. We can have more discussion in the st Andrews June meeting. I’m sure it will take some time but you will be convinced that my results are correct.

K.-H. Chae

He also shares this figure:

This is how the science sausage is made. As yet, there is no consensus.

22 thoughts on “Commentary on Wide Binaries

  1. If the rotational rigidity of galaxies is an emergent property of a complex system of starts, a galaxy strong emergent property, binary starts study will not provide any insights regarding that emergent property exactly like studying a pair of metallic atoms interaction will not provide insights on the rigidity of macroscopic solid metals.

    But empirical evidence reign supreme and we should bow down to it, but if binary studies keep providing “contradicting” results like the ones mentioned here that will only reaffirm the emergent character of galaxies rotational rigidity.


    1. I’m not sure it is fair to describe galaxies as rotationally rigid. Asymptotic rotation speeds are approximately constant, but that only pertains at large radii. There are many divers paths by which that constant speed is reached. Even then, the orbital period gets longer as one goes out just because the circumference grows even if the orbital speed does not decline. So there is a lot of differential rotation, with stars on the inside track lapping those in the outer lanes.


      1. Perhaps using “rigidity” is not a good analogy to describe galaxies flat rotational speed, but the point remains, the MOND behavior/regime appears to be a galaxy level properly, going above that level like galaxy clusters complexity level will break it or going down that level to planetary systems, binary star systems or systems way below galaxy complexity level will break it too.

        Going to levels different from galaxy complexity level will produce inconclusive/contradictory results, but again empirical evidence reigns supreme and only more precise and independent observations providing consistent results could provide a definitive answer.

        Thanks for your reply.


  2. Do you know if talks at the conference will be recorded and made available to the public? I don’t see anything about that on their home page.


    1. For a hammer everything looks like a nail and for the people pushing the dark matter fictional construct DM can explain almost everything.

      Obviously some of the results about binaries mentioned by Triton Station contradict DM, but inconsistent results should be expected when trying to use DM in almost any context, exactly like using epicycles in the Ptolemaic model of the solar system as many have observed before.

      Liked by 1 person

  3. I’ve long had what is perhaps a very naive question about the rotation rates of stars in galaxies. Has it been positively eliminated that galactic gas creates some cohesion to galaxies giving them some fluid rigidity that drags outer stars along faster than would be expected for an isolated star at a given radius orbiting per Newton? Especially given that galaxies have been rotating a long time which would give stars time to be caught up and dragged along?


        1. Precisely the idea of a galaxy level emergent viscosity, a system property, is what implies that MOND will be only valid at galaxy level complexity, going above it(galaxy clusters or beyond) or below it (planetary systems, binary systems or small start clusters) will deviate from it because that “viscosity” appears only at galaxy level.

          But once again only independent and consistent empirical evidence will provide the final answer.


      1. Re: Emergent Viscosity. IMHO this is off-topic for Stacy’s Wide-Binary post.
        But if Stacy will permit the digression to continue a little further, I’m going to call “crackpot” on that Emergent Viscosity paper. The paper postulates 2 new parameters B and C and a “vacuum viscosity” eta, hypothesized to be of the form eta = C/(B+r) , where r is the star’s distance from galactic center. Later (p6), they change their minds and find that “the best fitting function can be expressed in terms of an exponential function as v(r) = b[1 – exp(-r/c)], where b and c are constants which vary from one galaxy to another.” Their values of b and c in their table(2) vary widely, but they make no attempt to examine whether any correlation might exist between these parameters and other per-galaxy parameters (such as total baryonic mass, for example).
        IOW, they choose their fitting function a-posteriori based on what function best fits the data, and even then they have to rely on 2 per-galaxy constants of unexplained origin. There is no motivation for b and c from any underlying theory of viscosity. Rather, they postulate a function with 2 per-galaxy constants and call it “viscosity” arbitrarily. Sounds to me like just another type of dark matter fitting.
        Moreover, in their discussion section (p10) they speculate that this viscosity arises by “interaction of baryonic matter with virtual states of the vacuum”. This is just another way of saying “vacuum energy does it” (sigh). Then they say it’s “quite reasonable to expect that the viscosity of the medium will be […] some function of position”. Ordinary QFT vacuum is not position-dependent, but they’re effectively saying it varies significantly with galactic radius.
        Summary: just another convoluted attempt at some kind of Dark Matter fitting exercise.


        1. There are some things that made my eyebrows go up a bit (although I don’t really have the background for careful analysis). For instance, the line: “This makes the universe expands slower than expected.”

          I thought it was expanding faster than expected, so I’m not sure what they mean by drag there. (And the paper could certainly use a proofread by someone more familiar with English.)

          But, still, maybe the notion of something that pulls the outer parts of galaxies along faster than free orbital speed might not be entirely off the table? I never imagined it being a quantum phenomena; I wondered about the galactic gases. Could the rotation speed of the inner gas link to the outer gas and star systems?


          1. This is confusing. The original (1920s) surprise was that the universe was expanding at all. In retrospect, this shouldn’t have been surprising, but it was such an amazing thing that it was. Then there was the (1990s) surprise that the expansion rate appears to be accelerating instead of decelerating as it should, hence dark energy. Today, we have a tension between the expansion rate measured in the traditional way (H0 = 73) and that estimated from fits to the power spectrum of the CMB (H0=67). So you have to pay close attention to what is “expected” as well as what is observed to know whether any particular author thinks it is slower or faster than they expect.


  4. Sabine Hossenfelder tweeted out a few months ago that

    “We have a new paper, that is Tobias Mistele, @DudeDarkmatter & me. It’s about superfluid dark matter but highlights what I believe is a general problem of hybrid approaches. The issue is roughly speaking this. You introduce a new field that reproduces modified gravity in some regime and CDM in some other regime. Question is, do you couple it to photons or not. If you do couple it to photons you get a problem with reproducing observations from GW170817 that require photons to travel pretty much like gravitaitonal waves inside galaxies. If you don’t couple it to photons then kinematic measurements (inferred from the motion of stars and gas) will generically not match to lensing observations. Alas the data say they do.”

    Demystifier on Physics Forums than asked whether Hossenfelder’s statement also applies to bimetric theories as well, like Milgrom’s bimetric MOND:

    “What about bimetric theories, where different kinds of matter see different geometries?”

    What are your thoughts on this?


    1. Indeed, hybrid approaches (such as superfluid dark matter) do not include BIMOND or RelMOND. And in Deur’s approach the self-interaction term of gravity affects photons just like it affects other gravitons (it’s just second-order gravitons) so I’m thinking the same applies.


  5. I’m not backing or defending this article or their arguments but in the crackpot scale dark matter, (or even more things like superfluid dark matter) runs very high and it’s being used/milked everywhere by very “serious” people and in very “serious” mainstream frameworks.

    Obviously”crackpot” is a relative notion.


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