Mapping the Dark Matter

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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35 Responses to Mapping the Dark Matter

  1. Pingback: Tweets that mention Mapping the Dark Matter | Cosmic Variance | Discover Magazine -- Topsy.com

  2. 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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  4. sweetser says:

    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.

    Doug

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  6. Torbjörn Larsson, OM says:

    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.

  7. Lab Lemming says:

    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.

  8. Anders says:

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

  9. Darker Matter says:

    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.

  10. Charon says:

    “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!

  11. pheldespat says:

    Dark matter, huh? Dark matter has lots of problems with the famous Bullet Cluster and with Abell 520.

    http://www.scilogs.eu/en/blog/the-dark-matter-crisis/2010-07-30/but-the-bullet-cluster-…-proof-of-cold-or-warm-dark-matter-in-galaxy-clusters-is-but-a-myth

    http://en.wikipedia.org/wiki/Abell_520

    Also: http://www.physorg.com/print160726282.html

    “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.”

  12. Icarus2 says:

    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
    http://www.universetoday.com/13091
    Astronomers Find Black Holes Do Not Absorb Dark Matter
    http://www.universetoday.com/60422

    4)At the galactic plane, no evidence
    “No evidence for a dark matter disk within 4 kpc from the galactic plane”
    http://arxiv.org/PS_cache/arxiv/pdf/1

    5)At the galactic halo, no evidence
    Globular Cluster problem
    “Evidence Against Dark Matter Halos Surrounding the Globular Clusters MGC1 and NGC 2419”
    http://arxiv.org/abs/1010.5783

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

  13. Anders says:

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

  14. bittergradstudent says:

    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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  16. Peter Fred says:

    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.

  17. Ben Finney says:

    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.

  18. IGGY says:

    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?

  19. Doug Watts says:

    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.

  20. Mandy says:

    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.

  21. “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.

  22. Mark H says:

    @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.

  23. Peter Fred says:

    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.

  24. Valatan says:

    @Peter:

    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.

  25. Ben Finney says:

    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.