Mapping the Dark Matter

By Sean Carroll | November 13, 2010 8:19 am

Have any friends or colleagues who don’t believe in dark matter? Showing them this should help.

Dark Matter in Abell 1689

That ghostly haze is dark matter — or at least, an impression of the gravitational field created by the dark matter. This is galaxy cluster Abell 1689, in the constellation Virgo. (We feel compelled to add that information, in case you’re going to go looking for it in the night sky tonight or something.) It’s easy to see that the images of many of the galaxies have been noticeably warped by passing through the gravitational field of the cluster, a phenomenon known as strong gravitational lensing. This cluster has been studied for a while using strong lensing. The idea is that the detailed distribution of dark matter affects the specific ways in which different background images are distorted (similar to what was used to analyze the Bullet Cluster). Astronomers use up massive amounts of computer time constructing different models and determining where the dark matter has to be to distort the galaxies in just the right way. Now Dan Coe and collaborators have made an unprecedentedly high-precision map of where the dark matter is (paper here).

This isn’t all about the pretty pictures. We have theoretical predictions about how dark matter should act, and it’s good to compare them to data. Interestingly, the fit to our favorite models is not perfect; this cluster, and a few others like it, are more dense in a central core region than simple theories predict. This is an opportunity to learn something — perhaps clusters started to form earlier in the history of the universe than we thought, or perhaps there’s something new in the physics of dark matter that we have to start taking into account.

But the pretty pictures are certainly a reward in their own right.

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  • Daniel Fischer

    Readers in Germany might want to attend a “Great Debate” on Dark Matter next week, in which a vocal doubter and a leading researcher of DM will fight it out …

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  • sweetser

    Dark matter is an ironic hypothesis. For it to work, there must be staggering amounts of this stuff. What does it accomplish? It solves 2 problems. First, it generates super tiny accelerations. For the arms of a spiral galaxy, it will make an acceleration that is more than ten orders of magnitude SMALLER than what we see on this puny Earth. If one integrates over a hundreds of millions of years, that is enough to get around a galaxy. I do not know how much the gravitational lensing here is, but I would be surprised if it was > 10^-10g.

    The second problem has to due with the stability of the solutions. Spiral galaxies want to collapse, at least from a math standpoint. That is not consistent with what we see which are galaxies that last a good long time. I have only heard of the stability issue in the context of spiral galaxies, and would be interested to know if it still is an issue for other large structures such as clusters.

    I can only agree with the data: Newton’s law cannot explain super small accelerations. I do think it more likely that there is a tricky problem with the effects of classical gravitation, not a stupidly large amount of dark matter. I do have a specific idea in mind if anyone wants to email me off this blog.


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  • Torbjörn Larsson, OM

    Wow, so FSM is real!? What will Sean say … oh. DM.

    [So how about FSDM (Flying Spaghetti Dark Matter-monster)? I would think theorist’s nightmares could be cantering in that direction by now.]

    an ironic hypothesis … staggering amounts … galaxies

    I don’t know what “ironic” hypotheses are, unless it is those that concern substantial amounts of iron. But I do know that an amount of mass on the same order as the visible matter isn’t “staggering”, again unless you see the amount of visible matter in the visible universe as staggering (which it is, kinda).

    It isn’t galaxies that is the best test for DM as of yet AFAIU, but cosmology and lensing. And, perhaps not coincidentally, galaxies and clusters of them are the areas where we see the most difficulties. (I take it – layman here.) So I wouldn’t worry or raise pet theories as of now, at least on actual science blogs.

  • Lab Lemming

    Isn’t it the wrong time of year to look for dim objects in Virgo? Even if you do get up early enough, the dawn glare and low elevation will complicate observations.

  • Anders

    What would happen if a large chunk of dark matter would pass through the earth?

  • Darker Matter

    Dark matter — the scientists “God”. No evidence for its existence (and no, gravitational lensing is NOT a compelling argument) and essentially eliminated by more rigorous mathematical models being produced today.

    There’s more evidence for the tooth fairy.

  • Charon

    “What would happen if a large chunk of dark matter would pass through the earth?”

    Nothing. Dark matter has no (or extremely little) non-gravitational interaction with normal matter. Less than neutrinos, which zip through the Earth all the time.

    “and no, gravitational lensing is NOT a compelling argument”

    I assert by fiat that lensing is not compelling! I have no reasons for this! Except that I want this to be the case! Which is totally a compelling argument! To me!

  • pheldespat

    Dark matter, huh? Dark matter has lots of problems with the famous Bullet Cluster and with Abell 520.…-proof-of-cold-or-warm-dark-matter-in-galaxy-clusters-is-but-a-myth


    “Even if it does exist, dark matter would be unable to reconcile all the current discrepancies between actual measurements and predictions based on theoretical models. Hence the number of physicists questioning the existence of dark matter has been increasing for some time now.”

  • Icarus2

    1. Observation evidence for dark matter
    1) Galactic rotation curves
    2) Galaxy clusters and gravitational lensing
    3) WMAP
    4) Bullet cluster
    5) Structure formation

    2.In the Galaxy, No evidence for dark matter
    1) At the Earth, non-observation(Xenon100, CDMS-II…)
    2) At the solar system, non-observation

    3) At the center of galaxies, no evidence
    Greedy Supermassive Black Holes Dislike Dark Matter
    Astronomers Find Black Holes Do Not Absorb Dark Matter

    4)At the galactic plane, no evidence
    “No evidence for a dark matter disk within 4 kpc from the galactic plane”

    5)At the galactic halo, no evidence
    Globular Cluster problem
    “Evidence Against Dark Matter Halos Surrounding the Globular Clusters MGC1 and NGC 2419”

    Dark matter is clustered around galaxy. and centripetal force effect exist in the galaxy from dark matter halo out of the galaxy.

  • Anders

    @charon: even a chunk of dark matter the size of the moon or is dark matter not thought to be dense enough?

  • bittergradstudent

    I don’t get the animosity between the dark matter people and the modified gravity people–when you write down a modified gravity lagrangian, you’re essentially just writing down a lagrangian for Einstein gravity, and specifying that it is coupled to dark matter of a particular form–take something like TeVeS–how are the extra vector and scalar field NOT just dark matter particles?

    And yes, I know that you build that fiducial metric tensor from them, blah blah. All that’s saying is that their coupling to ordinary matter and gravity is a bit more complicated than ordinary couplings.

    No matter what you do, there’s a dark sector of physics, you expect it to be described with a Lagrangian, and you’re trying to get better constraints on that Lagrangian. What’s with the hostility?

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  • Peter Fred

    Have any of those underground detectors found experimental evidence for the dark matter after years of looking for it. The Ptolemaic system had strong observational evidence as does the dark matter hypothesis. The evidence for the Ptolemaic system turned out to be spurious because it was based on an artifact–the earth rotating on its axis every 24 hours. The observational support the dark matter idea could also be spurious and based on an artifact if LUMINOSITY and not mass mediates gravitational phenomena. Why don’t you scientists ask yourselves the question that has not been properly answered for the last 300 years: Does the sun’s mass attract the earth or does its its mass? This is a legitimate question. For my answer to this question: Check out my website where I describe 4 experiments which show a weight increase of test masses that have been placed between a 1000 W heat source and cold source that comprises a copper container filled with ice.

  • Ben Finney

    Have any of those underground detectors found experimental evidence for the dark matter after years of looking for it.

    Have any of your web searches returned answers to this question? I found two articles from my first search.

    My understanding is that the search is just beginning, and detection is not a matter of “there it is!”; rather, it’s a matter of statistical likelihood that an observation was caused by dark matter. It will take a lot of data analysis before anyone can say with confidence that observations of dark matter interacting with objects on Earth can be concluded.

    Why don’t you scientists ask yourselves the question that has not been properly answered for the last 300 years: Does the sun’s mass attract the earth or does its its mass? This is a legitimate question.

    It appears to be a grammatically un-parseable question as written.

    If you mean “Does the sun’s mass attract the Earth, or does the Earth’s mass attract the Sun?”, then the answer is either “Yes, both occur”, or “Neither, both distort the gravitational field”.

    Our current understanding of gravity is not one of objects attracting each other; rather, it is better explained by a field of gravity distorted by masses. Perhaps you’d like to read books on gravity written later than 300 years ago to get more information.

  • IGGY

    So, where’s a sample of this over-abundant dark matter?

    Or, is this just more laughable physics garbage that gives us the absurd big bang, 11th dimension, multiverses & string theory?

  • Doug Watts

    Neutrinos are dark matter. They are emitted all the time during beta decay from the rocks and dirt in your yard (well, actually anti-neutrinos). This is not controversial. But neutrinos don’t have enough mass to explain galactic rotation speeds. They are too light.

    So I’m not sure why there is such a hubbub about dark matter. We already accept that neutrinos and anti-neutrinos exist and pass through us by the billions every second and are … ahem … quite difficult to detect. So the existence of another particle, more massive, but equally elusive to detection, seems pretty reasonable for now. And does not require NASA cover-up conspiracies.

  • Mandy

    I’m not a scientist, but I do have a great interest in the history of science.

    That allows me to see direct parallels between dark matter and previous pronouncements such as epicircles or the aether. If history is a guide, we still have another 25 years of not finding anything before science gives up their green fairy goblins and figures out that approaches such as MOND, etc were a more rational direction to take.

    The thing is, scientists are just humans. They are as susceptible to preconceptions and confirmation bias as any other talking monkey.

    It took them half a century to accept the big bang, as it conflicted with their preferred view of a static perpetual universe. It certainly didn’t help, that the big bang theory was proposed by Leimatre, a roman catholic scientist priest.

    You still see this bias to this day, pick a random entry level online physics course from a random university. Chances are they will talk in great detail about Fred Hoyle deriding the theory and the origin of it’s name, but won’t mention for a second the original name of the theory being the Primordial Atom or the scholar-monk that originally did the math.

    The same thing going on in this instance, as every scientist knows Einstein was filled with awesome-sauce and his theory certainly doesn’t need modification! The phantom dark-matter that will never be found enables them to continue their belief system.

  • Phillip Helbig

    “It’s easy to see that the images of many of the galaxies have been noticeably warped by passing through the gravitational field of the cluster, a phenomenon known as strong gravitational lensing.”

    Actually, more or less by definition, warped images are weak lensing and multiple images are strong lensing (of course, multiple images of extended sources are usually (always?) warped).

    Weak lensing also usually refers to lensing of extended, rather than point, sources.

    One can have multiple images or not, point sources or not, and small angular scales or not. That makes 8 different fields which are all gravitational lensing but quite different with regard to theoretical methods, objects of study etc.

  • Mark H

    @Mandy and others who think scientists are deluding themselves with dark matter.

    The history of science tells us that revolutions are impossible to predict. Why are you so confident that dark matter is wrong and MOND is right? More importantly, why does the question have to be settled now? Is it so important to start laughing at epicycles as soon as possible?

    The world isn’t ending in 2012. We have time to gather more data.

  • Peter Fred

    Ben Finney said, “I found two articles from my first search.”

    Those two articles you cite make the statements that the underground experiments “may” and ”
    “could” have detected the dark matter. Rest assured that when and if any of these multimillion dollar detectors find reliable evidence for the dark matter neo-ether idea, that evidence will be posted in elaborate detail on Cosmic Variance. Since you choose to ignore my experiments that provide repeatable experimental support for the idea that luminosity mediates gravity, let me comment on the light bending picture that we are asked to show all our friends. First of all we do not know whether its some property of the mass of a body that is doing the bending or the heat of that body that is doing the bending. That is why I am not impressed with the picture.

    Second of all I am not impressed with the 1919 light bending study where a result twice that predicted Newtonian gravity was obtained. Soldner (not Newton) assumed that the light that traversed across the sun’s gravitational field traversed across a CONSTANT gravitational field. This assumption is as was unrealistic. As that light falls toward the sun ( due to either heat-based or mass-based gravity theory) the gravitational force changes i.e. a jerk occurs. A tidal force which occurs with light bending phenomena and the secular advance of the perihelion of Mercury involves a changing acceleration or a JERK. Thus, the starlight that grazes the sun jerks towards the sun–it does not just constantly accelerate towards it. Therefore with a jerk one should expect that a greater angle of bending would be observed than that predicted by Soldner where only constant acceleration was assumed.
    You have not seen this question raised in the literature have you? This is a legitimate point. Just a my question: “Is it the heat or the mass of a body that is doing the attracting (or warping of space” is a legitimate question. Why haven’t our esteemed scientists addressed these two legitimate questions? I do not think they really like dark matter or a preposterous universe where something comes out of the vacuum to cause cosmic acceleration. My guess is that they like all that stuff they memorized for years so that they could get a PhD. Having gained money and prestige from that PhD. its risky for their reputation to even consider that some parts of the textbook, all of which they memorized, could be wrong.
    So show my comments and photographs of my experiments to your friends who are doubtful about the dark matter and think of it and the dark energy as the neo-ether.

  • Valatan


    What, pray tell, is a ‘heat-based gravity theory’?

    And scientists don’t ask this question because there have been further light-bending studies done since 1919, with much greater precision (particularly when radio waves are used), all of which agree with General Relativity. See page 39 of Clifford Will’s article.

    Also, general relativistic derivations of the light bending do not and need not assume a constant acceleration of the light beam due to the effects of gravity. It is not especially hard to solve for appropriate null geodesics of the schwarzschild metric.

  • Ben Finney

    Those two articles you cite make the statements that the underground experiments “may” and ”could” have detected the dark matter. Rest assured that when and if any of these multimillion dollar detectors find reliable evidence for the dark matter neo-ether idea, that evidence will be posted in elaborate detail on Cosmic Variance.

    Yes. You say all that as though it somehow disagrees with what the current science on dark matter describes.

    If you ask “Have they found it?” and don’t like an answer of “maybe, within such-and-so degree of tolerance”, then you’re invited to find a different universe where science is bound to provide only certainties.

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  • Peter Fred

    @ Valatan asks, “What, pray tell, is a ‘heat-based gravity theory’?” Click Peter Fred above and it will take you to my paper on a “heat-based gravity theory” . There you will find all about how heat mediates the gravitational force.

    On your remarks, “It is not especially hard to solve for appropriate null geodesics of the schwarzschild metric”.

    Why get involved in non-Euclidean geometry if you can explain light bending and secular advance of the perihelion of Mercury the with Newtonian gravitational theory using the Euclidean concept of the jerk. Newton and Einstein were well aware that a Newtonian mass-based gravity theory required the unpalatable idea of action-at-a-distance. Einstein and everybody else thinks that this problem is adequately addressed by GR. But I ask you, “How is mass able to warp space?” This is as much of a”sterile ad hoc formalism” as that held by the Ptolemaics that the earth had some magical ability to cause all the objects in the Heavens to rotate around it in a 24 hour period.

    Since heat and luminosity can easily move through a vacuum, this action-at-a-distance problem disappears with a heat-based gravity theory. Since heat or luminosity is measured in units of power which is intimately related to acceleration, it is not difficult to further interpret just how heat goes about attracting mass. Thus, a heat-based gravity theory cannot be easily considered a “sterile ad hoc formalism”.
    I am “in your face” about a heat-based gravity theory because I am trying to get you to think how dumb a mass-based theory is and how much more logical and “close to experience” a heat-based one is.
    I have experiments that show that heat is attractive. They need to be replicated just as the cosmic acceleration studies had to be replicated because going to a heat based gravity theory is a big change as is the adopting of the idea of cosmic acceleration. ( I hope you click on my name and see the graph in my paper which shows that the onset of the “dimming of the universe” was shortly followed by the onset of “cosmic acceleration”! And if you realize the import of this “coincidence in time of these two cosmic events” you will have stumbled upon another plus for a heat-based gravity theory.

  • Valatan

    You can’t account for both time dialation and special relativity without non-Euclidean curvature–historically, this was Einstein’s motivation far more than the Mercury perehelion precession. The light bending was something that was not even previously predicted that was then observed to verify the original theory. You also need an explanation for why the Hulse pulsar seems to be radiating only in its quadrupole mode if you’re going to chunk GR over the 1919 tests of relativity. That paper I linked cites a large number of high-precision tests for relativity. An argument based around dimensional analysis isn’t extremely convincing in the face of a large number of quantitative predictions.

  • Peter Fred

    You can talk about how non-Euclidean geometry is needed to clear up some points about time-dilation and special relativity and how it is needed to account for the Hulse pulsar results that give some support for gravitational radiation. But non-Euclidean geometry has a hard time accounting for 95 % of the universe that is missing.

    Galaxies are so shaped that luminosity leaving at low angles from their center will be bent and cause luminosity at the edges of a galaxy to fall off as 1/r rather than as 1/r^2.
    If luminosity is attractive as my five experiments indicate, then we have a simple, close-to-experience explanation for the flat rotation curves of galaxies. This could help explain the dark matter problem. At my paper you will also see a graph which shows how shortly after the onset of the “dimming of the universe” the onset of cosmic acceleration occurs. If luminosity is attractive, then we have another simple, close-to-experience explanation for the dark energy problem. With this mass-based theory of General Relativity we have Harvard’s Christopher C. Stubbs saying, “Understanding the nature of Dark Energy is arguably the most profound outstanding problem in contemporary physics.”
    With a heat-based gravity theory this “profound outstanding problem” has a good chance of quietly going away with a very simple explanation.

  • Valatan

    Time dialation and special relativity cannot be reconciled without Non-Euclidean geometry. Having both effects is fundamentally incompatible with a flat spacetime. Both are known to exist to a very, very high precision. If you come up with a theory that explains complicated things without explaining simple things, and then posit a paper with a few plots and very little in the way of actual quantitative predictions, you’re being foolish. There is more evidence for dark matter than luminosity curves. Saying that anamolous galactic rotation curves happen due to heat makes the bullet cluster even more problematic than it is for MOND, where you can at least argue that the self-coupling of the MOND sector might cause decoupling of the lensing matter from the luminous matter.

    That paper by Will has literally hundreds of high-precision experiments. But you’re not interested in any of that. You just want to yell about experiments from 1919 and dark matter. Casually dismissing modern science as ‘you can’t explain 95% of matter’, without even considering that there are a lot of very solid predictions before that statement is made is completely insane. If I’m going to completely rewrite cosmology, for instance, I’d at least THINK about whether or not I’m completely screwing up nucleosynthesis.

    I’m done with this conversation.

  • Tissa Perera

    “Have any friends or colleagues who don’t believe in dark matter? Showing them this should help.”

    Not me Sean. Dark matter is one way to explain things, but that is not the only way. I have published
    an article at my web site to explain it in another way that corroborates the other idea of MOND gravity.
    Dark matter is still not found, and my extra forth space dimension reproduces MOND naturally. As I show, MOND gravity also produces gravitational lensing and therefore can be mapped, not for the presence of Dark matter but because of the presence of a 4th space . Unfortunately, the 4th space is hard to see or detect. So I call it the Dark Space.
    Which is true? Dark Matter or Dark Space? If dark matter is not found(so far not) may be we should start looking for the Dark space?
    /Tissa Perera

  • John Ramsden

    If Dark Matter exists, I reckon it’s backscatter from inside black holes, like mist round a waterfall.

    Some process within black holes (mass inflation possibly?) “measures” momentum with such precision that the Uncertainty Principle means the mass/energy position is uncertain enough that the mass can “leak” back out.

    That would perhaps explain why, despite its significant gravitational effects, it is so damned elusive!

    Mind you, the curious “loop” in one part of that image did give me pause for thought, unless it’s an artifact of the lensing calculation.

    For those not familiar with mass inflation (and references are remarkably sparse – even the mighty Wikipedia had only a place holder last time I checked), it’s a consequence of General Relativity that in certain conditions of highly curved spacetime inside a black hole, when a mere two photons meet a torrent of energy, comparable to all the mass/energy in the observable universe, is released in order to preserve causality. So it’s somewhat analogous a tiny speck of dust under the wheels of an electric train being able to produce a huge spark and a noise like a cannon shot!


  • John Ramsden

    Just a brief clarification of my preceding post – I wasn’t suggesting the mass leaves the interior entirely, in the same sense as Bekenstein-Hawking radiation is emitted from the event horizon (also on account of the Uncertainty Principle, in one of several equivalent interpretations).

    For a significant sized black hole, B-H radiation is mere gnat’s piss compared with the vast torrents of mass inflation that can occur within the hole, and I propose that the “peak probability” of the position (in so far as it comprises particles) remains within the interior with only the tail of the position uncertainty extending considerably outside the event horizon.

    Presumably a similar notion (for what it may be worth) could be expressed using the Uncertainty Principle in terms of other complementary pairs of variables, such as energy and time – If the supposed process regulates or “measures” energy very precisely for example, then one would expect a correspondingly large uncertainty in time, including travelling backwards in time and hence outward from the hole.

    Mind you, I gather antimatter, which is said to be matter travelling backward in time, is affected normally by gravity. So perhaps something doesn’t quite add up with the energy-time version of the idea. Oh well, perhaps I’d better leave the mysteries of dark matter to the experts!

  • heldervelez

    here is a first presentation of a model without no DM, no DE, no space expantion, no BB
    that matches both cosmic and local data. (BBT does not fit local data,…etc,etc..)

    A relativistic time variation of matter/space fits both local and cosmic data
    by Alfredo Oliveira.

    In a few words:
    How can we be sure that matter is not shrinking (Length, Mass and Time units vary thru time) instead of space expantion?
    Follow this path and contradictions evade.
    A question of a subtle new interpretation of data.
    Not the simplistic, obvious one that we know full of darkness.

    But the new model do not make directly any enlightment on DM. As BB is not there anymore, the evolution of of the universe at large scale is a new story, at odds with our present assumptions.

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About Sean Carroll

Sean Carroll is a Senior Research Associate in the Department of Physics at the California Institute of Technology. His research interests include theoretical aspects of cosmology, field theory, and gravitation. His most recent book is The Particle at the End of the Universe, about the Large Hadron Collider and the search for the Higgs boson. Here are some of his favorite blog posts, home page, and email: carroll [at] .


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