More Experimental Evidence for MOND

[strangerep]

The last paragraph of @strangerep's post #43 says [potentials add). It says compute the total conventional (i.e., Newtonian) field and then apply an interpolation function to it. That means adding the potentials linearly just like in Newtonian gravity, taking the gradient of the total (since that's the Newtonian field), and then applying an interpolation function.

Take care to keep track of the context. I was talking about a ##\nu(y)## (force-side) interpolation function, whereas Vanadium seems to be using a ##\mu(x)## (inertia-side) interpolation function. The problems involving composite objects that Vanadium alludes to are a good reason why force-side interpolation functions are to be preferred (imho).

I'll just have to take the time to read the [MOND] papers when I get a chance.

Yes, that's always a good idea.

Indeed I wish more referees of journal papers would actually "read the papers" (properly).

Anyway, I'll try to write up a more coherent presentation of the MOND tenets and the various ways of implementing them in the next few days, since there are sooooo many misconceptions about these 2 things and the distinctions between them -- including fresh42's misconceptions that (a) MOND doesn't predict anything (it does), and (b) that MOND is based on a length scale (it's not -- MOND is based on an acceleration scale).

 

[Structure seeker]

Huh? How do you figure that?

Yes it's not that simple but the idea (you can't just add accelerations) is clear.

The source problem is, I think, reading ohwilleke's statement that "MOND has the effect of increasing the acceleration towards ##a_0##" as "... to ##a_0##".

 

[Vanadium 50]

I goofed - it's a²/a0 and not a0. However, the point is the same - it's not linear.

 

[skynr13]

TL;DR Summary: SciTech Daily a recent paper on the orbital motion of wide binary stars shows deviations in gravity measurements that differ from either Newton or Einstein and may support MOND

https://scitechdaily.com/conclusive...heories-in-low-acceleration/?expand_article=1

A study on the orbital motions of wide binaries has uncovered evidence that standard gravity breaks down at low accelerations. This discovery aligns with a modified theory called MOND and challenges current concepts of dark matter. The implications for astrophysics, physics, and cosmology are profound, and the results have been acknowledged as a significant discovery by experts in the field.

I have read many articles in the past years that support MOND in determining gravitational interactions like you are pointing out. I believe in the next few years with more gravitational satellites coming on-line many Astrophysicists will demand a change to this system, permanently leaving Einstein's and Newton's theories out in left field.

 

[ohwilleke]

Since this is still a B-level thread, I dare to make a comment as a naive layman.

Firstly, MOND describes what is and makes predictions of what we measure. This is no valid argument in my mind. It is constructed to do exactly this. It is a minimal requirement. These models are accordingly fine-tuned. What lacks in my opinion is a good reason why nature should follow these tunings.

Secondly, the discussion reminds me of ether. Some medium that tells the central mass about the amount of mass far away in order to adjust its gravitation. Maybe I got it wrong, but I had the impression from reading the posts here, that it makes a difference whether two Jupiters surround each other very far away or a Jupiter and a tiny moon would. Or is all this just a different (from ##r^{-2}##) potential?

There is an aether variant of MOND which is discussed at Scholarpedia.

 

[ohwilleke]

I have read many articles in the past years that support MOND in determining gravitational interactions like you are pointing out. I believe in the next few years with more gravitational satellites coming on-line many Astrophysicists will demand a change to this system, permanently leaving Einstein's and Newton's theories out in left field.

There are indeed a lot of satellites coming on line (of all types) that have resulted in an absolute deluge of new data.

Gravitational wave satellites aren't particular important to the consideration of MOND and most other gravity modification theories. But, other telescopes which allow for more precise measures of the movement star within galaxy, galaxies, and galaxy clusters are absolutely critical for this effort and will help determine which theory is correct.

Newtonian gravity has been superseded by General Relativity (GR), but remains an excellent approximation of GR in many circumstances.

MOND proponents don't claim that GR or something very close to it is not the correct theory in strong gravitational fields, or that GR effects like gravitational lensing aren't correct. They simply claim that in circumstances involving weak gravitational fields where astronomers usually use a Newtonian approximation of GR, that MOND is a better description of reality.

 

[ohwilleke]

Sure you can, just add the potentials linearly and take the gradient of the total potential. That is what I understood was being done in the "modified gravity" version described earlier. My question is, if that's what is being done, there should not be any such thing as an "External Field Effect", nor should it make any difference whether we use, say, coordinates centered on the center of mass of a wide binary or coordinates centered on the center of mass of the Milky Way. But other posts in this thread have talked about an EFE and have at least implied that it does make a difference which coordinates we use. That's why I am confused about what MOND actually says.

The EFE is absolute a critical effect in MOND. For example, it is critical to predicting when a satellite galaxy will be a "no dark matter" galaxy, and when it will not be.

The EFE is not a function of coordinate systems. It is, however, a function of what the distribution of mass in the vicinity of a seemingly free falling system of masses bound by gravity is. For example, in a common MOND fact pattern, a low mass density diffuse galaxy behaves very different near another big massive galaxy than it does in isolation in the middle of a large void.

 

[ohwilleke]

Could there be an astrophysical explanation? I think that is likely. However, the battle lines have been drawn - you have people saying "gravity must be modified!" and others saying "the universality of a0 tells us nothing! I won't even look at it!" IMHO, neither path is the correct one.

This is indeed a key point. Even if the radial acceleration relation isn't caused by modified gravity, it is such a pervasive phenomena that the universe so tightly follows that if you have a dark matter particle explanation for dark matter phenomena, your dark matter particle explanation needs to replicate this observation about how matter in the universe behaves in what appears to be systems bound only by gravity.

 

[jedishrfu]

There is another related MOND thread started by @ohwilleke:

https://www.physicsforums.com/threa...ic-acceleration-imprint.1055101/#post-6925194

Discussing another paper on MOND as it relates to Globular Clusters and how the galactic acceleration is imprinted upon them.

 

[Dale]

many Astrophysicists will demand a change to this system, permanently leaving Einstein's and Newton's theories out in left field

That outcome isn’t even on the table for MOND.

First, for one theory to displace another it must be more general. MOND is not. MOND does not predict gravitational time dilation, precession of Mercury, gravitational lensing, frame dragging, gravitational waves, the CMB, etc. So MOND simply cannot replace GR, even in principle.

Second, suppose a new theory is more general, agrees with existing theories in all extant experimental domains, suggests new experimental domains, and is subsequently experimentally validated in those new domains. Even then the old theory is still used. The old theory remains valid in all of the experimental domains in which it was validated. So even if MOND could replace GR in principle, it wouldn’t in practice.

 

[Vanadium 50]

MOND does not predict gravitational time dilation, precession of Mercury, gravitational lensing, frame dragging, gravitational waves, the CMB, etc. So MOND simply cannot replace GR, even in principle.

I think that's a little bit unfair, as ΛCDM doesn't predict these things either,

If you say "GR does not have MOND as its Newtonian limit", everyone would agree with you. It's in the name. But there are at least three logical possibilities other than "MOND is just wrong"

1. The correct theory of gravity has GR as its high acceleration limit and MOND as its low acceleration limit.
2. GR is the correct theory of gravity, but inertia behaves differently at low accelerations.
3. There is a (very weird) 5th force that when you add it to GR at low accelerations you get MOND.

You might not like these possibilities - I sure don't - but they are there.

 

[Dale]

I think that's a little bit unfair, as ΛCDM doesn't predict these things either,

I agree that is unfair which is why I didn’t make that claim. Nor was that the claim I was responding to. So it is a little bit unfair to pretend that my claim was a little bit unfair.

 

[Vanadium 50]

In that case, I apologize.

 

[berkeman]

So it is a little bit unfair to pretend that my claim was a little bit unfair.

I'm getting dizzy...

 

[aheid]

I'll just have to take the time to read the papers when I get a chance.

Now I'm just a layman so your needs might be different, but I found the following article to have a nice review of the MOND theory. YMMV.

https://doi.org/10.3390/sym14071331 (open access)

From galactic bars to the Hubble tension: weighing up the astrophysical evidence for Milgromian gravity​

Astronomical observations reveal a major deficiency in our understanding of physics—the detectable mass is insufficient to explain the observed motions in a huge variety of systems given our current understanding of gravity, Einstein’s General theory of Relativity (GR). This missing gravity problem may indicate a breakdown of GR at low accelerations, as postulated by Milgromian dynamics (MOND). We review the MOND theory and its consequences, including in a cosmological context where we advocate a hybrid approach involving light sterile neutrinos to address MOND’s cluster-scale issues. We then test the novel predictions of MOND using evidence from galaxies, galaxy groups, galaxy clusters, and the large-scale structure of the universe. We also consider whether the standard cosmological paradigm (ΛCDM) can explain the observations and review several previously published highly significant falsifications of it. Our overall assessment considers both the extent to which the data agree with each theory and how much flexibility each has when accommodating the data, with the gold standard being a clear a priori prediction not informed by the data in question. Our conclusion is that MOND is favoured by a wealth of data across a huge range of astrophysical scales, ranging from the kpc scales of galactic bars to the Gpc scale of the local supervoid and the Hubble tension, which is alleviated in MOND through enhanced cosmic variance. We also consider several future tests, mostly at scales much smaller than galaxies.

 

[ohwilleke]

From M. Milgrom, Mon. Not. R. Astron. Soc. 437, 2531 (2014). MOND laws of galactic dynamics preprint (https://arxiv.org/pdf/1212.2568.pdf) pages 16-19:

"4.6.The external-field effect

Unlike ND, MOND is nonlinear even in the NR regime. It generally does not satisfy the strong equivalent principle; so effects of an overall acceleration on the internal dynamics of a system are generically expected. To be able to say what constraints the basic tenets impose on such effects I have to confine myself here to theories whereby only the instantaneous value of the external acceleration matters. This excludes from the discussion a large class of MI theories that are time nonlocal (Milgrom 1994). In these, the full(external)trajectory of the system enters, which complicates the discussion. Some of the possible consequences of such nonlocality are discussed briefly in Milgrom(2011),but what follows here does not apply to such theories.

Consider then a system of mass m(‘system m’), and extent r, that is falling in the field of a mother system with acceleration whose instantaneous value is g0. Assume that the theory and conditions are such that, to a good enough approximation, all the information about the mother system enters the dynamics within m only through g0. One can then write

a=a(m,r,a0,G,g0 ,n0 ,α), (15)

where a stands for the internal acceleration runs of elements of m, namely the full acceleration in the field of the mother system minus g0 (suppressing the dependence on position, time, and particle index). It is written as a function of all the available dimensioned independent parameters, as well as of n0 ,the unit vector in the direction of g0, and of α, which stands for the many dimensionless parameters that characterize the configuration, such as all the mass ratios, and all the geometrical parameters(angles, ratios of all distances tor, etc.). Here I am only interested in scaling laws of the dimensioned parameters–for example, in how |a| depends on the dimensioned system attributes–so I shall suppress the dependence on n0 and α. Since a/g0 is dimensionless, it can depend only on dimensionless quantities; so we can write, most generally

a=g0 F∗(η,θ),η≡mG r2g0 ∼gN g0 ,θ≡g0 a0 . (16)

When g0≪|a|, its effects can be neglected. So here I shall be interested in the opposite case, of external-acceleration dominance,g0≫|a|.35 The above choice of dimensionless variables is useful for this case. Clearly, F∗(0,θ)=0. So we are interested in the behaviour of F∗ to lowest order in η. We shall see below that external-field dominance requires η≪1 when θ≳1, and the SI condition η≪θ when θ≲1(in which case the whole problem is in the DML; η and θ both scale likeλ−1 underscaling); so we can write this condition generally as η≪min(1,θ).

We do not know that a MOND theory is necessarily expandable in powers of η near η=0. But assuming that it does, I write

a≈g0ηqf(θ),η≪min(1,θ). (17)

(I assume that q does not depend on θ; see below.)

To constrain q and f(θ) I now employ the basic tenets of MOND. The limit a0→0, namely whena0≪|a|≪g0, is strongly Newtonian for all accelerations, and is within the validity domain of eq.(17). For this region, g0 anda0 have to disappear from expression(17). This implies that q=1, and that|f(θ≫1)|∼1, such that(mG/r2)f(∞) is the internal Newtonian acceleration.

It is difficult to make general statements about the intermediate case where the two accelerations are of the same order field within m. This means that the internal dynamics is Newtonian for any value of gN when θ≫1; i.e.,also wheng N≪a0. In other words: whenever the external field is highly Newtonian and dominates over the internal field, the latter is necessarily Newtonian. This result holds also when q depends on θ, because then we still must have q(θ→∞)→1.

More generally, in as much as q=1 for all θ, we can write eq.(17)in its full validity domain (external-field dominance) as

a=mG r2 f(θ). (18)

This means that when the external field is dominant, the internal dynamics is always quasiNewtonian, in the sense that the accelerations scale as mG/r2, only with an enhanced effective constant Geff∼G|f(θ)|, and with not-quite-Newtonian geometrical aspects that stem from the fact that f has different geometric properties than f(∞): for example, f depends on the direction relative to n0 ,and on the theory at hand, while f(∞) does not.

When θ≪1 the whole system is in the DML, where the basic tenets dictate that eq.(18) becomes SI. Underscaling, θ scales like g0, namely θ→λ−1θ (since g0 is a DML acceleration of the mother system it scales as g0→λ−1g0). This means that f must become proportional to θ−1: f(θ≪1)≈θ−1¯f. We see then that f(θ) has the same asymptotic behaviours as 1/µ(θ), where µ is the interpolating function appearing in present MOND theories. If q does depend on θ, the EFE does not conform to the standard results.

For example, in the DML we could have 0<q(0)=1, in which case SI dictates f(θ≪1)≈θ−q(0)ˆf. Then a∼g0(gN a0/g2 0 )q(0)=g0(η/θ)q(0). We see that, as stated above, the condition for external-field dominance, a≪g0, whenθ<1, is indeed always η≪θ. For example, if q(0)=1/2, this gives the standard scaling of the MOND acceleration in isolated systems a∼(gN a0)1/2; i.e.,there is no EFE, except for effects in¯f(0). So the basic tenets lead to the standard EFEresults(indeed to an EFE) only if some additional analytic properties are assumed. The toy DML theory described by eq.(6), which satisfies scale invariance (but which does not combine with an appropriate Newtonian limit), does not lead to an EFE. The above analytic assumptions do hold in all the MOND formulations considered to date: e.g., in the original, pristine formulation in (Milgrom 1983),in the formulation of Bekenstein & Milgrom (1984), and in QUMOND(Milgrom 2010a). For example, in QUMOND, we can write schematically (ignoring the vectorial nature of the quantities involved)

a/g0 ∼ν[θµ(θ)+θη][µ(θ)+η]−1, (19)

where ν(y) is the QUMOND interpolating function, and µ(x) is such that ν[xµ(x)]µ(x)=1. We have µ(θ≪1)≈θ,µ(θ≫1)≈1; so we see explicitly why the condition η≪min(1,θ) is tantamount to a dominant external field. And, clearly the next to zeroth-order term is a/g0∼ η(1+ˆν)/µ(θ), where−1/2<ˆν<0 is the logarithmic derivative of ν.

In summary, the fact that an external field |g0|≫a0 renders the internal dynamics Newtonian, follows from only the basic tenets of MOND, provided that only the instantaneous external field enters the internal dynamics (not necessarily true in MI, time-nonlocal theories). This is relevant, for example, to experimental results in the laboratory and Solar system, and to the dynamics of star clusters near the sun. On the other hand, the specific form of the EFE when|g0|≪a0, even its very existence, is not strictly dictated by the basic tenets alone. Its basic features do follow under another plausible assumption, shared by all full-fledged theories considered to date: that the expansion power in eq.(17) does not depend on θ.

There is no EFE in the DM paradigm."

 

[vanhees71]

That outcome isn’t even on the table for MOND.

First, for one theory to displace another it must be more general. MOND is not. MOND does not predict gravitational time dilation, precession of Mercury, gravitational lensing, frame dragging, gravitational waves, the CMB, etc. So MOND simply cannot replace GR, even in principle.

Second, suppose a new theory is more general, agrees with existing theories in all extant experimental domains, suggests new experimental domains, and is subsequently experimentally validated in those new domains. Even then the old theory is still used. The old theory remains valid in all of the experimental domains in which it was validated. So even if MOND could replace GR in principle, it wouldn’t in practice.

Another argument is that GR is validated at high precision with Pulsar timing (usually evaluated in terms of Postnewtonian parametrization). Is there some paper, discussing in how far this is compatible with MOND?

 

[Vanadium 50]

Binary pulsars are at high acceleration, so they are in the GR limit, not the MOND limit.

 

[Vanadium 50]

One of the things that is either genius or annoying ("there's a fine line between clever and stupid") is that there is a neat separation between rotation curves and every other test of gravity.

 

[vanhees71]

In which sense?

 

[Vanadium 50]

If you had an overlap region, experiments could test both theories and see which does better, and/or map the transition between them. But you really don't.

 

[Ibix]

If you had an overlap region, experiments could test both theories and see which does better, and/or map the transition between them. But you really don't.

So just to clarify, in #89 you meant "...there is a neat separation between rotation curves and every other test of gravity that has so far been performed"? Or is there something that makes rotation curves unique beyond their dependance on really low gravitational acceleration?

 

[vanhees71]

This makes MOND very suspicious then. It should make clear predictions about the phenomenology that it is supposed to describe (rotation curves without "Dark Matter") and then it should be possible to be tested against standard GR + "Dark Matter".

I thought there are also galaxies with no or very little Dark matter, and there the rotation curves are discribed by GR with only the visible matter. If MOND is a description of gravity as a universal interaction, then how does it explain that in this case no modification against GR + visible matter occurs?

 

[Vanadium 50]

Or is there something that makes rotation curves unique beyond their dependance on really low gravitational acceleration?

I don't think so.

All the terrestrial tests? Done with g > a0.
All the solar system tests? Done with g > a0.

That's the problem. And thinks we "know" about gravity may or may not apply to MOND. We "know" gravity affects all materials equally. Do we know that for MOND? Not really - there we look at pretty much the same material all the time: a mix of mostly hydrogen and some helium.

In fact, if soneone found a clever way to test MOND on earth with lead weights, and it failed, a fair conclusion is "well you tested it with lead. Stars are made mostly of hydrogen/"

 

[Structure seeker]

I thought there are also galaxies with no or very little Dark matter, and there the rotation curves are discribed by GR with only the visible matter. If MOND is a description of gravity as a universal interaction, then how does it explain that in this case no modification against GR + visible matter occurs?

With the EFE, usually a big enough other galaxy is nearby or may be nearby to explain these. And these are not easy to explain in Lambda-CDM either, since galaxies should only appear at high enough dark matter concentrations there.

Except for elliptical galaxies, there simply ##g > a_0##

 

[Vanadium 50]

I thought there are also galaxies with no or very little Dark matter,

There are two. Or maybe three.

The amount of DM inferred depends on the distance. The distance of both is problematic. Indirect measures of their masses is also problematic.

One part of the astro culture I do not like is taking a distribution (spread can be either real, or measurement-driven, or both) and looking at the tails, and then declaring "Look! A new class of [whatever] !!" I understand why the culture developed this way, but I still do not like it.

Additionally, is difficult to strip a galaxy of its DM without disturbing its structure. It cannot be done in one interaction - it takes several. However, one does expect some DM variation and one sees that. Oddly, the variation always seems to match the MOND prediction.

 

[AndreasC]

Firstly, MOND describes what is and makes predictions of what we measure. This is no valid argument in my mind. It is constructed to do exactly this. It is a minimal requirement.

It is constructed to explain some galaxy rotation curves. That is similar to dark matter. BUT using it you can infer some other things that don't intrinsically have anything to do with these rotation curves, and indeed it seems it makes some correct predictions. The main problem is probably that it still can't explain some things without dark matter.

 

[ohwilleke]

McGaugh has an update on the wide binary debate.

 

[pinball1970]

McGaugh has an update on the wide binary debate.

I PMd you. This paper from the Gaia data,
Monthly Notices of the Royal Astronomical Society, stad3393, https://doi.org/10.1093/mnras/stad3393
Published: 3 November 2023
This goes against the MOND claims of the Chae paper.

 

[AndreasC]

Yep, this is the upcoming paper I alluded to with my previous post. Interesting to see what this means for MOND in the long run!

 

[PAllen]

Note this latest paper is by someone (Banik) who has long been a proponent of MOND, writing one of the major review papers in the field.

 

[Vanadium 50]

I don't see that as a problem. I have published several experimental papers supporting my side of a controversy. A few have even settled the controversy.

Am I biased? In don't know. I am certainly interested. We have controversy when people draw different conclusions from the same data. If N-1 experiments convice me but not everybody, and I do an Nth experiment that does convince everybody, where is the problem?

 

[Vanadium 50]

Where I do see a problem is, as I mentioned before, that we cannot test MOND in a terrestrial setting because the gravity of Earth obliterates the effect. But we can test it in wide binaries because for some reason the gravity of the planets does not obliterate the effect.

In Science, you and your worst enemy have to agree on what the predictions are. I don't think "Whatever Milgrom says it is" is a good basis for that.

 

[ohwilleke]

I've blogged Banik's new paper. If correct, it strongly disfavors simple toy-model MOND, but doesn't, for example, rule out Deur's gravitational self-interaction approach which replicates MOND in galaxy rotation curves, but does not predict a wide binary effect.

 

[Vanadium 50]

I will go a step further - since there is no agreement on either the data nor the predictions for wide binaries, it is not a good test.