Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

The case for [or against] dark matter

  1. Jul 23, 2015 #1

    Chronos

    User Avatar
    Science Advisor
    Gold Member

    This paper, http://arxiv.org/abs/1507.06282, The Duhem-Quine thesis and the dark matter problem, may be of interest to those curious about how and why dark matter has gained general acceptance by cosmologists
     
    Last edited: Jul 23, 2015
  2. jcsd
  3. Jul 29, 2015 #2
    After more than 50 years the direct search for dark matter has been negative which would make sense if dark matter does not exist.

    http://www.popsci.com/article/science/inside-hunt-dark-matter

    “I’ve been looking for dark matter for 23, no, 24 years now,” he says. And he is not alone; the search for dark matter has grown into a small industry, albeit one that does not yet have a product to sell. “Every experiment has reported essentially negative results. No one even knows for sure if the damn stuff really exists. Those fellows,” Gaitskell says, nodding to the pit, “know exactly where the gold is.” I realize now he is not feeling empathy for the miners. He is feeling envy."


    There more than a dozen separate astronomical observations that cannot be explained by the dark matter hypothesis and cannot be explained by a change in gravity.


    Complex holistic problems cannot be solved by guessing. It appears there are sufficient observations to solve the cosmological puzzles which is more complicate that is there or there not dark matter.

    There are close to a hundred significant astronomical anomalies and paradoxes, including structural anomalies, that indicate there are fundamental errors in the standard cosmological model, at the most basic level.

    An incorrect theory will generate paradoxes. If a theory is incorrect at a fundamental level, it blocks all progress.

    The standard approach when there are piles and piles of anomalies and paradoxes in a field, is to separate the observations from the theories and to start from square one using the observations to lead, to construct alternative theories.


    Comment:
    An example of a structural anomaly is in geology were there were many observations that indicated the continents had separated and that the continents moved. The solution to the geological anomalies has a new mechanism, continental drift.



    http://www.eso.org/public/news/eso1217/


    "Serious Blow to Dark Matter Theories?
    The most accurate study so far of the motions of stars in the Milky Way has found no evidence for dark matter in a large volume around the Sun. According to widely accepted theories, the solar neighbourhood was expected to be filled with dark matter, a mysterious invisible substance that can only be detected indirectly by the gravitational force it exerts. But a new study by a team of astronomers in Chile has found that these theories just do not fit the observational facts. This may mean that attempts to directly detect dark matter particles on Earth are unlikely to be successful.


    A team using the MPG/ESO 2.2-metre telescope at ESO’s La Silla Observatory, along with other telescopes, has mapped the motions of more than 400 stars up to 13 000 light-years from the Sun. From this new data they have calculated the mass of material in the vicinity of the Sun, in a volume four times larger than ever considered before.

    The amount of mass that we derive matches very well with what we see — stars, dust and gas — in the region around the Sun,” says team leader Christian Moni Bidin (Departamento de Astronomía, Universidad de Concepción, Chile). “But this leaves no room for the extra material — dark matter — that we were expecting. Our calculations show that it should have shown up very clearly in our measurements. But it was just not there!


    http://www.scilogs.eu/en/blog/the-dark-matter-crisis/2012-04-19/dark-matter-gone-missing-in-many-places-a-crisis-of-modern-physics[/URL]

    [URL]http://arxiv.org/pdf/1204.3377v1.pdf[/URL]

    [B] "Dark Matter is missing in the Local Universe[/B]

    In the work titled “[URL='http://arxiv.org/abs/1204.3377'][U]Missing Dark Matter in the Local Universe[/U][/URL]”, Igor D. Karachentsev has looked at a sample of 11,000 galaxies in the local Universe around the MW. He has summed up the masses of individual galaxies and galaxy-groups and used this to test a very fundamental prediction of LCDM.

    The idea is as simple as it is brilliant: cosmology has precise predictions as to what is the content of our universe. In particular, it predicts the density of matter to be Ωm,glob = 0.28 +- 0.03 (83 per cent of this in dark, 17 per cent in luminous matter). Now, to test this, all you have to do is to sum up all the mass within a certain volume of space, and you can estimate the actual density of mass within that volume. To be sure that your volume is representative, it needs to be large. If you only sum over, say, a sphere of 100 kpc in diameter, the density strongly depends on whether you have a galaxy in this volume or not. Karachentsev chose to use a volume with a radius of 50 Mpc around the MW. On this size-scale, the density is expected to fluctuate by only 10 percent, a reasonably low value in astronomy. The scale can thus be assumed to be representative and you should observe the mass density predicted by LCDM.

    Except that you do not.

    Karachentsev reports that the average mass density is only Ωm,loc = 0.08 +- 0.02, a factor of 3-4 lower than predicted and can not be explained by the uncertainties in the data or prediction. As most of the mass-content in the Universe is supposed to be dark matter, this means that most dark matter is missing in this volume."


    Normal matter interacts gravitational with “dark matter”, so dark matter can loss or gain energy from the galaxy. Unfortunately for the “dark matter theory”, hydro-dynamic simulations, fundamentally disagree with real galaxies. The simulations create a model disc that is an order of magnitude smaller than what is observed. This discovery, which is called the “angular momentum catastrophe”, was made 8 years ago. There is no solution to the angular momentum catastrophe, which is not surprising; however, as more detail data and observations concerning spiral galaxies shows structures that could not possibly have been created by the interaction of “dark matter” and normal matter.

    It should be noted that the ‘angular momentum catastrophe” problem and the “missing satellites problem” is leading some researchers to state that dark matter does not exist which is interesting as LCDM theory will need to change. The “angular momentum catastrophe” and the missing satellites problem” are not the only fundamental disagreements with the “dark matter” theory and reality.

    This is an example of an anomalous observation that indicates galaxies do not form from mergers and stray gas clouds. How galaxies do form explains the angular momentum anomaly and the velocity anomalies.

    [URL]http://www.sciencedaily.com/releases/2014/07/140721100418.htm[/URL]

    [SIZE=6][B]"Mysterious dance of dwarf galaxies may force a cosmic rethink[/B][/SIZE]
    The discovery that many small galaxies throughout the universe do not 'swarm' around larger ones like bees do but 'dance' in orderly disc-shaped orbits is a challenge to our understanding of how the universe formed and evolved. The researchers believe the answer may be hidden in some currently unknown physical process that governs how gas flows in the universe, although, as yet, there is no obvious mechanism that can guide dwarf galaxies into narrow planes."


    [URL]http://www.nature.com/nature/journal/v511/n7511/full/nature13481.html[/URL]

    "Velocity anti-correlation of diametrically opposed galaxy satellites in the low-redshift Universe

    Recent work has shown that the Milky Way and the Andromeda galaxies both possess the unexpected property that their dwarf satellite galaxies are aligned in thin and kinematically coherent planar structures1, 2, 3, 4, 5, 6, 7. It is interesting to evaluate the incidence of such planar structures in the larger galactic population, because the Local Group may not be a representative environment. Here we report measurements of the velocities of pairs of diametrically opposed satellite galaxies. In the local Universe (redshift z < 0.05), we find that satellite pairs out to a distance of 150 kiloparsecs from the galactic centre are preferentially anti-correlated in their velocities (99.994 per cent confidence level), and that the distribution of galaxies in the larger-scale environment (out to distances of about 2 megaparsecs) is strongly clumped along the axis joining the inner satellite pair (>7σ confidence). This may indicate that planes of co-rotating satellites, similar to those seen around the Andromeda galaxy, are ubiquitous, and their coherent motion suggests that they represent a substantial repository of angular momentum on scales of about 100 kiloparsecs."
     
    Last edited by a moderator: May 7, 2017
  4. Jul 30, 2015 #3

    Chronos

    User Avatar
    Science Advisor
    Gold Member

    I have little faith in popsci presentations - I view they have too many publishing agendas to satisy to be unbiased and reliable. If you choose to appeal to peer reviewed papers, please clarify which ponts you find interesting. A smorgasborg of dissenting papers on dark matter is unsatisfying. They are as common as UFO sightingts.
     
  5. Jul 30, 2015 #4
    Summary:

    The observations do not support the assertion that there is dark matter and do not support the assertion that general relativity (gravity) is incorrect. It is a fact that there are multiple galaxy anomalies that requires an explanation. The list of galaxy anomalies is growing year by year.

    It seems that it is time to move on and to start the search for alternatives. This area of cosmology (galaxy formation and evolution) should be in a crisis which a good thing, a normal step in fields of science if the objective is to solve a problem, rather than to talk endless about incorrect toy models which accomplishes nothing.

    The below list is a sample of the observations that support the assertion that it is very, very, likely that dark matter does not exist.

    1) Direct dark matter detection, negative after 50 years search. One cannot find what does not exist. If dark matter does not exist there is no progress until other solutions are looked for. The solution is not changing gravity. When there are multiple fundamental observations that do not support a theory, it is better to have no theory rather than an incorrect toy theory which blocks all progress.

    It is a big deal to create a new particle or whatever that does not exist. There are negative consequences for both cosmology and for fundamental physics, if dark matter does not exist and we have spent 50 years before looking for other explanations.

    The point is there does appear to be fundamental physics issues, something that will advance fundamental physics with the correct solution to a very long list of galaxy formation and evolution anomalies.

    2) Solar system area search for dark matter, negative for dark matter.

    3) Local universe search for dark matter, negative for dark matter.

    4) Observed spiral galaxy central form, does not agree with simulations for galaxy formation/evolution with dark matter. Cusp problem.

    5) Satellite galaxy crisis. There are two few satellite galaxies (there should based on simulations be many hundreds of satellite galaxies rather than roughly 30 in the Milky Way and Andromeda). It has been found in the last five years, that the satellite galaxies that are observed in multiple galaxies (including the Milky Way and Andromeda) are located in a narrow plane. The narrow plane of satellite galaxies cannot be explained by the dark matter mechanism or by galaxy formation by mergers. The initial reaction to finding a plane of satellite galaxies for the Milky Way and Andromeda was to appeal to a weird interaction of the two galaxies, although there was no observational evidence for that assertion. The finding that a plane of satellite galaxies based on observations appears to be ubiquitous for all spiral galaxies, (>7σ confidence), published in Nature, is a big deal.

    i.e. Galaxy growth by mergers should produce a sphere of satellite galaxies not a narrow plane of satellite galaxies. There are two anomalies.

    1)There are multiple observations that support the assertion that something physical is stopping mergers. i.e. Mergers should produce a sphere of satellite galaxies. If mergers were common then there would be spheres of satellite galaxies. No sphere of satellite galaxies supports the assertion that something physical is stopping/inhibiting mergers.

    2) Secondly what is creating the plane of satellite galaxies?
     
  6. Jul 30, 2015 #5

    Chronos

    User Avatar
    Science Advisor
    Gold Member

    1. The search continues. Negative results tend to motivate more ingenious search methods.
    2 Given we don't know what it is we are seeking, that result is unsurprising, You rarely find chicken on the beef counter
    3. See above
    4. Speculation - how many 'spiral galaxy central forms' do we have for comparison?
    5. Our count of satellite galaxies is obviously incomplete, given we keep discovering new ones every year.
    6. Objection. The presumed plane of satellite galaxie is inadequately supported by facts in evidence - see 5. The obvious question is how many spiral galaxies are there for which we have a reliable count of satellite galaxies?
     
  7. Jul 30, 2015 #6
    It is interesting to first study and try to understand/document the implications of key new and old forgotten anomalous observations (this step is not part of the current cosmological theory paradigm, there are now extraordinarily complete, highly developed, astronomical observations/analysis to work with) and then just for fun try to formulate a high level solution to the problems. No wild guesses, but play with all theoretical assumptions in your own sandbox. The observations are the guide to the correct solution.

    As noted a structural anomaly is different than an anomaly that simply cannot be explain by a theory, in this case the theoretical entity 'dark matter'. The implications of the anomalous galaxy observations (there are now dozens and dozens of anomalous observations, I have picked a couple and will add a few more that are easiest to explain to a general audience and that point to the correct solution) is deeper than they falsify the dark matter hypothesis. Changing general relativity (law of gravity) also will not explain the observations. The choice should not be two incorrect hypotheses. No answer is better than an incorrect answer, as that forces/enables/allows people to look at the observations to look for an answer that is at least on the correct page.

    The finding of a structural anomaly requires a new mechanism to explain, rather than a rework of the dark matter hypothesis and/or galaxy model simulations. The spiral galaxy observations support the assertion that some of the most basic assumptions concerning how spiral galaxies grow and evolve are incorrect.

    The following paper notes the astonishing discovery (this was most certainly not expected) that six spiral galaxy parameters are tightly controlled which is not possible for a dark matter hierarchical galaxy formation theory/mechanism The galaxy parameters in question should be random and highly hence highly variable, not tightly controlled.

    The paradoxical implications of this discovery is most easily seen by comparing how observed spiral galaxy angular momentum for different spiral galaxy masses compares to theoretical angular momentum for different spiral galaxy masses.

    Spiral galaxy angular momentum was long ago hypothesized to originate from gas cloud torque when the galaxy initially formed. Based on this mechanism, as noted below, the galaxy angular momentum should be random, primarily determined by random circumstances (gas cloud torque) when the galaxy formed and hence should be completely independent of galaxy mass. (i.e. The spiral galaxy would gain mass in the future via mergers maintaining roughly its initial angular velocity as it grows).

    What is observed is spiral galaxies have more angular momentum directly proportional to their mass. The implication of this finding is that spiral galaxies mysteriously gain angular momentum as they grow in mass. This is a Goldlock's type problem. Not too much or too little angular momentum. Think of random mergers and the conservation of angular momentum.

    There are three questions/anomalies:

    1) As noted, there must be some mechanism that stops/inhibits the merger of spiral galaxies. Wet mergers (random mergers of two spiral galaxies with stars) will gradually change spiral galaxies into elliptical galaxies or elliptical like galaxies and will produce random sized galaxy bulges. That is not observed. Wet mergers will most definitely not cause the angular momentum of a galaxy to increase in direct proportion to the spiral galaxy's mass.


    What is observed is that spiral galaxies show a continual tightly controlled spectrum of growth with a bulge that grows in direct proportion to the mass of the super large object in the center of almost all galaxies.

    What that indicates is the grow of the spiral galaxy is somehow connected with the grow of the super massive object at the center of almost all galaxies.

    2) If spiral galaxies do not grow by wet mergers, how then do they grow?

    3) How do spiral galaxies gain angular momentum in direct proportion to their mass?


    http://www.nature.com/nature/journal/v455/n7216/abs/nature07366.html

    http://arxiv.org/ftp/arxiv/papers/0811/0811.1554.pdf


    "Galaxies appear simpler than expected

    Galaxies are complex systems the evolution of which apparently results from the interplay of dynamics, star formation, chemical enrichment, and feedback from supernova explosions and supermassive black holes1. The hierarchical theory of galaxy formation holds that galaxies are assembled from smaller pieces, through numerous mergers of cold dark matter2,3,4. The properties of an individual galaxy should be controlled by six independent parameters including mass, angular-momentum, baryon-fraction, age and size, as well as by the accidents of its recent haphazard merger history. Here we report that a sample of galaxies that were first detected through their neutral hydrogen radio-frequency emission, and are thus free of optical selection effects5, shows five independent correlations among six independent observables, despite having a ....

    ... This implies that the structure of these galaxies must be controlled by a single parameter, although we cannot identify this parameter from our dataset. Such a degree of organization appears to be at odds with hierarchical galaxy formation, a central tenet of the cold dark matter paradigm in cosmology6.

    ...Consider spin alone, which is thought to be the result of early tidal torquing. Simulations produce spins, independent of mass, with a log-normal distribution. Higher-spin discs naturally cannot contract as far; thus, to a much greater extent than for low-spin discs, their dynamics is controlled by their dark halos, so it is unexpected to see the nearly constant dynamical-mass/luminosity ratio that we and others14 actually observe. Heirarchical galaxy formation simply does not fit the constraints set by the correlation structure in the Equatorial Survey."
     
  8. Jul 30, 2015 #7
    Well it took about 50 years to find the Higgs boson, and we knew what to look for there...
     
  9. Jul 30, 2015 #8
    'Dark matter' is just a name for an unknown 'something' which evidently exists, because it has observable interactions gravitationally.
    It doesn't interact with other fundamental forces, but it is called 'matter' because everything else we know of which interacts gravitationally has mass.
    We don't know what it is, we have some ideas and clues, but it definitely is 'something' which exists.

    Personally I haven't given up on the idea of MACHO's - we really don't have much clue about how many 'failed' brown dwarf protostars are out there or 'dark' solar systems which our present technology is unable to see. Primordial micro black holes can't be ruled out either.
     
    Last edited: Jul 30, 2015
  10. Jul 30, 2015 #9
    Of course it is not how long one looks for something but rather has the search determined that it is very, very likely that what one is searching for does not exist.


    It is bad thing rather than a good thing if cosmology theory advancement has been on hold for 50 years while people search for dark matter and dark energy and attempt to build theoretical models using dark matter and dark energy to attempt to explain galaxy morphology and evolution, if it is a fact that dark matter and dark energy do not exist.

    It is important to be able to recognize when the parrot is dead.

    John Horgan in his book "The end of science" created the term 'ironic' science to describe sciency work (string theory and cosmology) that may have no basis in reality. Ironic science looks like science and can go on forever, but will never converge on the truth if it is a fact that it concerns theoretical entities that have no basis in reality.

    There are more string 'theories' than there are atoms in the universe. There is no definition as of yet as to what is or is not a string. There is as of yet no string 'theory' in that someone has created a mathematical model that makes predictions, that is a 'theory'. String theory is the term used to describe those how create and discuss mathematical models (more than 100,000 string 'theory' papers have been written, very productive field of work if the objective is to write and discuss 'papers') that might one day be a theory.

    String 'theory' appears to be very, very, advanced alchemy. The other more traditional approach to solving scientific problems is to look at the observations as a guide to the correct 'theory'. The mathematical model follows that step, rather than leads that step.

    The medieval 'alchemy' is an example of primitive ironic science. If dark matter and dark energy do not exist, that is advanced ironic science.

    The point is it time to relook at the entire cosmological problem as it is a fact that there are more and more anomalies concerning every aspect of the standard cosmological theory components due to multi spectrum data at all redshifts.


    The Frenk Principle:

    "If the Cold Dark Matter Model does not agree with observations, there must be physical processes, no matter how bizarre or unlikely, that can explain the discrepancy."


    The Strong Frenk Principle: (2 versions)

    1: "The physical processes must be the most bizarre and unlikely..."

    2: "If we are incapable of finding any physical processes to explain the discrepancy between CDM models and observations, then observations are wrong."

    - George Efstathiou


    https://www.astro.umd.edu/~ssm/mond/DMOct05.pdf
     
  11. Jul 30, 2015 #10
    If you convinced about that then you will have to publish a theory which can explain the data better than existing models can.
     
  12. Jul 30, 2015 #11

    ohwilleke

    User Avatar
    Gold Member

    The terminology that I like to use is to say that it is beyond dispute and definitively established that dark matter phenomena exist. Galactic rotation curves, lensing, the success of the lamdaCDM model in describing the observed universe with respect to the parameters it describes, etc. all overwhelmingly support the existence of something causing dark matter phenomena other than GR as currently formulated and applied, and other than the SM forces and particles.

    It is also true that we haven't solved the problem of what causes dark matter phenomena. There are several sub-types of dark matter particle based theories and a few modified gravity theories that make a good go of it, but there is no precise consensus solution that has been definitively established to solve all of the dark matter phenomena and to out perform all of the other approaches. There are at least several dozen other dark matter particle and modified gravity approaches that were considered seriously at one point or another and have been more or less definitively ruled out at this point.

    We are perilously close to being overconstrained and ruling out all of the current candidates, although I am hopeful that another solution would come to the fore if that happened. I have my own subjective handicapping of the final contenders that are being tested now. The good news is that there is a lot of data that has not yet been gathered or has not been analyzed with sufficient computing power, but can be gathered or computed within the lifetimes of most of the people reading this comment, which will materially improve our ability to discriminate between the accuracy of different proposed solutions, and to narrow the parameter space of dark matter phenomena explanations. Dark matter research is not in the potential dead end situation of the poor HEP physicists who could very well be facing a desert in which no beyond the Standard Model physics turns up at any experiment within the capacity of mankind to build for many decades to come. The path to get the information we need to answer dark matter questions is clear and we have come a long way already since we started looking. We may need to invest some serious money in space based telescopes and detectors to get the answers they need, but the book is still wide open and available to read to anyone with the right instrumentation.

    Also, the assertion about a lack of a solar system signal is just flat wrong. As I explained in a recent post at my blog: "If dark matter exists in the solar system, it should exist in such a homogeneous distribution at such a low density that it is undetectable through solar system gravitational dynamics. The total amount of dark matter in the solar system (if it exists) has a mass comparable to a single medium sized asteroid, but it is spread evenly throughout the spherical space centered at the Sun and including the entire solar system out to Pluto and beyond. Dark matter adds up at the galactic level but has only insignificant relevance to solar system gravitational dynamics."

    The trick is to recognize that you have to use a reasonable estimate of the galactic dark matter halo distribution and not simply assume naively that DM is distributed in the same proportions it has to ordinary matter in the universe as a whole in the Milky Way galaxy. Inferred DM amounts relative to luminous matter differ systemically between different kinds of galaxies and galactic clusters.
     
    Last edited: Jul 30, 2015
  13. Jul 30, 2015 #12
    :wideeyed:Maybe in 1000 years we will have domestic cleaning appliances which work by using this dark stuff.
     
  14. Jul 30, 2015 #13

    ohwilleke

    User Avatar
    Gold Member

    There are lots of perfectly reasonable dark matter models (e.g. singlet sterile neutrino dark matter that don't oscillate with other neutrinos) in which it shouldn't be possible to detect dark matter directly. All direct dark matter detection experiments assume that dark matter has some cross-section of interaction with other ordinary matter through some force, with the weak force interactions of the neutrino typically used as a benchmark. No direct dark matter detection experiment has excluded dark matter particles under about 1 GeV or cross-sections of interaction consistent with a particle that interacts with ordinary matter only via gravity and Fermi contact forces (for which the cross-section of interaction from the signal would be drowned in the baseline of the cross-section of interaction of the background neutrinos).

    There shouldn't be enough dark matter in the solar system or a sufficiently non-homogeneous distribution of it, to be discernible from solar system gravitational dynamics. The total amount of dark matter expected for the solar system has a mass on the order of a small asteroid and would be spread fairly evenly throughout the solar system.

    False. Lensing data and the dynamics of objects in the local universe clearly indicate that there are dark matter pheneomena in the local universe.

    This is a problem with a particular dark matter paradigm called "Cold Dark Matter" which presumes thermal relic dark matter particles with masses on the order of 10s to 100s of GeVs that interact via the weak force and gravity, but not the strong or electromagnetic forces, for which supersymmetry theories provided multiple plausible candidates.

    But there are a variety of ways to solve that problem. One is to use a lighter dark matter candidate (mass ca. keV) which is called "Warm Dark Matter". Another is to assume that there is a medium range force that causes dark matter to interact with other dark matter (but not with ordinary matter) via roughly MeV mass bosons with roughly the strength of the electromagnetic force.

    This is also as good a place as any to explain that "Cold Dark Matter" as used in the lambdaCDM model's definition is a false friend which means something different when a CDM model is used to explain, for example, galactic rotation curves.

    As used in the lambdaCDM model, a dark matter particle includes every kind of particle that would qualify as warm dark matter (WDM) or cold dark matter, but in other contexts, CDM refers exclusive to thermal relic dark matter with mass of about 5-10 GeV or more.

    This too is a problem particular to the "Cold Dark Matter" paradigm that can be solved with either lighter "Warm Dark Matter" or with self-interacting dark matter.

    The amount of galaxy group scale structure that you see in a system is essentially a function of the mean velocity of dark matter particles which in a thermal relic scenario is basically a function of dark matter particle mass. The lighter your dark matter particles, the fewer satellite galaxies you will have on average. They heavier your dark matter particles the more satellite galaxies you will have on average. This is one of the most obvious instances where tweaking one parameter, dark matter particle mass, of your model can fit it to observation without further difficulties.

    The assumption that galaxy growth should produce a sphere of satellite galaxies is not well established. The observation is notable, but it isn't obvious evidence one way or the other because the status quo evolution isn't terribly well understood in this respect.

    More generally, there are a number of serious methodological issues with the simulations that are used to compare universes with hypothetical varieties of dark matter with observation, some of which rescue otherwise troubled DM theories, and others of which are challenging for DM theories:
    1. Some of the cusp problem, for example, is due to the failure of simulation models to consider gravitational interactions with ordinary matter (often simulations are run in dark matter only universes).
    2. Simulations routinely disregard GR effects and simply reply on Newtonian gravity. This is less bad that you would naively assume and it certainly isn't necessary to use full fledged GR to do an accurate numerical model at this scale, but there are some post-Newtonian tweaks that arise as a result of the differences between GR and Newtonian gravity to be discernible at this scale which should be incorporated, which is hard but not impossible as computers become more powerful. For example, GR clearly treats a rotating disk of matter differently than Newtonian gravity does, even though the differences are rather subtle.
    3. Simulations usually make some pretty unrealistic assumptions about galaxy formation that make dark matter models seem to perform better than they would if realistic assumptions were used instead.

    It isn't entirely clear how the various models will shake out as these issues with the computer simulations are resolved.

    And, there are a few things that DM theories do quite well, such as predict the dynamics of "RAVE" stars in the Milky Way galaxy that are significantly above or below the plane of the Milky Way's spiral galaxy.

    On the other hand, there are some things that DM theories generically (i.e. without regard to parameters like DM particle mass or known problems with simulation methods) do rather poorly. For example, DM theories generically under predict the proportion of spiral galaxies that lack a bulge. They generically fail to predict that the ratio of dark matter to luminous matter in elliptical galaxies is lower when they are more nearly spherical, and higher when they are less spherical. They predict much more scatter between the ratio of dark matter to luminous matter in spiral galaxies relative to their size than is actually observed if galaxy formation is just a product of random collisions of clumps of matter in the early universe according to GR alone. DM theories, generically, fail to predict the fine level wavelike texture of the distribution of stars in spiral galaxies and elliptical galaxies; instead they predict a smooth texture which is not observed.

    Systems like the Bullet Cluster disfavor many (but not all) modified gravity alternatives to dark matter, but also place some serious constraints on dark matter parameters.

    For example, the most prominent modified gravity theory, MOND, does a wonderful job of predicting (in advance) galactic rotation curves of every kind of galaxy from dwarf to elliptical with just a single experimentally fixed parameter. This by itself strongly hints at a fairly simple mechanism to explain this dark matter phenomena since one degree of freedom can explain far more than one might naively expect. But, this toy model phenomenological model also has a variety of known flaws: (1) it isn't relativistic although a relativistic extension of it called TeVeS exists, (2) it systemically underestimates the magnitude of dark matter pheneomena in galactic clusters, (3) it does a poor job of predicting the dynamics of RAVE stars, (4) it is inconsistent with the Bullet Cluster in its original form, and (5) it requires a slight tweak when there are two galaxies with heavily overlapping gravitational fields. But, another modified gravity theory, called MOG (for modified gravity) by Professor Moffat, lacks problems (1), (2) and (4) and hasn't been tested against RAVE star dynamics or (5) at this point.

    I've seen some quite impressive early efforts to modify gravity by assuming that conventional GR using Einstein's equations incorrectly models the self-interactions of the gravitational field, which implies that the strength of modified gravity effects should be driven almost entirely by the overall mass of the system, and the extent to which a system is not spherical, which would not require any new experimentally measured parameters not already derivable from standard GR. This model also explains at least some observed dark energy effects and hence also the "cosmic coincidence" problem of why the amount of matter, inferred dark matter, and inferred dark energy are all of the same order of magnitude. But, there is a lot of work yet to be done to turn that into a workable completely articulated modified gravity theory.

    The key bottom line point is that evidence that strongly indicts any particular dark matter or modified gravity model to explain dark matter phenomena doesn't necessarily mean that particle based dark matter paradigms or modified gravity model paradigms are failures in all of their many variations.
     
    Last edited: Jul 30, 2015
  15. Jul 30, 2015 #14
    betzalel,

    Dark matter or changes to general relativity do not explain the correlated structure of spiral galaxy parameters which is a paradox. The dark matter papers ignore the correlated structure paradox.

    Clouds of dark matter particles will not cause Goldlock's increases of spiral galaxy rotational (not too much or not too little) in direct proportion to the spiral galaxy's mass.

    There is a continual unexplained evolution in spiral galaxy properties. Why? What is causing the increase in rotational speed as the spiral galaxy grows in size.

    The solution must explain what is observed. Note there are other anomalies concerning how spiral galaxy grow.

    1) As noted, there must be some mechanism that stops/inhibits the merger of spiral galaxies. Mergers (random mergers of two spiral galaxies with stars) will gradually change spiral galaxies into elliptical galaxies or elliptical like galaxies and will produce random sized galaxy bulges. That is not observed. Mergers will most definitely not cause the angular momentum of a galaxy to increase in direct proportion to the spiral galaxy's mass. What is observed is that spiral galaxies show a continual tightly controlled spectrum of growth with a bulge that grows in direct proportion to the mass of the super large object in the center of almost all galaxies.

    What that indicates is the grow of the spiral galaxy is somehow connected with the grow of the super massive object at the center of almost all galaxies.

    2) If spiral galaxies do not grow by mergers, how then do they grow?

    3) How do spiral galaxies gain angular momentum in direct proportion to their mass?



    http://www.nature.com/nature/journal/v455/n7216/abs/nature07366.html

    http://arxiv.org/ftp/arxiv/papers/0811/0811.1554.pdf

     
  16. Jul 30, 2015 #15
    What makes you think this?
    It''s generally accepted our that our galaxy will merge with Andromeda galaxy in the very long run.
     
    Last edited: Jul 30, 2015
  17. Jul 30, 2015 #16

    ohwilleke

    User Avatar
    Gold Member

    This is simply inaccurate. The correlated one dimensional structure of spiral galaxy parameters, first described with as the Tully-Fischer relation, historically, was the primary motivation for the original modified gravity theory, MOND, and essentially all subsequent modified gravity theories address this issue.

    Dark matter models address this problem as well, although with not quite such a tight fit to the galactic mass as modified gravity models. Mostly this arises because dark matter halos are less capable of maintaining distinct form than ordinary matter due to their lack of non-gravitational interactions. In dark matter models, the distribution of ordinary matter in galaxies is homogenized by the powerful role that dark matter halos (which tend to be fairly homogeneous at any given size) exert on the formation of galaxies and the distribution of ordinary matter within them. There is also gravitational feed back in the other direction from the baryonic matter to the dark matter halo, in which the disk-like distribution of baryonic matter's tugs the shape of the dark matter halo away from a sphere towards a rugby ball shape.

    Let me put this another way. Dark matter models became popular because in crude simulations they crudely reproduced what we observed, with some very simple starting assumptions. They do work. Particular versions of them may not perfectly fit the data, but the extent to which a very simple version of them can come reasonably close to reality with relatively arbitrarily chosen parameters shows why these models are promising, even if there is more fine tuning of the model that is necessary to fit the better resolved data matched to better simulations, than one would have initially hoped.

    It is one thing to say that this particular model or that particular model has a flaw, and it is another to indict the entire paradigm with objections that really only apply to particular models and are not generic.

    As noted by another poster earlier, this is a constructive model building process. The only way somebody gets a turn to come to the podium and say something that people will listen to is to propose an alternative to the status quo that is better. Saying that there is a problem with a model is just another way of saying that the model needs to be modified to produce a better result. Until the problem is solved once and for all, one has to keep trying to formulate better and better models until we get it right.

    Currently, we are fortunate to have work done on two separate paradigms which both seem promising: particle based dark matter paradigms, and modified gravity paradigms. If you can come up with a third paradigm that is better than either of the other two or some combination of them, more power to you. Tell us all.
     
  18. Jul 30, 2015 #17
    betzalel,
    As noted the correct solution must explain the correlated spiral galaxy properties as noted in the Disney paper which is a paradox. None of the dark matter papers or galaxy simulation papers discuss the Disney finding as it is paradox. The correct solution cannot be dark matter or changing gravity. (See my comment below.)

    The correct solution must in addition to explaining the correlated spiral galaxy properties must explain how spiral galaxies grow (their means of grow cannot be mergers, as a merger will not produce a giant bulgeless galaxy) and why they do not show evidence of mergers.

    Something is stopping/inhibiting spiral galaxies from merging and somehow they still grow.

    Note as the spiral galaxies grow and age they gradually develop a bulge whose size is directly proportional to the mass of the super massive object in their core.

    There is a third mystery. If spiral galaxy growth is not by mergers how does the super massive object in their center grow?

    http://arxiv.org/abs/1009.3015


    http://arxiv.org/abs/1001.4542v1

    .
     
  19. Jul 30, 2015 #18

    ohwilleke

    User Avatar
    Gold Member

    The argument that there is bulgeless galaxies is essentially an argument for favoring modified gravity theories over dark matter theories. The expectation of near universal bulges is particular to galaxy formation scenarios observed in particle dark matter models. It is not an argument that there is no explanation for this pattern under any theory.
     
  20. Jul 31, 2015 #19
    betzalel,

    Problem 1: Analysis supports the assertion that there is a precise angular momentum gain (not random angular momentum gain) as the spiral galaxy gains mass. How is that possible with mergers? It appears dark matter and/or modified gravity does not enable spiral galaxies to precisely gain angular momentum as they gain mass via mergers. The paradox is the standard model for galaxy growth assumes galaxies grow by mergers. Mergers however produce random changes in angular momentum not precise changes in angular momentum. A precise gain in angular momentum is required to produce the tight correlated set of spiral galaxy parameters which Disney discovered.

    Could you provide a paper link and a Coles Note explanation as to how spiral galaxies gain angular momentum as they gain mass via mergers using dark matter or modified gravity? What you have said in this forum does not agree with Disney's comment.

    http://arxiv.org/ftp/arxiv/papers/0811/0811.1554.pdf

    Problem 2: Evidence that a set of galaxies (bulgeless disc galaxies) grow without mergers. Two related issues. 1) Why are there no mergers? 2) If these galaxies did not have mergers how then did they grow?

    How is the mass getting into the galaxy in a manner such that a bulge is not produced?

    Note the bulgeless galaxies form a continual set that has larger and larger tiny bulges. This supports the assertion that the set of bulgeless galaxies gradually develop a bulge and become standard spiral galaxies. The size/mass of the spiral galaxies bulge is unexplainably directly proportional to the mass of the super massive object which is in the center of almost every galaxy. The super massive object somehow also gains mass in some mysterious way as the bulgeless galaxies gain mass without mergers.

    http://arxiv.org/abs/1009.3015

    Problem 3: There are no type B dwarf satellite galaxies (dwarf satellite galaxies that are spherically orientated about the spiral galaxy as opposed to type A which are located in a plane about the spiral galaxy) which is further support that there are no mergers and in addition this author assertions shows as there are no type B dwarfs that dark matter does not exist. Note modified gravity does explain the above.

    http://arxiv.org/abs/1204.2546v2

    Communication is serial and is not instantaneous. There needs to be time for thought about the problem and time to understand what the problem is. This problem is holistic. It is obvious or it should be obvious that the observations will point to the correct solution. The trick to solving the problem is to first at the look at the observations, think about the observations, understand the implications of the observations and then propose a solution. There are piles and piles of related paradoxes.

    Issue for a couple of other threads.

    There is another set of paradoxes concerning the downsizing of the super massive object mass that is in the center of almost ever galaxy with redshift (the mass of the objects in the center of galaxies get smaller with redshift rather than larger with redshift, the super massive objects shrink with time as it is assumed greater redshift objects are older objects, where did the old large super massive objects go?) and quasar evolution with redshift.
     
  21. Jul 31, 2015 #20
    The observational fact that there is a correlated structure of spiral galaxy parameters has astonishing implications.

    It is not possible to fine 'tune' a dark matter or a modified gravity model to create a continuum of spiral galaxy angular momentum that is directly proportional to spiral galaxy mass if spiral galaxies grow by mergers, due to conservational of angular momentum applied to the random merger process.

    The following explains why that assertion is correct based on fundamental physics.

    Disney asserts in his paper:

    The specialists have ignored the implications of Disney's finding, that there is a precise continuum of increasing angular momentum of spiral galaxies, with increasing galaxy mass. That observation fact rules out both 'Heirarchical' mechanisms as the principal method by which galaxies gain mass and grow and also requires a non gravitation mechanism to produce the precise angular momentum increase correlating with galaxy mass. A dark matter or modified gravitational model will produce a log-normal distribution of angular momentums.

    Does everyone understand conservation of angular momentum? Does everyone understand Disney's comment " Simulations produce spins, independent of mass, with a log-normal distribution"?

    I have always been curious what produces galactic angular momentum. The standard theory is that differential speeds of gas clouds when the galaxies first condense sets the angular momentum. Subsequence mergers may add or reduce that angular momentum depending on the random spatial arrangement of the merger. This type of mechanism will produce a log-normal distribution of angular momentum rather than a precise continuum of angular momentum that increases with galaxy mass.


    Fundamentals of Physics Extended Fifth Edition H.R.W.

    Page 289

    http://arxiv.org/ftp/arxiv/papers/0811/0811.1554.pdf

     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: The case for [or against] dark matter
  1. On Dark Matter (Replies: 5)

  2. Dark matter (Replies: 2)

  3. Dark matter (Replies: 28)

Loading...