Questions about dark matter/energy

[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.

 

[Ken G]

It seems like you are agreeing with me; you just don't like calling this process "picking a model with the simplest parameterization".

No I do like calling it that; that's exactly what I did call it.

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.

Perhaps we are saying the same thing then, it sounded like you were contracting what I said. But that's what I said, it was an example of what Planck did. I'm merely wondering if MOND is in a similar boat at the moment-- picking a simple parametrization, and waiting for the Schroedinger equation to come along later.

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.

I'm not disputing any particular meaning for the word "model," that is of no consequence to me. At issue is simply the paths progress can take, which sometimes flow from looking for the simplest parametrization first, and then later trying to understand why it worked-- if it did. In other words, it is often not necessary to decide what model one prefers, on any kind of physical grounds or any philosophical basis at all, one can simply follow the parametrization that seems to be getting more bang from its buck than would be expected if it was a dead end. Some MOND followers are seeing reasons to think the simple a_o type parametrization is getting such bang, at least to a surprising degree. So that could be that phase of the process.

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

I certainly agree that none of us have the necessary crystal ball to know where it will go, the issue is not to predict the future but rather to frame the status of the present. I think the main error to avoid is to try to decide if one prefers a solution involving a new parametrization of gravity, versus a solution that involves invisible particles. There is no reason to prefer anything, we just look under all the streetlights first, preferences are simple biases. An individual has to decide where to put their energy, that's true, but people mistake preferences for evidence sometimes. It's like some people pointing at the Bullet Cluster and other people pointing at other clusters that behave differently, when does evidence get confused with preference?