Is Dark Matter Really a Cold, Collisionless Fluid?

[ohwilleke]

From one of the papers you cited:

https://arxiv.org/abs/1407.7544

That last part of the sentence is critical. Baryonic behavior for compact systems is monumentally, absurdly complicated. Both supernovae and high-mass black holes can have galactic-scale impacts, and both have massive modeling problems.

The specific reason why they focus on faint dwarf galaxies is because these systems are likely to be less-impacted by such things. But even there the simulations are both complicated and highly contingent on uncertain initial conditions.

First, the dark matter phenomena, from which a dark mater halo could be inferred, in faint dwarf galaxies, was accurately predicted with MOND about 35 years ago. It was one of the very first predictions made by the theory.

Second, as noted by Brooks, a lot of progress has been made since 2014 in modeling self-interacting dark matter with baryonic effects. She has noted this in at least one of her papers.

Third, if the simulations are both complicated and highly contingent on uncertain initial conditions, why isn't the observed outcome of those processes more complicated or more diverse?

If all of the data in a range of applicability from binary star systems to the largest individual galaxies and systems of central galaxies and galaxy satellites can be explained with a one line non-relativistic formula and a single parameters, then either (1) the universe is extremely finely tuned in advance at high Z to fit every known galaxy, or (2) your model is missing something really huge because none of that complexity or formation history matters in influencing the relationships of bodies in these gravitationally bound systems. I respectfully suggest that option 2 and not option 1 has to be correct.

We know, as a matter of rigorously demonstrated empirical fact that 100% of effects attributed to dark matter at galaxy or galaxy-satellite galaxy system scales or less can be fully described from the current distribution of baryonic matter, a single universal physical constant, and a one line formula, without any regard to the history of the formation of that gravitationally bound system. The magnitude of the observed deviations from this relationship are, in every case, no greater than measurement error (not all astronomy measurements, especially of very distant objects, are terribly precise). There are NO OUTLIERS!

We also know with very high confidence that none of the three Standard Model forces (electromagnetism, the strong force, and the weak force) has anything to do with this relationship, and that the observed effects can not be explained by General Relativity without resorting to either dark matter particles, or a gravitational modification (including "fifth forces" that interact with baryonic matter), or some combination of the two.

This is a big problem for dark matter particle theories.

Generically, in any theory with dark matter particles that form halos that give rise to dark matter phenomena, the history of how a galaxy came to be formed should matter. There is no reason in a dark matter particle theory why a galaxy with a more or less identical distribution of baryonic matter to another galaxy should have exactly the same dark matter halo, unless you formulate an additional second theory that explains that. But, the observational reality is that this that a system's formation history is irrelevant to the behavior of gravitationally bound systems.

Indeed, while the absurdly simple MOND formula does not generalize to galactic clusters, even there, the dark matter phenomena which are observed can be discerned from the baryonic matter distribution in the galactic cluster alone, albeit, with a different relationship. This can be hypothesized in terms of a formation process, but it is experienced as a set of tight phenomenological relationships between baryonic matter distributions and inferred dark matter halo size and shape.

We study the total and dark matter (DM) density profiles as well as their correlations for a sample of 15 high-mass galaxy clusters by extending our previous work on several clusters from Newman et al. Our analysis focuses on 15 CLASH X-ray-selected clusters that have high-quality weak- and strong-lensing measurements from combined Subaru and Hubble Space Telescope observations. The total density profiles derived from lensing are interpreted based on the two-phase scenario of cluster formation. In this context, the brightest cluster galaxy (BCG) forms in the first dissipative phase, followed by a dissipationless phase where baryonic physics flattens the inner DM distribution. This results in the formation of clusters with modified DM distribution and several correlations between characteristic quantities of the clusters. We find that the central DM density profiles of the clusters are strongly influenced by baryonic physics as found in our earlier work. The inner slope of the DM density for the CLASH clusters is found to be flatter than the Navarro--Frenk--White profile, ranging from α=0.30 to 0.79. We examine correlations of the DM density slope α with the effective radius Re and stellar mass Me of the BCG, finding that these quantities are anti-correlated with a Spearman correlation coefficient of ∼−0.6. We also study the correlation between Re and the cluster halo mass M500, and the correlation between the total masses inside 5 kpc and 100 kpc. We find that these quantities are correlated with Spearman coefficients of 0.68 and 0.64, respectively. These observed correlations are in support of the physical picture proposed by Newman et al.

Antonino Del Popolo et al., "Correlations between the Dark Matter and Baryonic Properties of CLASH Galaxy Clusters" (August 6, 2018) (the prior works by Newman, et al., being extended are A. B. Newman, T. Treu, R. S. Ellis, D. J. Sand, C. Nipoti, J. Richard, and E. Jullo, The Density Profiles of Massive, Relaxed Galaxy Clusters. I. The Total Density Over Three Decades in Radius, ApJ 765 (Mar., 2013) 24, [arXiv:1209.1391] and A. B. Newman, T. Treu, R. S. Ellis, and D. J. Sand, The Density Profiles of Massive, Relaxed Galaxy Clusters. II. Separating Luminous and Dark Matter in Cluster Cores, ApJ 765 (Mar., 2013) 25, [arXiv:1209.1392]).

The problem with a dark matter particle theory is not that the reality is too complicated to model. The problem is that far too many factors, that should matter, turn out to be completely irrelevant in practice for reasons that no one has yet managed to articulate.

Note, to be clear, I'm not saying that it is impossible that there is some process of galaxy formation that does produce such a tight relationship. But, whatever that process is, it simply can't be that complicated and it absolutely can't be very initial conditions dependent. If it is a chaotic system as that term is defined in mathematics (i.e. end states are highly sensitive to initial conditions) it has to be one with a very strong attractor to the MOND relationship. Any theory that lacks that property is wrong.

Therefore, the claim that galaxy formation is too complicated so dark matter particle theories should be excused for not having a galaxy formation theory that accurately predicts dark matter halo shapes and sizes falls on deaf ears.

 

[ohwilleke]

It's not just that. Modified theories of gravity don't just affect cosmological observations. They affect all observations where gravity is involved. It's very, very difficult to find any modified theory of gravity that makes different predictions about cosmological observations (galaxy rotation curves, expansion of the universe) but doesn't make predictions different enough to be already falsified about other domains that involve gravity (such as the solar system).

Moffat's MOG theory does that.

 

[PeterDonis]

Moffat's MOG theory does that.

The abstract of the paper on this that you linked to earlier starts with:

"Since general relativity (GR) has already established that matter can simultaneously have two different values of mass depending on its context,"

which already makes me skeptical--what are they talking about?

 

[ohwilleke]

The abstract of the paper on this that you linked to earlier starts with:

"Since general relativity (GR) has already established that matter can simultaneously have two different values of mass depending on its context,"

which already makes me skeptical--what are they talking about?

They are doing two separate things.

One is that they are comparing about half a dozen leading phenomenological modified gravity theories with relativistic generalizations to the experimental data (which validates Moffat), which they use as a benchmark for their own modified gravity theory's performance.

Secondly, they are trying a different kind of gravity modification themselves in which each particle has a rest mass and a dynamical mass, in a manner analogous to the way that a particle has both a rest mass from the perspective of a local, co-moving observer, and a relativistic linear momentum, in special relativity. They formulate their gravitational modification from the perspective of the ordinary matter that is giving rise to the modified gravitational pull.

For what it is worth (briefly and just as full disclosure of where I am coming from to avoid misconceptions or suspicions, not because I am advancing this personal theory here at Physics Forums), I don't really like their approach, even though the comparison of other theories that is done with the evidence is very useful. Mechanistically, I personally think that the MOND relationship arises because there are second order quantum gravity effects that are only material in very weak gravitational fields (as measured by the amount of acceleration induced locally by gravity), and I personally think that the source of the second order quantum gravitational effects is a subtle flaw in how general relativity models the self-interactions of gravitational fields with themselves. (I'm not claiming that I came up with either of those ideas myself.)

The latest of the series of articles articulating this approach is this one (whose abstract incidentally, is incorrect to the extent that it says that this result is "consistent with General Relativity" as it is currently formulated and applied, even though it adheres to the core axioms used to formulate GR):

Numerical calculations have shown that the increase of binding energy in massive systems due to gravity's self-interaction can account for galaxy and cluster dynamics without dark matter. Such approach is consistent with General Relativity and the Standard Model of particle physics. The increased binding implies an effective weakening of gravity outside the bound system. In this article, this suppression is modeled in the Universe's evolution equations and its consequence for dark energy is explored. Observations are well reproduced without need for dark energy. The cosmic coincidence appears naturally and the problem of having a de Sitter Universe as the final state of the Universe is eliminated.

A. Deur, "A possible explanation for dark matter and dark energy consistent with the Standard Model of particle physics and General Relativity" (2017). Some of the earlier articles in the series (almost all of which, if not all of which, were published in reputatable peer reviewed academic journals although the papers are not widely cited) are A. Deur, "Self-interacting scalar fields in their strong regime" (November 17, 2016); Alexandre Deur, "A correlation between the amount of dark matter in elliptical galaxies and their shape" (28 Jul 2014); A. Deur, "Implications of Graviton-Graviton Interaction to Dark Matter" (May 6, 2009) and A. Deur, "Non-Abelian Effects in Gravitation" (September 17, 2003). One of the better and more intuitive introductions to the idea is in this power point presentation.

The 2014 article also makes this notable empirical observation motivated by and motivating his approach:

We discuss the correlation between the dark matter content of elliptical galaxies and their ellipticities. We then explore a mechanism for which the correlation would emerge naturally. Such mechanism leads to identifying the dark matter particles to gravitons. A similar mechanism is known in Quantum Chromodynamics (QCD) and is essential to our understanding of the mass and structure of baryonic matter.

The fact of the matter is that nobody has given Deur's work a through enough independent vetting to validate his analysis beyond the almost back of napkin calculations in his own papers (his day job is as a QCD physicist in a national lab, see, e.g., here, and when I corresponded with him and asked why he didn't do more to develop this, he stated that it came down to funding and time, he isn't paid to study gravity, he's paid to study hadron behavior). Still, conceptually, this approach, whether or not it needs tweaks in detail, is very convincing and appealing, and his disadvantage as an outsider to GR physics is also an advantage as it frees him of group think and provides him with a lot of mathematical tools that QCD physicists used to working with non-abelian systems that don't renormalize at low energies know and the GR physicists working with classical GR equations do not. But, I fully recognize that this could end up being too good to be true. If I had a few million dollars to spend, I'd fund a research collaboration with him and some other handpicked physicists so we could devote sufficient resources to find out.

 

[PeterDonis]

they are trying a different kind of gravity modification themselves in which each particle has a rest mass and a dynamical mass

Yes, I get that they are considering modified theories of gravity with that property. What I don't get is their claim that I quoted that General Relativity has that property. And the fact that they make that claim in the first sentence of their abstract as though it were obvious makes me skeptical.

 

[ohwilleke]

Yes, I get that they are considering modified theories of gravity with that property. What I don't get is their claim that I quoted that General Relativity has that property. And the fact that they make that claim in the first sentence of their abstract as though it were obvious makes me skeptical.

My understanding is that they are simply referencing special relativity in the example that I provided when they say ""Since general relativity (GR) has already established that matter can simultaneously have two different values of mass depending on its context[.]" This language may be a bit sloppy, but the gist of that statement read in that manner is true.

 

[PeterDonis]

My understanding is that they are simply referencing special relativity

That doesn't help since SR doesn't have matter with "two different values of mass depending on context" either.

the gist of that statement read in that manner is true.

How?

 

[Chronos]

I am willing to concede that collisionless DM is a less than ideal fit for current observational evidence. I consider it an effective approximation. We are still struggling to identify the players in the dark sector much less draw any conclusions about their interactions.

 

[Adrian59]

I have been following this thread and there is a lot of in depth analysis of the relative merits of dark matter theories v alternative gravity theories. In trying to make sense of this I have constructed a concise summary which I append to my entry. I am sure some cells will provoke discussion and I am showing this in an attempt to gain some clarity and in no was as a final word on the current state of cosmology.

 

[MikeinSpain]

Talk of dark matter and with billions of clusters, galaxies, nebulae, etc, to choose from, there is the Bullet Cluster and...nothing.

Large groups of stars act as though they are controlled by light matter only and we see no weird shapes that cannot be accounted for by said light matter.

DM was once said to be in the halo of a galaxy, but where are the ring doughnut galaxies as in DM outmasses light matter by a factor of six to one?

 

[AgentSmith]

How certain are scientists that the solution to the dark matter problem is a new, unknown particle (or more)? Theories that eliminate the need for new particle(s) and suggest modifications to the current understanding of gravity seem to get less attention. Why is that?

Alternative theories have failed verification attempts of one sort or another. Something is out there that has mass but does not interact with the electromagnetic force. We can always hope that a TOE or quantum gravity will resolve the problem, but...

 

[Adrian59]

(I say "to existing Core Theory applied using existing methods" to recognize a third possibility that could exist instead of, or in addition to the dark matter particles and modified gravity, which is that some of all of dark matter phenomena could arise from the possibility that using a Newtonian approximation of GR in lieu of complete GR to do galaxy plus scale estimates of behavior under GR could be less negligible than previously realized due to previously unrecognized flaws in how the discrepancy between a Newtonian approximation and a GR calculation is estimated. One or two people are argued this in published papers that were largely refuted, but neither side's analysis was really rigorous and extraordinarily careful so this isn't an impossibility.)

In your first section I think that the point you made in parentheses (as quoted) about a possible third way ie one that doesn’t involve dark matter or modifying gravity was a path that I think should be pursued. I think there are likely flaws in simplistically applying Newton Law of gravity to galaxies. It works for the inner solar system with 8-9 bodies in orbit but when there are hundreds of billions of orbiting bodies it may be that we need a different perspective.

Within the dark matter particle paradigm, the evidence has ruled out a lot of the potential dark matter particle parameter space. For example, both MACHOs like red dwarfs and stellar sized or larger black holes, and the hundreds of GeV WIMPs that interact via the weak force as anticipated in electroweak scale supersymmetry theories, have been all but ruled out experimentally.

In your second section, I was pleased that someone else is saying what is surely obvious that early GeV WIMPs have virtually been ruled out by experiment: Xenon 100, LUX 2013, Darkside 50, LUX 300 day results in 2016 and now the preliminary results of Xenon 1T in 2017 have found no evidence of a Gev WIMP from the first theoretical predictions.

Future experiments are planned but it appears unlikely that the marginal extension of the parameter space will show up anything new. I am not against continuing these studies but I am not holding my breath.

The dark matter idea appears not to be a unified theory but a collection of theories where one single example fails to produce all the required results.

 

[Adrian59]

Missing from this are two important facts:
1. Modified gravity models have so far utterly failed to explain the CMB data (here's one formal argument along these lines from 2011, which I'm pretty sure remains valid to this day: https://arxiv.org/pdf/1112.1320.pdf).
2. The above argument asks you to disregard observations of comparatively simple systems (e.g. the CMB) and instead focus on comparatively complex systems (e.g. galaxies). Systematic uncertainties are far, far more likely to muddle our understanding of complex systems.

I have had a copy of this paper by Scott Dodelson for some time, so it is not new to me. I find it highly suspect. The graph is quite spectacular, too much so to be valid; it is after all a model.

You are apparently more in awe of simple solutions that convoluted arguments. Well if simple relationships from direct observation are what you are looking for the the Tully Fisher graph produced by Stacy S. McGaugh, Federico Lelli, and James M. Schombert, in 2016 in their paper ‘The Radial Acceleration Relation in Rotationally Supported Galaxies’ should be of interest to you! The mechanics of rotating galaxies may be complex but the fact that there is a tight relationship between galactic baryonic matter and rotational velocity is a serious challenge to dark matter theories.

If the only thing dark matter theories can do better that other theories is to explain esoteric signals from the relic of the big bang, then it has failed. I think it is far better to get a good theory for current (ie t=0) astronomical phenomenon than minor (1/100,000) fluctuations in the CMBR.

 

[kimbyd]

I have been following this thread and there is a lot of in depth analysis of the relative merits of dark matter theories v alternative gravity theories. In trying to make sense of this I have constructed a concise summary which I append to my entry. I am sure some cells will provoke discussion and I am showing this in an attempt to gain some clarity and in no was as a final word on the current state of cosmology.

View attachment 229165

The Tully-Fisher relationship is definitely predicted by LCDM (https://arxiv.org/abs/1204.1497). Whether the low scatter is fully-explained is somewhat less certain, but this definitely warrants at worst a "maybe" rather than a "no".

 

[kimbyd]

I have had a copy of this paper by Scott Dodelson for some time, so it is not new to me. I find it highly suspect. The graph is quite spectacular, too much so to be valid; it is after all a model.

The CMB is an incredibly rich source of data. There's nothing to be suspicious about. This is why I always draw these discussions back to the CMB data.

 

[Adrian59]

The CMB is an incredibly rich source of data. There's nothing to be suspicious about. This is why I always draw these discussions back to the CMB data.

We are slightly at crossed purposes here. It wasn't the CMBR data I was saying was suspect it was the Scott Dodelson paper you referenced I regard as suspect. You I am sure will disagree but let me put the argument differently - if the graph that Scott Dodelson draws is so correct and it does show a massive disparity between CDM and modified theories then why was that not the end of the argument?

 

[Adrian59]

The Tully-Fisher relationship is definitely predicted by LCDM (https://arxiv.org/abs/1204.1497). Whether the low scatter is fully-explained is somewhat less certain, but this definitely warrants at worst a "maybe" rather than a "no".

I had a look at your new reference but slightly got disillusioned at part 2, Method as I quote, 'The basic semi-analytic methodology employed in this work is
adapted from M98 (Sect. 2). As the starting point, I take a spiral galaxy with a particular stellar mass. The Mvir-Mstar relation from the Halo Abundance Matching performed in RP11 (eq. 5) is then used to calculate the virial mass of the surrounding dark halo.

Surely, if your using an analytical method to calculate the dark halo from the stellar mass then is it any wonder that the Tully-Fisher relationship remains intact?

 

[kimbyd]

I had a look at your new reference but slightly got disillusioned at part 2, Method as I quote, 'The basic semi-analytic methodology employed in this work is
adapted from M98 (Sect. 2). As the starting point, I take a spiral galaxy with a particular stellar mass. The Mvir-Mstar relation from the Halo Abundance Matching performed in RP11 (eq. 5) is then used to calculate the virial mass of the surrounding dark halo.

Surely, if your using an analytical method to calculate the dark halo from the stellar mass then is it any wonder that the Tully-Fisher relationship remains intact?

The BTFR compares the baryonic mass estimated from luminosity to the gravitational mass determined from stellar velocities.

 

[kimbyd]

We are slightly at crossed purposes here. It wasn't the CMBR data I was saying was suspect it was the Scott Dodelson paper you referenced I regard as suspect. You I am sure will disagree but let me put the argument differently - if the graph that Scott Dodelson draws is so correct and it does show a massive disparity between CDM and modified theories then why was that not the end of the argument?

You mean the matter power spectrum estimate? Eh. It might be possible to do a little bit better than is presented there. But really, this is why very, very few physicists are still on the "no dark matter" train.

Edit: And if you doubt it, why not see if any people pushing modified gravity models have results for the matter power spectrum in their model?

 

[kimbyd]

Incidentally, McGaugh has presented just that as recently as 2014:
https://arxiv.org/abs/1404.7525

Even in MOND, apparently, the issue pointed out by Dodelson is very clear. McGaugh only compares the models without reference to the data, and hand-waves away the discrepancy claiming that at low redshift the discrepancy should disappear in MOND (without explaining why or modeling it). To me, this is a giant cop-out: structure formation will only impact the small-scale behavior. There are large discrepancies between MOND and dark matter models even on pretty large scales.

 

[Adrian59]

Incidentally, McGaugh has presented just that as recently as 2014:
https://arxiv.org/abs/1404.7525

Even in MOND, apparently, the issue pointed out by Dodelson is very clear. McGaugh only compares the models without reference to the data, and hand-waves away the discrepancy claiming that at low redshift the discrepancy should disappear in MOND (without explaining why or modeling it). To me, this is a giant cop-out: structure formation will only impact the small-scale behavior. There are large discrepancies between MOND and dark matter models even on pretty large scales.

That is all very well but I am not a supporter of MOND and frequently when I question the dark matter paradigm the response is what about MOND? I not sure if this is a ploy to shift the argument to a safer place for a dark matter proponent rather than meet the question head on.

 

[Adrian59]

You mean the matter power spectrum estimate? Eh. It might be possible to do a little bit better than is presented there. But really, this is why very, very few physicists are still on the "no dark matter" train.

Edit: And if you doubt it, why not see if any people pushing modified gravity models have results for the matter power spectrum in their model?

How about, 'Comment on “The Real Problem with MOND” by Scott Dodelson', arXiv:1112.1320 by J. W. Moffat and V. T. Toth ref arXiv:1112.4386 [astro-ph.CO].

 

[kimbyd]

How about, 'Comment on “The Real Problem with MOND” by Scott Dodelson', arXiv:1112.1320 by J. W. Moffat and V. T. Toth ref arXiv:1112.4386 [astro-ph.CO].

Their argument appears to be that the data set doesn't have the resolution required to distinguish the baryonic oscillations which occur in the modified gravity model.

That's fine, but that just argues for a reanalysis of the data which is designed to focus in on the baryonic oscillations themselves, and to include more than one old data set (2006). I don't buy Moffat and Toth's argument that we don't yet have enough data. I do buy that the way in which data is often processed might hide this effect, but the raw galaxy data sets are quite large. I'm quite sure that they could bin the data in such a way that these oscillations would be made more apparent, and there's a ton more data that could be included.

 

[ohwilleke]

I have been following this thread and there is a lot of in depth analysis of the relative merits of dark matter theories v alternative gravity theories. In trying to make sense of this I have constructed a concise summary which I append to my entry. I am sure some cells will provoke discussion and I am showing this in an attempt to gain some clarity and in no was as a final word on the current state of cosmology.

View attachment 229165

I would call unmodified GR a "no" for galaxy rotation and Tully-Fisher. I would say that TeVeS and MOG are both compatible with the Standard Model of Particle Physics, in the sense of not adding new particles although that is really untrue for all five including unmodified GR. Not sure why you think TeVeS fails in terms of gravitational redshift. On cluster dynamics DM is a "maybe" not a "yes". MOG would be a maybe or yes on a reasonable cosmological simulation.

Also MOG really refers to a specific modified gravity theory of Moffat, while f(R) is a different modified gravity theory. MG is the commonly used abbreviation for "modified gravity" theory, in general.

 

[ohwilleke]

The dark matter idea appears not to be a unified theory but a collection of theories where one single example fails to produce all the required results.

Just so.

 

[ohwilleke]

The Tully-Fisher relationship is definitely predicted by LCDM (https://arxiv.org/abs/1204.1497). Whether the low scatter is fully-explained is somewhat less certain, but this definitely warrants at worst a "maybe" rather than a "no".

No one has every produced a DM fit to the Tully-Fisher relationship without an extra 16 parameters highly tuned to the actual reality. Further, the attempts to fit the Tully-Fisher relationship with LCDM are attempts at post-diction not examples of prediction.

 

[Adrian59]

Their argument appears to be that the data set doesn't have the resolution required to distinguish the baryonic oscillations which occur in the modified gravity model.

That's fine, but that just argues for a reanalysis of the data which is designed to focus in on the baryonic oscillations themselves, and to include more than one old data set (2006). I don't buy Moffat and Toth's argument that we don't yet have enough data. I do buy that the way in which data is often processed might hide this effect, but the raw galaxy data sets are quite large. I'm quite sure that they could bin the data in such a way that these oscillations would be made more apparent, and there's a ton more data that could be included.

Maybe we are moving towards a consensus. You appear to have dropped your original position that a modified theory hasn't been tried to explain the matter power spectrum, as it has. Your final point about reanalysis with a larger data set appears correct but I still don't think the original graph drawn by Scott Dodelson is correct.

Doing some background searching I have found that Scott Dodelson is much more open to alternative theories than I thought, only hearing him second hand. Speaking in 2006 he says, “To definitively claim that dark matter is the answer, we need to find it,” Dodelson explained to PhysOrg.com. “We can do this in one of three ways: produce it in the lab (which might happen at Fermilab, the soon-to-start LHC, or ultimately the International Linear Collider), see a pair of dark matter particles annihilate and produce high energy photons (there are about a half dozen experiments designed to look for this), or see a dark matter particle bump a nucleus in a large underground detector (again, about 10 experiments are looking for this). Until one or more of these things happen, skeptics are still allowed'.

Also, speaking more recently he says, ' We're living in an era of cognitive dissonance. There is all this cosmological evidence for the existence of dark matter, but over the last 30 years, we've run all these experiments and haven't found it. My bet is that we're looking at things all wrong. Someone who's 8 years old today is going to come around and figure out how to make sense of all the data without evoking mysterious new substances.'

 

[ohwilleke]

The point about the 21cm data is more impressive with a diagram.

The predicted 21cm absorption with dark matter (red broken line) and without (blue line). Also shown (in grey) is the signal observed by EDGES.

 

[kimbyd]

The point about the 21cm data is more impressive with a diagram.

View attachment 229279

The predicted 21cm absorption with dark matter (red broken line) and without (blue line). Also shown (in grey) is the signal observed by EDGES.

We'll see. It's too early to be very confident in those results. Also, the red dotted curve is just incorrect: the dark matter contribution to that graph depends critically upon what type of dark matter is assumed. Milli-charged dark matter at 100MeV could still explain the data as it stands, or there might be something else that hasn't been considered.

But there's a fair chance the data is just incorrect. That's usually the case when a new observational window is opened on the universe. It's usually best to be patient with this sort of thing.

 

[Adrian59]

I would call unmodified GR a "no" for galaxy rotation and Tully-Fisher. I would say that TeVeS and MOG are both compatible with the Standard Model of Particle Physics, in the sense of not adding new particles although that is really untrue for all five including unmodified GR. Not sure why you think TeVeS fails in terms of gravitational redshift. On cluster dynamics DM is a "maybe" not a "yes". MOG would be a maybe or yes on a reasonable cosmological simulation.

Also MOG really refers to a specific modified gravity theory of Moffat, while f(R) is a different modified gravity theory. MG is the commonly used abbreviation for "modified gravity" theory, in general.

Some interesting responses: we were both involved in a thread last year when the issue of a paper by Carrick and Cooperstock was discussed. I have found your remarks then from 26/10/17:

‘I am pretty sure neither Cooperstock (in a classical GR framework) or Deur (in a quantum gravity framework) are actually applying mainstream GR to get their results and actually subtly deviate from mainstream interpretation of GR. But, the fact that both investigators can get such impressive results with such very, very subtle tweaks to GR interpretation is in my view very promising. It is not at all obvious that conventional GR does not have a subtle flaw or two that make a big difference at large scales in weak gravitational fields.

Everyone agrees that Newtonian gravity has to be modified. GR is a Newtonian gravity modification. The question is whether GR has to be modified.’


So, I would propose that unmodified GR can explain galaxy rotation curves or at least there should be a ‘maybe’ in the box. Correct me if I am wrong about the Tully-Fisher relationship but since it is a relationship between baryonic matter and galaxy rotation, maximum orbital velocity any theory that doesn’t have putative extra mass should agree with this relationship. The issue with TeVes comes from a paper by Wojak et al ref: ‘Gravitational redshift of galaxies clusters as predicted by general relativity.’ Nature volume 477, pages 567–569 (29 September 2011).

The reason that I elected to signify that TeVeS and MOG didn’t fit the standard model is because don’t these theories postulate extra fields which presumably if they exist will need an extension of the standard model of physics. I've noted your final point on nomenclature.

 

[ohwilleke]

So, I would propose that unmodified GR can explain galaxy rotation curves or at least there should be a ‘maybe’ in the box. Correct me if I am wrong about the Tully-Fisher relationship but since it is a relationship between baryonic matter and galaxy rotation, maximum orbital velocity any theory that doesn’t have putative extra mass should agree with this relationship.

The Tully-Fischer relationship is a trivial corollary of the radial acceleration relationship which is basically another version of MOND. You need either DM or MG for it to work. It is completely inconsistent with unmodified GR without dark matter.

The reason that I elected to signify that TeVeS and MOG didn’t fit the standard model is because don’t these theories postulate extra fields which presumably if they exist will need an extension of the standard model of physics. I've noted your final point on nomenclature.

The pedantic point would be that GR, a classical theory, is still incompatible with the Standard Model, because there isn't a quantum gravity theory that is necessary for them to be theoretically consistent. This is a non-trivial big deal, but surely not what you intended to indicate.

Even GR or vanilla quantum gravity require extra gravitational fields beyond the SM (two of them in theories with a cosmological constant (GR) and/or dark energy (vanilla quantum gravity)).

I would normally think of TeVeS and MOG as equally compatible with the SM as GR because all of the fields added in each case are purely gravitational. GR with a cosmological constant has a scalar field and a tensor field. TeVeS and MOG have a scalar field, a tensor field and a vector field. Vanilla quantum gravity has a scalar dark energy field and a tensor graviton. MOND isn't relativistic and is admittedly an incomplete toy model, and so it doesn't really make sense to talk about it in that sense.

The non-gravitational part of the resulting complete theory wouldn't be changed in any of them except as necessary for a quantum gravity version (all of the popular live theories except Deur's are at their root classical theories).

This isn't really perfectly true, however. There is basically a solid consensus that GR, vanilla quantum gravity, TeVeS, MOG, MOND, Deur's approach, and any other attempt to integrate any remotely realistic theory of gravity in any form with the Standard Model in a quantum gravity form, changes the beta functions (which change the values of these constants with respect to energy scale as they run) of all of the experimentally measured constants of the Standard Model (all 12 fermion mass constants, 3 boson mass constants, all 4 CKM matrix constants, all 4 PMNS constants and all 3 coupling constants).

This isn't a huge impact on the SM, but it would potentially impact, for example, gauge unification, which might happen in an alternative to the SM without gravity, but not one with it, or it could cause the SM coupling constants to unify (in principle, I haven't run the numbers on that and the exact form of the modified beta function is not a consensus issue even though the need to modify it in some manner when gravity is included is a consensus issue).

Indeed, given that all of the variations from GR take place in the weak field limit (with possible quantum gravity differences that would be the same for quantum versions of all realistic modified gravity theories), the way that any integration of quantum gravity or modified quantum gravity impacts the SM beta functions would probably be experimentally indistinguishable, even though there would be some slight differences.

 

[Adrian59]

The Tully-Fischer relationship is a trivial corollary of the radial acceleration relationship which is basically another version of MOND. You need either DM or MG for it to work. It is completely inconsistent with unmodified GR without dark matter.

I didn't see the Tully-Fisher relationship as similar to a galaxy rotation curve as you seem to be implying. Of course that is the underlying problem that both dark matter and MOND are trying to solve. As I am not a supporter of MOND, it is always with care that I reference Stacy McGaugh but non the less his paper from 2016 entitled ‘The Radial Acceleration Relation in Rotationally Supported Galaxies’ is an observational paper and as such can be read independently of Stacy McGaugh's preferred solution of MOND. I understood the observations to show a tight relation between baryonic mass and rotational velocity. It is surely meant to question the validity of dark matter as in order for dark matter theories to be compatible with these observations one has to invoke complex 'feed back' mechanisms. To my way of thinking any theory that can explain galaxy rotation velocities without dark matter will be compatible with these observations.

The pedantic point would be that GR, a classical theory, is still incompatible with the Standard Model, because there isn't a quantum gravity theory that is necessary for them to be theoretically consistent. This is a non-trivial big deal, but surely not what you intended to indicate.

Exactly.

Even GR or vanilla quantum gravity require extra gravitational fields beyond the SM (two of them in theories with a cosmological constant (GR) and/or dark energy (vanilla quantum gravity)).

I also fully agree with this but moving on would you not agree that within the theoretical framework of SM a field requires a boson (I don't know if a future theory of quantum gravity can dispense with this requirement) but look how desperate the physics community was in its search for the Higg's boson.

However, returning to an earlier post in this thread, you alluded to being a supporter of a version of MG. I don't know if you are prepared to say which and why if it is not too off topic for this thread.

 

[kimbyd]

It is surely meant to question the validity of dark matter as in order for dark matter theories to be compatible with these observations one has to invoke complex 'feed back' mechanisms.

I don't think I'll ever understand why this is an objection. The feedback mechanisms are either there or they aren't. They shouldn't be dependent upon any unknown physics. Every aspect of these feedbacks should be measurable through a combination of observations and simulations. The only aspect of the feedbacks that relies upon unknown physics is the degree to which dark matter interacts (both with itself and with normal matter). Everything else just depends upon understanding in detail the normal matter within galaxies.

When I hear somebody complaining about the inclusion of these feedbacks being necessary, I interpret that as a statement akin to, "That math is too complicated, so I don't believe you." If adding more complicated effects to the simulations improves the fits, that's a good thing. If those more complicated effects depend upon some parameters that aren't yet measured, but are measurable, then that's even better: it provides an additional test to the theory. If somebody comes up and says they have an answer that doesn't require that hard work, I'm going to be immediately suspicious.

 

[Adrian59]

I don't think I'll ever understand why this is an objection. The feedback mechanisms are either there or they aren't. They shouldn't be dependent upon any unknown physics. Every aspect of these feedbacks should be measurable through a combination of observations and simulations. The only aspect of the feedbacks that relies upon unknown physics is the degree to which dark matter interacts (both with itself and with normal matter). Everything else just depends upon understanding in detail the normal matter within galaxies.

When I hear somebody complaining about the inclusion of these feedbacks being necessary, I interpret that as a statement akin to, "That math is too complicated, so I don't believe you." If adding more complicated effects to the simulations improves the fits, that's a good thing. If those more complicated effects depend upon some parameters that aren't yet measured, but are measurable, then that's even better: it provides an additional test to the theory. If somebody comes up and says they have an answer that doesn't require that hard work, I'm going to be immediately suspicious.

I am partly in agreement. Einstein's general theory of relativity (as a graduate of physics I have struggled with connection coefficients and Riemann Tensors) is far more complex than Newton's theory of gravity but because it makes correct predictions it is regarded as the correct theory. However, epicycles never worked. So complexity is allowed if it ticks all the boxes. As Albert Einstein himself said, ‘Everything should be made as simple as possible, but not simpler.’

 

[kimbyd]

I am partly in agreement. Einstein's general theory of relativity (as a graduate of physics I have struggled with connection coefficients and Riemann Tensors) is far more complex than Newton's theory of gravity but because it makes correct predictions it is regarded as the correct theory. However, epicycles never worked. So complexity is allowed if it ticks all the boxes. As Albert Einstein himself said, ‘Everything should be made as simple as possible, but not simpler.’

Right. The difference is that epicycles were an idea that had no physical process behind them. Furthermore, it turned out that epicycles were just a way to do Fourier transforms on the path, and as such they can fit any closed trajectory provided enough epicycles were used. It's not terribly infrequent that people examine data today using methods that are conceptually similar to epicycles, in that they amount to nothing but curve fitting as the parameters involved have no connection to any physical model.

General Relativity is also an interesting comparison. In an important way, it isn't actually more complex than Newtonian gravity. Conceptually, it's as simple as, "What is the simplest possible non-trivial theory of gravity as space-time geometry?" A super-rough sketch of the logic is this:
1. To ensure the theory doesn't depend upon coordinates, the equations of motion can only depend upon coordinate-free parameters.
2. The only coordinate-free parameter available is the Ricci scalar R.
3. The simplest non-trivial function of R is $A + BR$, with A and B being constants (by convention, A and B are often expressed in terms of $\Lambda$, $G$, and $c$).
4. Use the above as the Lagrangian of the space-time geometry, which is added to the matter Lagrangian.
5. Be careful that all calculations take into account that space-time is curved.

That's it. That's all that General Relativity is. But the consequences of the theory can be inordinately complex at times. The construction above has an incredible simplicity to it, but the calculations one has to perform to determine what happens as a result of the theory can be tremendously challenging. It's really easy to make mistakes in the theory because it breaks a lot of our intuitions about how space-time behaves, and the choice of coordinates can have unexpected consequences if coordinates are not handled carefully (this is why many calculations try to avoid using coordinates at all).

GR really is the holy grail of a scientific theory. It has so few parameters, and so few assumptions, but explains so much. Most things that we try to understand scientifically end up being way more complicated in practice. The reason why galaxies are complicated is because baryonic matter is incredibly complicated. I see no problem with simulations of galaxies requiring 16 parameters (or however many parameters they fit). But those parameters absolutely should be things that are tied to physical processes that actually happen within galaxies, and we should be able to test the veracity of each and every one of those parameters through a combination of small-scale simulations and observations.