Dark Matter does not exist, what do you think?

[ohwilleke]

MOND was invented purely to match the galaxy rotation curves that were observed. If MOND were written down before these observations and *then* predicted the observed rotation curves then that would be something different (you know, like DM predicts the cosmic web as observed with SDSS, or predicts the CMB anisotropies).

MOND actually did just that. It was invented purely to match the galaxy rotation curves in spiral galaxies, turned out to fit existing data about elliptical galaxies not use to fit the original theory, and then proved to successfully fit dwarf galaxies for which there was no data when the theory was invented. MOND at least as good a record, if not a better one, than DM theories, in being predictive.

 

[CKH]

ohwillleke,

That's an interesting, objective synopsis of the situation.

This casts real doubt on the likelihood that a baroque self-interacting DM theory can work, creates a very high bar for WDM theories to live up to.

Exactly. We can dream up all sorts of exotic particles and theories of gravity, but why bother if a little normal dark matter can do the trick? Analyses show that relatively small additional amounts of dark baryonic matter in the extended disks explain galaxies just fine (with Newtonian gravity). Papers about this are ignored (in the sense that there are few citations and no refutations).

Confidence in the successes of CDM at the cosmological level seems to blind theorists to the obvious. Pavel Kroupa is quite vocal about this, but his arguments fall on deaf ears. He is branded as a misguided supporter of the admittedly ugly theory MOND, while in fact his arguments against non-baryonic matter are independent of MOND as a valid theory of gravity.

There are claims that observations prove that no significant amounts of unseen normal matter exist,. These ignore how little we actually know about the behavior of very cold molecular hydrogen (2.7K is cold even for H2) and how difficult it is to detect. Exploration of possible forms of normal matter is minimal while we spend billions looking for non-baryonic matter that is still hypothetical. Baryonic matter is physically far more complex than CDM. I would not be surprised if cold H2 has a tendency to clump into small, self-preserving, dense clouds that are difficult to observe. The vast extents of HI that we can observe may be nothing more than a atmosphere in which substantial quantities of H2 are embedded.

 

[julcab12]

ohwillleke,
There are claims that observations prove that no significant amounts of unseen normal matter exist,. These ignore how little we actually know about the behavior of very cold molecular hydrogen (2.7K is cold even for H2) and how difficult it is to detect. Exploration of possible forms of normal matter is minimal while we spend billions looking for non-baryonic matter that is still hypothetical. Baryonic matter is physically far more complex than CDM. I would not be surprised if cold H2 has a tendency to clump into small, self-preserving, dense clouds that are difficult to observe. The vast extents of HI that we can observe may be nothing more than a atmosphere in which substantial quantities of H2 are embedded.

...This is bit problematic. There are a lot of ways to distinguished baryonic behavior(interactions) both direct and indirect. We understand it from particle physics, observations-spectroscopy, formulated models such as GR and so on.

Hydrogen-PP. We know for the fact that hydrogen emits and absorbed radiation in diff energy levels(UV- visible light-infra)-- That's the usual red color shown in galactic photos (H-alpha). We can plot its wavelengths as the universe expands and leaves absorption lines. In one of the absorption lines a 'thing' is moving to quickly and ignoring the CMB's hot/cold spots and forming a small scale structure. Well normal structure don't do that. Keep in mind we are talking about interactions here from the slices of absorption lines.

Are you suggesting some phenomenon of hydrogen that acts like DM? If so, For all we know. The lowest energy state of hydrogen acts like a normal matter does.

Other evidences such as "Rotation-curves and galaxy-cluster mass measurements show the detailed distribution of matter in those objects, the amount of mass far exceeds the observed mass ---> most mass is non-observed."

http://physics.stackexchange.com/questions/26778/how-do-we-know-dark-matter-is-non-baryonic

 

[Torbjorn_L]

What MOND has going for it is that it is a very simple one parameter formula

That MOND is ad hoc makes it even more problematic than physically motivated "modified gravity".

MOND's success within its domain of applicability

It can predict galaxy behavior superficially naturally since it is a curve fit. But it - like all other modified gravity theories - doesn't make sense, since then it can't predict galaxy collisions (or other gravity effects). There is no physics continuity, except an extraordinary claim without evidence - "just because". Morally, they fail before they start.

With latest standard cosmology models, they do all that and predict galaxy emergence and detailed structure to boot, from first principles. [ http://www.illustris-project.org/ ] MOND is competed away long since, it is fringe. The OT article shows that, it is attempting to tear down current observations with the erroneous idea that 'hence MOND'. Cue pseudosciences, like 'design' creationism.

I'm not sure failed ideas fits on PhysicsForum as such. Maybe if there is a historical discussion?

 

[CKH]

...This is bit problematic. There are a lot of ways to distinguished baryonic behavior(interactions) both direct and indirect. We understand it from particle physics, observations-spectroscopy, formulated models such as GR and so on.

Hydrogen-PP. We know for the fact that hydrogen emits and absorbed radiation in diff energy levels(UV- visible light-infra)-- That's the usual red color shown in galactic photos (H-alpha). We can plot its wavelengths as the universe expands and leaves absorption lines. In one of the absorption lines a 'thing' is moving to quickly and ignoring the CMB's hot/cold spots and forming a small scale structure. Well normal structure don't do that. Keep in mind we are talking about interactions here from the slices of absorption lines.

Cold H2 is so nearly undetectable that, even though we know it must be present, we rely on CO to guess how much is there. We do not know whether H2/CO ratios hold, particularly beyond the luminous parts of galaxies. HI is much easier to detect and we do know it exists far beyond the luminous disk. Is it alone?

Are you suggesting some phenomenon of hydrogen that acts like DM?

Only in that we cannot always detect hydrogen. In the cold H2 form, it is highly transparent and has very little emission. Otherwise its properties are nothing like non-baryonic DM.

If so, For all we know. The lowest energy state of hydrogen acts like a normal matter does.

I'm not sure what your point is here. The question is how does it behave in the cosmos? Can it condense? We know it can because we have H2 planets in our own solar system. We also know that large molecular clouds of H2 form even near the center of the Galaxy where radiation is high. What happens in the outer disk where average densities are low and temperatures are below 14K, even approaching 3K? I don't believe that macho surveys have covered this territory.

We know the basic properties of the elements, but that has not allowed us to predict life. Knowledge of properties of hydrogen in certain states does not imply that we know how it behaves under all conditions.

Other evidences such as "Rotation-curves and galaxy-cluster mass measurements show the detailed distribution of matter in those objects, the amount of mass far exceeds the observed mass ---> most mass is non-observed."

We don't have a very good idea what accounts for the large unseen mass in clusters. All we see are lots of galaxies and some very thin X-ray emitting plasma. Whatever is not emitting substantial radiation, we don't see. That does not imply that the only possibility is non-baryonic matter.

Rotation curves are an entirely different story. Modest amounts of normal dark matter in the outer disks of spiral galaxies can account for the rotation curves. In the CDM view, huge halos are required, 5 times as massive as the visible matter. On the other hand, rotation curves have been shown to be highly sensitive to the visible baryonic matter. How can that be when CDM is 5 times as massive and barely interacts with baryonic matter? A realistic DM theory of disk galaxies requires a mysterious cooperation between baryonic and non-baryonic matter (this is called the disk-halo conspiracy). The cusps expected in DM distributions are missing. Disk galaxies obey simple relationships like Tully-Fisher that are at best difficult to explain with DM.

So far no suitable non-baryonic particle has been found. Supersymmetry, which is suppose to predict an appropriate particle, is failing to show up at the LHC. Elaborate experiments searching for DM have turned up empty.

http://physics.stackexchange.com/questions/26778/how-do-we-know-dark-matter-is-non-baryonic

I don't believe that baryonic matter has been constrained into non-existence, excluding that which is luminous. There is some evidence (see Extreme Scattering Events) that small (AU scale) dense clouds of H2 exist in large numbers. These clouds are small enough to go undetected in spectroscopic surveys.

An unmentioned motivation for non-baryonic matter is to make the LCDM theory of the CMB and nucleosynthesis work. The theory allows only a limited amount of baryonic matter, but it also needs much more mass to work. The extra mass is assumed to have essentially no effect on nucleosynthesis so non-baryonic, non-interacting matter is proposed to make the theory work.

In the above thread, someone claims that any solid bodies that might account for additional mass must be as small as asteroids otherwise they would have been detected. Then why have we not detected the hypothesized trillions of solid objects in the Oort cloud? We are still finding large objects in the outer solar system. These are in front of our noses on a galactic scale and illuminated by the nearby sun

 

[CKH]

That MOND is ad hoc makes it even more problematic than physically motivated "modified gravity".

You are missing the point. Even if MOND is wrong as a theory of gravity, the fits demonstrate that rotation curves respond to the baryonic matter distribution. Whatever non-baryonic DM exists must either be distributed like baryonic matter or somehow closely coordinated with baryonic matter. Neither is likely for collisionless, non-baryonic matter.

With latest standard cosmology models, they do all that and predict galaxy emergence and detailed structure to boot, from first principles. [ http://www.illustris-project.org/ ] MOND is competed away long since, it is fringe. The OT article shows that, it is attempting to tear down current observations with the erroneous idea that 'hence MOND'. Cue pseudosciences, like 'design' creationism.

Sorry, but LCDM has not predicted the detailed structure of galaxies nor has it yet provided a firm theory of galactic evolution. It has great success with large scale structure and the CMB. A theory of galactic evolution in the context of LCDM (merger theory) has been put forward, but there are problems.

Please, omit the name calling. Serious scientists, well respected in the community, see MOND as telling us something is wrong with the DM model of galaxies. It doesn't matter if MOND fails as a theory of cosmology. What matters is what it tells us about galaxies and the problems this raises for DM as a model.

I'm not sure failed ideas fits on PhysicsForum as such. Maybe if there is a historical discussion?

Isn't this a discussion about observational science? Are MOND's observational fits to be dismissed for some reason? MOND isn't history yet as much as some would like it to be. If LCDM was successful at explaining everything there would be nothing to discuss. The fact is that LCDM has many problems on small scales and these are important to address, even if it means considering ideas that contradict some tenets of LCDM.

Would you prohibit discussion of the problems with LCDM? Plenty of scientist in the mainstream recognize that there are problems and are publishing papers about them as we speak.

Besides, this discussion is not about replacing LCDM with some other theory of cosmology (no one has a replacement to offer). This is about testing LCDM on small scales and trying to figure out what's going on. It is not obvious.

 

[Jonathan Scott]

You are missing the point. Even if MOND is wrong as a theory of gravity, the fits demonstrate that rotation curves respond to the baryonic matter distribution. Whatever non-baryonic DM exists must either be distributed like baryonic matter or somehow closely coordinated with baryonic matter. Neither is likely for collisionless, non-baryonic matter.

I agree with all this in general, but don't forget that MOND uses a modified formula for gravity, where the bit that matters is an extra term of the form $\sqrt{a_0 Gm/r^2}$. This means that the surprisingly accurate fit for MOND is based on the existing baryonic matter but a modified law of gravity. This means for example that DM cannot match the MOND formula by simply multiplying up the baryonic matter in some way. The amount of matter for MOND purposes is of course estimated using mass to luminosity ratios. The fit in most cases is generally quite good assuming fixed values for M/L, but sometimes an even better fit can be assumed by using spectral characteristics to identify variations within a galaxy.

 

[CKH]

Right, MOND is applied as a theory of gravity acting only on the visible matter. But perhaps MOND can also be interpreted as an empirical relationship that accounts for some matter that we have not detected.

The idea is that the distribution of visible matter is highly correlated in some way with additional matter in the outer disk that we have not detected. Such a correlation is likely considering the consistency in the structure of disk galaxies that leads to strong relationships like Tully-Fisher. This additional normal dark matter may reside in the HI areas of the disk, mostly outside of the visible disk. The question is can such an unseen distribution account for the success of the MOND formula which is based only on the visible matter? This might explain the MOND fits while at the same time falsifying it as a theory of gravity. I have not yet seen a paper that directly addresses this question. I'm still looking.

Of course standard non-baryonic DM must also explain the MOND fits, but this seems more difficult because it implies that DM halos and baryonic matter share some firm interrelationship even though their properties are completely different and their interaction is weak.

One the other hand, I was just reading some papers that claim to explain disk galaxies in another way. They assume that there is additional dark matter associated with the HI disk. They assign a mass ratio of this hypothetical dark matter relative to the measured HI and show that this is consistent with the rotation curves. The obvious candidate is H2. So far the only paper I've seen that contradicts this possibility claims that there is a disk stability problem. The specific assumptions of this author that would lead to instability are not detailed in the paper.

 

[Torbjorn_L]

Even if MOND is wrong as a theory of gravity, the fits demonstrate that rotation curves respond to the baryonic matter distribution.

It is inconvenient to have this discussed in several threads. Here is what I said earlier today in response to you:

There is no miracle and no significance beyond a fit. And we would have to have a fit between a symmetric gravitational potential well and a symmetric rotation curve, similar to how we have to have a fit between a sink drain and a draining water surface. The miracle would be if we didn't find a steady state rotation curve to fit against.

The first happens to be called "MOND" by some, the latter is called a "swirl" by many. In neither case does it mean the forces involved are exceptional. It means we have found an expected steady state. Maybe we should go out and call for an exceptional SWIRL theory of drainage water surfaces, because we don't see that precise surface much elsewhere. Or maybe not.

Baryonic matter is what is observed by those rotation curves, that is the only significance here, not the steady-state itself.

Please, omit the name calling. Serious scientists, well respected in the community, see MOND as telling us something is wrong with the DM model of galaxies.

It was not name calling, it was a description.

I don't question the seriousness of fringe scientists, if anything they need more of ot.

Nor do I question that they are well respected on other matters or before they adopted or not left the fringe. However I would question if they stay well respected on that matter.

Isn't this a discussion about observational science?

If MOND has passed to unusable fringe, it is a historical discussion.

In any case, the OP was whether or not DM is accepted as existing (it is), not whether or not MOND was once fruitful (it wasn't).

The OP is answered, so maybe you should open a thread on MOND for those who are interested. Or at least, can you extract this fringe topic from all but one thread where you find it most interesting for you?

 

[CKH]

There is no miracle and no significance beyond a fit. And we would have to have a fit between a symmetric gravitational potential well and a symmetric rotation curve, similar to how we have to have a fit between a sink drain and a draining water surface. The miracle would be if we didn't find a steady state rotation curve to fit against.

If these fits were that trivial and unrelated to the issue of dark matter halos, there would be no interest in them. You seem to be unaware that these fits are important to mainstream scientists. They require explanations in the context of mainstream theory. Many mainstream scientists are interested and seeking explanations in the context of CDM.

MOND as a theory of gravity may be considered fringe since there are relatively few supporters. There are strong arguments against MOND as a theory of gravity. However, the empirical relationships discovered in disk galaxies are observational facts of interest to everyone.

Tully-Fisher for example is an empirical relationship that is an observational fact. Such relationships beg theoretical explanations.

 

[julcab12]

Hi CKH,

... Ah. Ok. I remember an article in H2 -- molecular h in the past but i haven't got much into subject. All I've read is that their seem to be a problem on the CBR readings that doesn't fit in the adiabatic model of primordial fluctuations inclusive of universe of baryons and photons.

That's the time when the current view of DM is some sort of massive nuetrino.

I got little luck googling reliable article on H2-as dark matter and just found 1.

http://ned.ipac.caltech.edu/level5/Combes3/Combes4.html.

..I just don't like how they treat MOND although it is invaluable to some extent. Talking about high expectations..

 

[ohwilleke]

That MOND is ad hoc makes it even more problematic than physically motivated "modified gravity". It can predict galaxy behavior superficially naturally since it is a curve fit.

Why?

It necessarily follows that if MOND is a good fit over the entire range of dark matter phenomena up to elliptical galaxies with a single parameter, that any actually correct theory must closely reproduce this result and have approximately the same relationships of the variables and same numbers of degrees of freedom for all dark matter phenomena at these scales. If a dark matter or modified gravity theory doesn't reproduce MOND at galactic scales, it is necessarily wrong.

Any additional degrees of freedom and dependency on other factors than those found in the MOND formula must necessarily have negligible phenomenological impact at galactic scales. And, since MOND doesn't work at cluster scales, any correct theory must have significant scale dependence that becomes significant at some point between the ellipical galaxy scale and the small galactic cluster scale.

These are pretty strong hints regarding what kind of theory scientists should look for to explain dark matter phenomena.

Also keep in mind - MOND was developed with a curve fit to one narrow subtype of galaxies and only much later found to be accurately in other circumstances. And, there is nothing at all intuitive about the fact that it is possible to fit the entire range of dark matter phenomena in all galaxies with just one experimentally set parameter. No dark matter theories out there do that, and certainly no theory explaining dark matter at the time that MOND was proposed did that.

But it - like all other modified gravity theories - doesn't make sense, since then it can't predict galaxy collisions (or other gravity effects).

Hence my careful statement that it works in its domain of applicability. Newtonian gravity is just great for all sorts of purposes, even though it doesn't predict black holes or frame dragging or that light can be bent by gravity. MOND works everywhere that Newtonian gravity does and also accurately reproduces observation at the galactic scale where Newtonian gravity does not.

Similarly, the fact that the proton-neutron-electron model of the atom omits many of the aspects of the Standard Model that have been experimentally confirmed, doesn't mean that it isn't perfectly useful in the domain of doing chemistry. The perturbative QCD approximates the Standard Model and experimental results well in the ultra-violet, but fails dismally in the infrared, but it is still used every day at the LHC.

The fact that the initial version of MOND from he early 1980s is a non-relativistic toy model with a limited domain of applicability, doesn't mean that it is not a worthwhile theory to understand and apply in the appropriate circumstances.

There is no physics continuity, except an extraordinary claim without evidence - "just because". Morally, they fail before they start.

MOND has all sorts of evidence in the form of galactic behavior in systems where there was no observational evidence before it was proposed that have matched its predictions and which were not predicted by dark matter models.

Your complaint is not really that it is an extraordinary claim without evidence. Your really beef is that a mechanism by which the proposed law comes into being is not provided. But, this isn't any different than other aspects of physics, like non-local entanglement effects, which are empirically demonstrated to happen and are accurately described by equations, but which our primate brains evolved to have different intuitions about the physical world derived from the problems we faces as hunting and gathering primates have a hard time accepting as real.

Lots of physics is "just because" notwithstanding perfectly good reasons why Nature (the jerk, not the journal) shouldn't behave as it does. The strong force CP violation parameter should "naturally" be 1 and not 0. The mass constants and mixing angles of the Standard Model take the values they do (in the Standard Model) just because. There are three kinds of quark color charges and eight color charge variants of gluons because that is what we observe, even though it is perfectly possible to imagine physics (and indeed Lattice QCD theorists routinely do imagine physics to extrapolate to the physical parameters) with a different number. It is "natural" for there to be SUSY sparticles at the TeV scale, but that isn't what we see.

Nature has no obligation to act in a manner we find to be "moral" and a theory that efficiently reproduces observations in its domain of applicability is perfectly respectable and adds insight, even when it doesn't work outside of its domain of applicability.

With latest standard cosmology models, they do all that and predict galaxy emergence and detailed structure to boot, from first principles. [ http://www.illustris-project.org/ ]

The problem, as noted above, is that the standard cosmology model, lamda CDM, does no such thing. It doesn't reproduce the correct number of satellite galaxies (it predicts too many). It does not correctly reproduce the shape of dark matter halos inferred from observations of galaxies. It predicts more inferred dark matter subhalo structure than is observed. It does not correctly predict the ansiotropic distribution of satellite galaxies around central galaxies. It does not get the timing of galaxies with modern levels of metallacity right. It predicts that dark matter should be possible to detect directly in circumstances where it has been experimentally excluded by direct detection experiments. It predicts a cross-section of interaction between dark matter particles at odds with what was observed in the bullet cluster (the lamda CDM cross-section would be lower).

At scales larger than central galaxies with satellite galaxies, lamda CDM does a good job. But, this doesn't mean that this model isn't seriously flawed. Indeed, the domain of applicability of lamda CDM and the domain of applicability of MOND are almost completely disjoint and between the two cover the entire realm of observed phenomena. The fact that the mechanisms in each case are completely different strongly suggests that there is a major conceptual leap that has been missed somewhere and that both theories are wrong, not just MOND.

MOND is competed away long since, it is fringe. The OT article shows that, it is attempting to tear down current observations with the erroneous idea that 'hence MOND'. Cue pseudosciences, like 'design' creationism.

I'm not sure failed ideas fits on PhysicsForum as such. Maybe if there is a historical discussion?

Lots of MOND papers are produced every year. Probably as many as Loop Quantum Gravity or any particular inflation theory. It is not merely a matter of historical interest, because the idea of explaining dark matter phenomena by tweaking the laws of gravity rather than inventing new particles and force carrying bosons continues to be a viable approach to investigate. All serious MOND proponents admit that the original model is flawed, but that doesn't mean that the generally theoretical approach of looking at modifications of gravity laws that could reproduce observational evidence, as opposed to the dark matter paradigm is discredited.

UPDATE: Who added the function that censors "bad words"?

 

[CKH]

http://ned.ipac.caltech.edu/level5/Combes3/Combes4.html.

It's an interesting possibility that there is more H2 out there that we do not see. Both the baryonic and non-baryonic dark matter theories have problems directly detecting the mass. However, at least baryonic matter (mostly H) is known to exist.

That article is old but mentions the "future" FUSE mission which has since made measurements. I'm told that it failed to find large quantities of H2 by looking for UV absorption lines. However, if H2 exists as small dense clouds in the outer disk, such detection is very unlikely anyway.

You might also be interested to know that a least one paper has claimed that Extreme Scattering Events (ESEs) may not be the result of small dense clouds but rather coincidences in normal variations of the ISM along a line of sight. I'm skeptical that this idea actually works as an explanation because of the sharp cutoffs seen in ESEs.

Meanwhile, yet another dark matter detector has failed to detect anything: http://arxiv.org/abs/1410.0653.

 

[Fek_]

http://www.thedailybeast.com/articl...ark-matter-from-space-station-experiment.html

Nothing from satellite yet either.

The experiment, which has been collecting particles for several years, has foundsomething interesting: an elevated number of positrons, the antimatter counterpart of electrons. However, fascinating as it is, AMS-02 isn’t seeing what we’d expect if dark matter was the culprit. While we can’t yet rule out the possibility that dark matter is responsible, what evidence there is seems just as likely to point to another source.

Well, to answer the question whether the positron excess seen by ANS is due to dark matter annihilating the halo or to some nearby pulsars injecting pairs in the ISM, we need to collect more data at higher energies, to see eventual cutoffs in the spectra. I can tell you that in last energy bin we get just 72 positrons. In addition, we need to see if there is some excess in antiprtons too.
I'm in the AMS group doing the data analysis, by the way :)
My feeling is that the excess is due to the unipolar inductor in one or more nearby pulsars.

 

[wolram]

Nice to see you Fek

 

[Chronos]

The trouble with dark matter detection efforts is not knowing what it is. Some studies have focused on detection of dark matter decay products, despite the fact we do not know if it actually decays. Other studies look for interaction effects, despite the fact its ability to interact is unknown. Maybe it both decays and interacts, but, coaxing a signal out of the error bar level has thus far eluded us. So long as we don't know what we are looking for, knowing when we find it is a problem. We have a hay stack that appears to weigh far more than it should. We don't know if that extra weight is in the form of needles, noodles, or whatever. We are, however, pretty sure its not all cow pie. Keep in mind it took us 25 years to experimentally detect neutrinos, despite the fact we knew what we were looking for.

 

[ohwilleke]

The trouble with dark matter detection efforts is not knowing what it is."

True enough. In the entirely plausible scenario in which dark matter has no coupling to Standard Model bosons, and no interactions with Standard Model fermions other than Fermi contact forces (i.e. the rule that no two fermions can be in the same place at the same time), and gravity is the only connection between a dark sector and a SM sector, then direct detection of dark matter ought to be impossible.

At best we can hope to rule out dark matter that interacts with the SM sector by a means other than gravity with direct detection experiments (or to detect dark matter that interacts with the SM sector, although that parameter space grows ever smaller).

 

[CKH]

In lieu of direct detection, I wonder if the Gaia mission can tells us anything about the amounts and distribution of dark matter in the Galaxy?

 

[TumblingDice]

In lieu of direct detection, I wonder if the Gaia mission can tells us anything about the amounts and distribution of dark matter in the Galaxy?

Interesting thought. I wonder if creating the 3D model they hope for would find any 'problems' resolving the data due to DM. We already know about galaxy rotational speed issues, so there may be little to add to that. But if there are varying densities of DM within galaxies, might that cause measurable lensing anomalies? Anomalies large enough to result in problems justifying/explaining the complete 3D modeling of apparent relative motions between all stars measured in the same galaxy?

Could Gaia see curveballs like these in the data it collects from its solar L2 vantage point? That's a lot of number crunching, assuming the data contains important information within the detection limits of the mission. From wiki:

Each celestial object will be observed on average about 70 times during the mission, which is expected to last five years. These measurements will help determine the astrometric parameters of stars: two corresponding to the angular position of a given star on the sky, two for the derivatives of the star's position over time (motion) and lastly, the star's parallax from which distance can be calculated.

 

[Torbjorn_L]

FWIW, DM has survived yet another test and MOND has been blown out of the water again.

A new measurement of dark matter in the Milky Way has revealed there is half as much of the mysterious substance as previously thought. ...

"When you use our measurement of the mass of the dark matter the theory predicts that there should only be three satellite galaxies out there, which is exactly what we see; the Large Magellanic Cloud, the Small Magellanic Cloud and the Sagittarius Dwarf Galaxy."

University of Sydney astrophysicist Professor Geraint Lewis, who was also involved in the research, said the missing satellite problem had been "a thorn in the cosmological side for almost 15 years."

[ http://www.sciencedaily.com/releases/2014/10/141009091600.htm ]

If these fits were that trivial and unrelated to the issue of dark matter halos, there would be no interest in them. You seem to be unaware that these fits are important to mainstream scientists.

They may have been as I described earlier, but not now. And in retrospect I, at least, wonder why as they are trivial.

They require explanations in the context of mainstream theory. Many mainstream scientists are interested and seeking explanations in the context of CDM.

Now you are confusing the peculiar behavior of the rotation curves with their existence.[/QUOTE][/QUOTE]

 

[Torbjorn_L]

DM first:

At scales larger than central galaxies with satellite galaxies, lamda CDM does a good job.

That description is wrong, see my previous comment. The satellite galaxies was the only structural problem, now solved. Remaining observational tension is with the cores of galaxies. But they have exotic physics (SMBHs), so are not well understood.

Historical theory:

It necessarily follows that if MOND is a good fit over the entire range of dark matter phenomena up to elliptical galaxies with a single parameter, ... Your complaint is not really that it is an extraordinary claim without evidence.

Yes, that is what would be expected with a steady-state fit.

Lots of MOND papers are produced every year. Probably as many as Loop Quantum Gravity or any particular inflation theory. It is not merely a matter of historical interest,

In what world would mentioning LQG be support for other fringe theories? It isn't even a physical theory, it is math (lack dynamics). Something that it shares with MOND by the way.

Your claim that MOND isn't rejected by the consensus and thus has played out its role historically is not supported by remaining activity, which I have already noted (fringe). For an analogous situation, there is also a lot of group selection papers produced despite the fact that it has met a similar destiny. The frantic actrivity of the few supporters tell of a) lack of peer review (I suspect most would be arxiv papers) and b) lack of quality. You would want to cite number of researchers and number of quotes outside the MOND fringe,

 

[Jonathan Scott]

The Tully-Fisher relation and Faber-Jackson relation are empirical observational results which apply to galaxies. The MOND formula is effectively another related and somewhat more specific empirical observation - that galaxies apparently obey a rule such their rotation curves work as if there were an additional gravitational force as given by the MOND formula, which is derived purely from the visible matter (in the same way as the other relations).

Neither the original MOND theory nor the closely-related TeVeS make much sense from a physics point of view. STVG (MOG) seems better put together, but still appears to be a set of arbitrary mathematical rules rather than an explanation.

I'm not aware of any independent statistical analysis of whether the MOND curve fit could be explained by some other systematic effect, but from the examples I've seen over the years, the predicted and observed rotation curves match spectacularly well for such a simple idea, including handling types of galaxies that had not yet been observed when MOND was invented.

If these results are anywhere near as striking as they appear to be, any theory of how galaxies work needs to include an explanation of why the rotation curves appear to match the MOND formula (which also implies the other relations), and should certainly not be in conflict with this result. As the MOND results are based purely on the visible matter (with no free parameters in principle apart from the universal MOND acceleration parameter), this seems to be quite a challenge for DM to explain. There are of course some parameters involved in for example deciding the distance of a galaxy and the mass to luminosity ratio, but those apply for all theories, and the MOND formula doesn't require any special adjustments to give good results.

 

[CKH]

Does anyone know how the "missing satellite problem" has been reduced from hundreds missing to just a few and has now been solved by making our Galaxy less massive?

Torbjorn_L,

This "blows MOND out of the water" is an odd conclusion since MOND also claims much less total mass than usually assumed in DM halos.

There are many qualifications noted in this paper: ON THE SHOULDERS OF GIANTS: PROPERTIES OF THE STELLAR HALO AND THE MILKY WAY MASS DISTRIBUTION which is the basis for that news blurb. The words "assume", "assuming" and "assumption" appear 42 times in the paper. The word "seem" or "seems" appear seven times. See the last few paragraphs of the conclusion for some qualifications. The paper assumes a DM halo, of course.

LCDM is having significant problems in the local universe. Papers claiming to solve one or more (usually by appealing to baryons) are becoming common. Whether real progress has been made is unclear, since no one has gathered all of the new assumptions together into an extended theory that can be scrutinized.

Take for example the cusp problem. Lately we have some papers claiming that SNs couple to dark matter and flatten out the cusps while others say even with 100% efficient coupling (which is absurd) there is insufficient energy to do so.

Before you jump on a bandwagon claiming all is solved, bear in mind that supporters of LCDM are writing papers nearly daily theorizing about variations on CDM in an attempt to solve local problems. They wouldn't be doing this if the problems were so easily solved. There is huge human investment in LCDM so in the face of contradictions it's not surprising to see many claims of solutions.

No number of failed attempts at direct detection will disprove dominant non-baryonic matter. As long as it remains undetected, theorist can freely speculate about its properties in an attempt to support the theory.

Hopefully Gaia will have much to add to the observations on which this paper is based. It would be really nice to have some hard evidence one way or another about local non-baryonic dark matter. Unfortunately we will just have to wait.

 

[Drakkith]

Thread closed for the moment. Mentors, please see the report.

 

[Drakkith]

After review by the mentors, the decision is that this thread stay locked.