Sabine Hossenfelder on the search for new particles

[Maarten Havinga]

Then just to get each other right, how open do you consider your mind if it gives just small amounts of time to ponder about what it means for cosmology that almost all MOND predictions have come true?

 

[Vanadium 50]

lmost all MOND predictions have come true?

Um...no.

It is true that MOND does better than is generally acknowledged. But it is also true that it works on galactic scales and nowhere else. In a grumpy mood, I might even say it works for rotationally supported galaxies and nowhere else.

 

[OscarCP]

Um...no.

It is true that MOND does better than is generally acknowledged. But it is also true that it works on galactic scales and nowhere else. In a grumpy mood, I might even say it works for rotationally supported galaxies and nowhere else.

Hmm .. I believe I am reasonably open-minded, maybe a bit more than the average citizen these days, But as to MOND: are there scads of papers of no particular distinction being published on MOND these days?

And with that, I'll leave you two to get on with this.

Also with some food for thought:

https://academic.oup.com/mnras/article/327/2/557/1040696

 

[ohwilleke]

Um...no.

It is true that MOND does better than is generally acknowledged. But it is also true that it works on galactic scales and nowhere else. In a grumpy mood, I might even say it works for rotationally supported galaxies and nowhere else.

Or smaller. It doesn't work in galaxy clusters. It needs to be generalized to work in strong fields and at a cosmology scale.

 

[PAllen]

Or smaller. It doesn't work in galaxy clusters. It needs to be generalized to work in strong fields and at a cosmology scale.

I've never really expressed my view on this, but here goes.

First, there is a general critique of retrodiction made by some (including strongly by Hossenfelder when it suits her purpose, e.g. in a tirade against the LIGO team) -that I disagree with. Retrodiction differs from prediction only in the order of observation event vs theory. This cannot, in general, have fundamental significance, IMO (it is an accident of history). What does matter is whether a theory simply adds an element to an existing model to account for an otherwise unexplained observation, versus a theory arising independently that also explains a prior anomaly. GR is an example of the latter with respect to perihelion advance, MOND is an example of the former with respect to galaxy rotation curves.

BUT... MOND has had some successes beyond its initial construction, and the dark matter required to make GR 'work' has become more complex in its requirements and at least strained by absence of direct observation (though some lensing effects could be considered direct observation). Further, we know that MOND in its initial form cannot be a possible correct theory because it simply modifies a theory with limited application (Newtonian gravity). But the totality of difficulties with GR+plus dark matter combined with some real successes of MOND strongly, IMO, suggests that modified relativistic gravity theories are a promising research avenue.

 

[vanhees71]

Um...no.

It is true that MOND does better than is generally acknowledged. But it is also true that it works on galactic scales and nowhere else. In a grumpy mood, I might even say it works for rotationally supported galaxies and nowhere else.

AFAIK it also doesn't work on galactic scales, because there are galaxies containing almost no dark matter, i.e., where the amount of matter according to its gravitational action and its luminosity agree with each other, i.e., there's no deviation from standard Keplerian motion, i.e., it cannot be a deviation from standard gravitational interaction as assumed by MOND that explains the deviations observed in galaxies containing dark matter (in the standard interpretation).

 

[Vanadium 50]

because there are galaxies containing almost no dark matter

Well, maybe.

DF2/DF4 appear to have no dark matter. That's what the paper title says, anyway. Now,there is definitely some weirdness going on. For technical reasons I don't entirely understand, the amount of DM inferred depends on the distance, and there is considerable uncertainty in that - enough so DF2 might actually be enhanced in DM, although that's not the best fit.

What story best fits the data has been ping-ponging in the literature for a while.

The "no DM" story has a theory problem as well. The story is that a local overdensity of DM attracted more DM and hydrogen, and the hydrogen formed stars, and then the DM was stripped away in an interaction. This interaction somehow did not disturb the spiral structure or the rotation curve, nor trigger star formation.

I'd feel better drawing conclusions if we had more examples, in different parts of the sky observed and analyzed by different teams. I said much the same thing four or five years ago when this first came out.

 

[ohwilleke]

AFAIK it also doesn't work on galactic scales, because there are galaxies containing almost no dark matter, i.e., where the amount of matter according to its gravitational action and its luminosity agree with each other, i.e., there's no deviation from standard Keplerian motion, i.e., it cannot be a deviation from standard gravitational interaction as assumed by MOND that explains the deviations observed in galaxies containing dark matter (in the standard interpretation).

Incorrect.

The galaxies with no inferred dark matter seen so far are explained by the External Field Effect (EFE) in MOND (which was predicted to occur at the outset back in 1983).

MOND effects only arise when not only the matter source for the gravitational field, but also the gravitational field of neighboring bodies (like primary galaxies in a satellite galaxy system) combined are below its threshold acceleration a₀. So, MOND predicts that satellite galaxies sufficiently close to primary galaxies will appear to have no dark matter, which is what is observed.

For example, Stacy McGaugh, one of the leading MOND astrophysicists explained at his Triton Station blog that (emphasis and bracketed material mine):

The importance of the EFE in dwarf satellite galaxies is well documented. It was essential to the a priori prediction of the velocity dispersion in Crater 2 (where MOND correctly anticipated a velocity dispersion of just 2 km/s where the conventional expectation with dark matter was more like 17 km/s) and to the correct prediction of that for NGC 1052-DF2 (13 rather than 20 km/s). Indeed, one can see the difference between isolated and EFE cases in matched pairs of dwarfs satellites of Andromeda. Andromeda has enough satellites that one can pick out otherwise indistinguishable dwarfs where one happens to be subject to the EFE while its twin is practically isolated. The speeds of stars in the dwarfs affected by the EFE are consistently lower, as predicted. For example, the relatively isolated dwarf satellite of Andromeda known as And XXVIII has a velocity dispersion of 5 km/s, while its near twin And XVII (which has very nearly the same luminosity and size) is affected by the EFE and consequently has a velocity dispersion of only 3 km/s.

Similarly, consider this quotation from a journal article:

The small velocity dispersion observed in the two group UDGs NGC 1052-DF2 and NGC 1052-DF4, inferring dynamical masses close to their stellar masses, was initially interpreted as a challenge for MOND (van Dokkum et al. 2018, 2019a). Indeed, the dynamical effect attributed to DM in the CDM model, and to a modification of the gravitational law within MOND in isolation, would be absent. But taking the EFE into account removes or significantly lessens the tension (Famaey et al. 2018; Kroupa et al. 2018; Müller et al. 2019; Haghi et al. 2019b).

From J. Freundlich, et al., "Probing the radial acceleration relation and the strong equivalence principle with the Coma cluster ultra-diffuse galaxies" arXiv:2109.04487 (September 9, 2021) (published at 658 A&A A26 (2022).

Another possibility, although it hasn't come up so far, is that MOND's domain of applicability when it comes to predicting rotation curves and dynamics of galaxies is limited to galaxies in equilibrium without far more sophisticated mathematical analysis than is generally done. A deviation from naive MOND predictions for rotation curves in a galaxy can be an indication that the galaxy is far out of equilibrium, although this situation hasn't yet come up in a case of an isolated galaxy with no apparent dark matter.

Quoting Stacy McGaugh again discussing a "Starts With A Bang" article by Ethan Siegel about the dwarf galaxy Segue 1:

In order to estimate the dark matter mass, one assumes that a system is in dynamical equilibrium. That’s usually a good assumption. Here, it is a terrible assumption.

Segue 1, and very nearly all of the so-called ultrafaint dwarfs, are deep in the potential of the Milky Way where they are subject to strong tidal forces. This violates the assumption of equilibrium, in any theory. There is an eternal energy source: the stars are not just responding to their own gravitational field (and that of ‘their own’ dark matter). Thus it is likely that the motions of the stars have been stirred up by the external field so that the dynamical mass is overstated.

In the dark matter galaxy formation picture, one expects small galaxies like this to be accreted by larger galaxies like the Milky Way. In that process, they are tidally stripped. First the outer parts of their dark matter halo, then down to the stars, then ultimately they’re shredded completely. There’s no good way to tell how far along this process Segue 1 is, but it and the other ulrtafaints dwarfs are the poster children for hierarchical accretion.

In MOND, I had initially thought this was a huge problem (see https://arxiv.org/abs/1003.3448). The external field effect, by itself, does not explain this observation. Long story short, it turns out that tidal effects are even stronger in MOND, and the assumption of dynamical equilibrium certainly does not hold. So – same problem.

There is one difference: in MOND, there is a quantitative criterion for when an object is not in equilibrium (see https://arxiv.org/abs/astro-ph/0005194). All of the ultrafaints, including Segue 1, fail to meet this criterion [ed. i.e. they are out of equilibrium according to the quantitative test]. There is no chance that the measured velocity dispersion reflects the equilibrium value of an isolated system. Indeed, one can see the onset of this effect in the data (see Figs 6 and 7 of arxiv:1003.3448). From that perspective, this is another successful prediction of MOND: it not only predicts correctly the velocity of stars in equilibrium systems, it also tells you when it can’t.

There is no equivalent criterion in dark matter. If things don’t work out, we infer that the system is out of equilibrium. The difference is that MOND tells you when this must be invoked. All the famous cases (e.g., And XIX, Crater 2, and a half dozen others whose names I don’t recall offhand) that are now considered to be out of equilibrium in dark matter were predicted in advance by MOND.

 

[ohwilleke]

Well, maybe.

DF2/DF4 appear to have no dark matter. That's what the paper title says, anyway. Now,there is definitely some weirdness going on. For technical reasons I don't entirely understand, the amount of DM inferred depends on the distance, and there is considerable uncertainty in that - enough so DF2 might actually be enhanced in DM, although that's not the best fit.

What story best fits the data has been ping-ponging in the literature for a while.

The "no DM" story has a theory problem as well. The story is that a local overdensity of DM attracted more DM and hydrogen, and the hydrogen formed stars, and then the DM was stripped away in an interaction. This interaction somehow did not disturb the spiral structure or the rotation curve, nor trigger star formation.

I'd feel better drawing conclusions if we had more examples, in different parts of the sky observed and analyzed by different teams. I said much the same thing four or five years ago when this first came out.

The technical issue is basically that the distance impacts the precision of your estimate of the angle of inclination of the galaxy with respect to an observer on Earth and these two things, in turn, impact estimates of rotation speed. (And most people don't realize just how crude a lot of individual galaxy level astronomy measurements are - at that scale there historically at least hasn't been much precision.)

The most common DM particle explanation of no DM galaxies is tidal stripping from a neighboring body, but that almost always implies a primary galaxy nearby that also implicates the external field effect of MOND.

There have been one case so far purporting to find a no DM galaxy that is isolated in space from other large bodies (Galaxy AGC 114905, which is discussed in a December 2021 paper published the scientific journal MNRAS), which would rule out both the external field effect in MOND and "recent" (by galaxy standards) tidal stripping.

But later analysis has concluded that there was a very high probability that this was a false positive due to experimental error in measuring the inclination of the galaxy and that its highly irregular shape also injected significant unaccounted for theoretical error into the inferred DM estimate (which was made on the assumption of a much more typical baryonic mass distribution than the galaxy in question). See J. A. Sellwood and R. H. Sanders, "The ultra-diffuse galaxy AGC 114905 needs dark matter" arXiv:2202.08678 (February 17, 2022) (submitted to MNRAS) https://doi.org/10.48550/arXiv.2202.08678.

A no DM galaxy that is isolated in space would be more of a blow to MOND than to particle DM. It is less of a blow to particle DM theories because tidal stripping could have happened a very long time ago with the source of the stripping having drifted away from the no DM galaxy many billions of years ago. Still, it would be a problematic outlier in either theory.

It also bears noting that the tidal stripping mechanism used to explain such galaxies in DM context is itself is somewhat problematic unless there is evidence of tidal stripping of non-DM components of the galaxy that should behave in a similar way to DM, like interstellar hydrogen.

 

[haushofer]

Dark energy my be also ad hoc...

It's not. General principles of General Relativity dictates the cosmological constant should be included in the Einstein Field Equations.

 

[OscarCP]

It's not. General principles of General Relativity dictates the cosmological constant should be included in the Einstein Field Equations.

The value is ad hoc, not the constant itself (maybe).
It is ad hoc in the way the charge of the electron is, because the value is an experimental fit to the relevant data, not one stemming exactly from a fundamental principle's corresponding basic equation.
Although Einstein introduced the CC in an ad hoc way, as he saw it, to keep the Universe static, then removed it, then said: "It is OK, just put it there with some appropriate value. What value? Are you asking me?"

https://en.wikipedia.org/wiki/Cosmological_constant

 

[Vanadium 50]

Sure but do we really want "ad hoc" answers? "Nature just works that way" can be used to answer any question. Why are atoms neutral? Why is the weak force left-handed? Why does carbon hybridize? Why don't electrons in atoms radiate? Why are metal specific heats what they are?

All could be answered with a shrug and "nature just works that way."

 

[OscarCP]

Sure but do we really want "ad hoc" answers? "Nature just works that way" can be used to answer any question. Why are atoms neutral? Why is the weak force left-handed? Why does carbon hybridize? Why don't electrons in atoms radiate? Why are metal specific heats what they are?

All could be answered with a shrug and "nature just works that way."

Well ... I don't think that's how it's done.

 

[artis]

The value is ad hoc, not the constant itself (maybe).
It is ad hoc in the way the charge of the electron is, because the value is an experimental fit to the relevant data, not one stemming exactly from a fundamental principle's corresponding basic equation.
Although Einstein introduced the CC in an ad hoc way, as he saw it, to keep the Universe static, then removed it, then said: "It is OK, just put it there with some appropriate value. What value? Are you asking me?"

https://en.wikipedia.org/wiki/Cosmological_constant

I definitely agree about the cosmological constant being "ad hoc", but is the charge of the electron really ad hoc too?

I mean sure , the electron as far as we know doesn't have any internal structure and basically just is and that is not a satisfying answer for where it gets it's charge but the charge itself is definitely identifiable in a rather straight forward way and also in a precise way. In fact we did it more than 100 years ago.

Unlike the cosmological constant prediction which changes depending on which theory you are in, the electron charge is a real value, empirically determined and doesn't change does it?
Isn't that "first principles" enough?

 

[OscarCP]

I definitely agree about the cosmological constant being "ad hoc", but is the charge of the electron really ad hoc too?

I mean sure , the electron as far as we know doesn't have any internal structure and basically just is and that is not a satisfying answer for where it gets it's charge but the charge itself is definitely identifiable in a rather straight forward way and also in a precise way. In fact we did it more than 100 years ago.

Unlike the cosmological constant prediction which changes depending on which theory you are in, the electron's charge is a real value, empirically determined and doesn't change does it?
Isn't that "first principles" enough?

I meant that there is no equation where the electron charge appears either as a fundamental constant in it, much as π can only be "π, the modulus of the exponential in Euler's identity" for example. And also that neither does the electron's charge show up as a function of other fundamental constants.

The electron's charge can only be obtained empirically, no matter to how many exact figures. As can be, in principle, the Cosmological Constant in GR.
The former has been pointed out as one of the ways in which quantum physics can be said to be incomplete.

Unlike the speed of light in vacuum that is determined by fundamental electromagnetic properties of "empty" space, for example:

https://www.sciencealert.com/why-is-the-speed-of-light-the-speed-of-light

Quote: "Maxwell's equations fixed the electric and magnetic properties of empty space, and after noting that the speed of a massless electromagnetic radiation wave was very close to the supposed [from empirical esults] speed of light, Maxwell suggested they might match exactly.

It turns out Maxwell was right, and for the first time we could measure the speed of light based on other constants in the Universe."

And, by the way, Einstein seems to have been right again: https://www.scientificamerican.com/...heory-just-passed-its-most-rigorous-test-yet/

 

[vanhees71]

Sure but do we really want "ad hoc" answers? "Nature just works that way" can be used to answer any question. Why are atoms neutral? Why is the weak force left-handed? Why does carbon hybridize? Why don't electrons in atoms radiate? Why are metal specific heats what they are?

All could be answered with a shrug and "nature just works that way."

I think the natural sciences on a certain level indeed just answer such questions by "nature just works that way". However, and this makes the natural sciences useful, it doesn't simply shrug and answers in this way for any single phenomenon we observe in Nature. That would be hardly worth the effort to learn it. For Rutherford that would be just "stamp collecting" but not "science" (which for him was just "physics" ;-)).

Your examples are "answered" by QT in general and its application to charged particles and the electromagnetic interaction (why don't electrons in atoms radiate? Why does carbon hyridize? Why are metal specific heats what they are?) or the Standard Model of elementary particle physics (Why are atoms neutral? Why is the weak force left-handed?).

In the latter case the "shrugging", however starts pretty soon. "Why are atoms neutral?" is answerd by the observed fact, and in the Standard model it's mathematically realized by assigning the charge pattern of the quarks and leptons as observed using the, also observed (V-A)-structure of the weak interaction, which answers "Why is the weak interaction left-handed". The formalism of non-Abelian chiral gauge symmetry tells us that this charge pattern is not completely arbitrary, because corresponding anomalies must cancel, but the observed charge pattern, plugged into the Standard model (including also the additional color-degree of freedom for quarks and the -1/3 and +2/3 elementary charges for each of the three generations) is not the only possibility, i.e., the so (over-)successful Standard Model is based on both pretty general principles (mostly symmetry principles a la Noether's theorem 1 (global symmetries and conservation laws) and theorem 2 (local gauge symmetries + locality/microcausality), but why it has the specific particle/field content it has we can only answer by shrugging an saying, "because that's how (almost) all observations fit into a pretty nice theoretical scheme".

 

[OscarCP]

I agree with vanhees71 and would add, in reply to the rhetorical opening question in Vanadium 50's comment, that we don't want ad hoc answers, but sometimes is all we can have, at least for now. We might hope that this will become unnecessary when some new theoretical breakthrough happens, but meantime we need the ad hoc and just try to make it better, if we can. For example fitting more and better data to it.
Also, there is a difference, I think between "ad hoc" and "Nature just works that way"; not one of substance, but of attitude. One implies, or can imply, that perhaps for now that is all that can be said. The other, that there is no need to worry about this because nothing can be done. "So, accept it -- like death, or taxes -- and move on."
The ad hoc should not be identified with the true final answer, assuming there is one; instead, it should be seen as a caste and, one would hope, temporary veil thrown on our ignorance. It is even possible that "for all we know", in some cases it could also be the true answer. But that is, unless and until there is an explicit and testable proof that it is so, no more than a conjecture (cf. "Dark Matter").

 

[ohwilleke]

In the latter case the "shrugging", however starts pretty soon. "Why are atoms neutral?" is answerd by the observed fact,

This follows pretty much as a derived property of other things we know (to oversimplify the EM force is stronger than the weak force and gravity and wants to get to net zero, in a sort of weak parallel to QCD confinement, which can be derived heuristically and which can probably be derived rigorously), but I agree that the other cases are more descriptive than predictive.

 

[OscarCP]

And as well as that, I tend to think it is also because, if not neutral, all atoms, nuclei excepted perhaps, would be unstable, either flying apart or combining with other equally electrically unbalanced atoms (i.e. ions) something contradicted by the perfectly observable fact that most atoms, if left in peace, (i.e. not treated energetically by us or by things such as cosmic rays, etc.) are stable, yes? Or, because if not, we would not be here to discuss this? Or anything?
That would be an ad hoc assumption, no question about it, but a much more serious one than the accepted value of the charge of the electron, or the existence of Dark Matter, because most of us can live pretty good and useful lives without knowing why the electron has the charge it has, or what is it exactly, or at all, but could not do any of that if atoms were not neutral. Or quick to form neutral, stable combinations with other atoms.

Why they are stable may be also, as pointed out by ohwilleke, theoretically explicable, so definitely not ad hoc, as far as the mentioned theory goes.

 

[ohwilleke]

A new observation provides another teaching moment for the phenomena that the Op-Ed in the Guardian called out.

The paper Zhi-Chao Zhao, Yong Zhou, Sai Wang, "Standard physics is still capable to interpret ∼18 TeV photons from GRB~221009A" arXiv:2210.10778 (October 16, 2022), is something I'd like to see more of, a careful thoughtful effort to explain unusual observations with Standard Physics whenever possible (it concludes that this is only about a two sigma statistical fluke).

But the observation of thousands of 18 TeV photons from GRB221009A that this paper explains with Standard Physics has, however, also produced many ambulance chasing papers with new physics explanations that are poorly motivated, such as:
* "Light speed variation from GRB 221009A"
* "Axion dark matter from first-order phase transition, and very high energy photons from GRB 221009A"
* "Parameters of axion-like particles required to explain high-energy photons from GRB 221009A"
* "Lorentz invariance violation induced threshold anomaly versus very-high energy cosmic photon emission from GRB 221009A".

 

[DennisN]

Further, observed baryon asymmetry make one wonder why this is so.

For what it's worth (coming from a non-physicist like me) I personally think that (1) the baryon asymmetry and (2) the cosmological constant problem are two of the most exciting questions in physics (both of which Hossenfelder considered "illdefined/bad" problems if I understood her correctly).