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Hubble Space Telescope Detects Ring of Dark Matter 176

mknewman wrote with a link to a story on the NASA site indicating that they may have finally found dark matter using the Hubble telescope. We've discussed the stuff a few times in the last year, with the Hubble actually mapping out the dark matter in the universe in January. This, though, may be our first 'sighting' of the elusive substance. "NASA will hold a media teleconference at 1 p.m. EDT on May 15 to discuss the strongest evidence to date that dark matter exists. This evidence was found in a ghostly ring of dark matter in the cluster CL0024+17, discovered using NASA's Hubble Space Telescope. The ring is the first detection of dark matter with a unique structure different from the distribution of both the galaxies and the hot gas in the cluster. The discovery will be featured in the June 20 issue of the Astrophysical Journal."
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Hubble Space Telescope Detects Ring of Dark Matter

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  • by Anonymous Coward on Thursday May 10, 2007 @06:22PM (#19075771)
    I heard it circles Uranus.
  • by QuantumG ( 50515 ) <qg@biodome.org> on Thursday May 10, 2007 @06:23PM (#19075781) Homepage Journal
    except, of course, all the astrophysicists who often pointed out that exactly this kind of discovery was just around the corner.

    • by lgw ( 121541 )
      But how do you see dark matter with a telescope? It's not dark in the sense that light doesn't pass through it, it's dark in the sense that light doesn't interact with it at all. I guess we'll have to wait until 5/15 to see what the science is behind the headline.
      • Re: (Score:3, Informative)

        by Rycross ( 836649 )
        I'd assume you'd "see" it by observing how it interacts with massive bodies around it, like planets, stars, gas clouds, etc.
        • by lgw ( 121541 )
          You might "see" it as a gravitational lensing effect. Of course, that's far less sensational then claiming to see dark matter, as all you can really be sure of it that you're seeing a gravitational lensing effect (and this wouldn't really be news, as there have been a few gravitational lensing effects attributed to dark matter already).

          Seeing the gravitational effect on the massive bodies around it really wouldn't be news, as that's were all these dark matter theories have been coming from for years, with
          • by Thing 1 ( 178996 )

            Seeing the gravitational effect on the massive bodies around it really wouldn't be news [...]

            Agreed; but, seeing a star turn into dark matter, well, that would be news indeed.

            Perhaps it's my paranoid upbringing :), but I can easily envision dark matter simply being stars surrounded by Dyson spheres [wikipedia.org] or Matrioshka brains, [wikipedia.org] using up the entirety of the star's output.

            That might appear to us as merely a gravitational lensing effect, since we would not detect any electromagnetic frequencies.

            So, if we watc

            • As has been mentioned elsewhere, Dyson spheres would emit detectable blackbody radiation, therefore DM cannot be explained by such constructs.
          • IIRC, the theory of dark matter came from calculations that showed that the universe should have more mass than can be accounted for by visible matter. Showing that there is gravitational lensing supports that previously only mathematically proven theory.

            The news isn't that gravitational lensing was observed, but the shape of the area of dark matter. FTFA:

            The ring is the first detection of dark matter with a unique structure different from the distribution of both the galaxies and the hot gas in the clus

        • The problem I've always had with dark matter is the idea that we first suggested the phenomenon as an explanation for why our current theories of gravity do not work on the galactic and above scale. Now, after we've determined that there must be some theoretical type of matter which contributes mass but does not interact with electromagnetic waves, we've determined that we can observe it through the same phenomena which originally led us to postulate its existance. Does that strike anyone else as slightly b
          • Yeah - you're not alone, but lets just say there are plenty of people out there who really want to see some dark matter, so rest assured they'll be seeing dark matter for years to come. Eventually one of the many other theories that suggest there is something wrong with our gravitational models - rather than some thing 'wrong' with the universe - will be revisited and the long dead discoverer posthumously awarded a Nobel prize.
          • Yeah, it's a shame that observations of the Bullet Cluster [wikipedia.org] provide strong evidence of dark matter. Unless, of course, you can come up with a better explanation... so far, none of the popular MOND theories can, so you have your work cut out for you.

            Incidentally, I'm sure many people thought the idea of the atom was a hack. I mean, come on, an invisible particle we can't see, but is a building block for all matter in the universe? It's insane!! Unfortunately, personal aesthetics must take a backseat to ev
            • Why should I have to come up with a better theory in order that I be allowed to criticize our current theories?

              And since we discovered Dark Matter via gravitational interaction, anything that uses gravitational interaction to demonstrate the existance of dark matter is inheriently flawed circular logic. Another poster mentioned the Cosmic Microwave Background as another source of evidence, which I do buy. But when you come up with a new theory based on observations of gravity, you can't then turn around and
          • by lgw ( 121541 )
            Dark matter theories based on observed gravitational anomolies couldn't really be confirmed by additional observences of the same effect. However, studies of the cosmic microwave background radiation give confirmation of dark matter theories from completely unrelated observations.

            In a nutshell, the CMBR represents a snapshot of our universe at a certain point in its early history. At that point, around 80% of matter was clearly something that didn't interact with light (nor electrical charge), but did int
      • by TMB ( 70166 ) on Thursday May 10, 2007 @06:54PM (#19076153)
        Given that the press conference isn't until May 15, I can't say for sure, but based on the brief blurb on the NASA website, it's almost certainly a gravitational lensing measurement.

        It's true that dark matter doesn't interact directly with light, but it does curve space (ie. generate gravity), which light travels through. So light feels the gravitational effect of dark matter, a phenomenon known as "gravitational lensing". Essentially, the images of background galaxies going through a concentration of dark matter become magnified and distorted.

        I don't know whether this is a strong lensing or weak lensing measurement. In strong lensing, the distortion is extreme and the images of the background get stretched into long tangential (and radial, though they're rarer) arcs like this [nasa.gov]. In the case of weak lensing, the distortion in any one image is small, but all images in a certain area are distorted coherently so you can statistically disentangle the signal.

        Given the distorted images of the background galaxies, you can determine what mass distribution was responsible for those distortions, thereby producing a "mass map". It appears that in this case (again, based on the brief blurb), the mass map shows some sort of ring-like structure that isn't seen at any other wavelength (which non-dark matter would produce).

        • Wow. Looking at the sheer number of galaxies in that photograph, thinking about how many stars are in each of those galaxies, realizing how huge a star is, and then reflecting upon the fact that the photograph you posted was but a minuscule part of the visible sky... it makes me feel really really small.
          • Yep. It gives me the shivers, sometimes.

            Have you ever seen the Hubble Deep Field [hubblesite.org] image, though? I've been using that as my wallpaper for a while now; a jpeg named "the galaxies like dust".

            It's amazing.
    • I'm biased, but... (Score:3, Interesting)

      by Shag ( 3737 )
      ... there's damned little you can do with Hubble (other than observe in the ultraviolet, and honestly, when was the last time you heard about that capability leading to some huge discovery?) that you can't do with a reasonably large terrestrial scope.

      Hubble is, by today's terrestrial standards, small. Its resolving power is limited, even in the relative vacuum of space, by the size of its mirror, the size, age and design of its instruments, and so on.

      Yes, Hubble finds stuff. But it doesn't find disproport
      • So at the end there you're really saying "There's damned little you can do with Hubble that you can't do with a terrestrial telescope so long as you don't mind not working at all if the weather sucks or it's not day time."

        Sounds like Hubble is still useful to me. Even if we get a new one up there.
      • Earth telescopes with optical resolving power comparable to Hubble have to use adaptive optics [wikipedia.org] of some sort. Because adaptive optics requires a guide star of some sort — to measure the deformation caused by Earth's atmosphere and physically re-shape the mirror to cancel that deformation — you can't just point the telescope in any direction you choose. There has to be a bright star in the field of view for the adaptive optics to measure. Artificial guide stars, which use lasers that are reflec

  • pic (Score:5, Funny)

    by antiaktiv ( 848995 ) on Thursday May 10, 2007 @06:28PM (#19075851)
    Screenshot or it didn't happen!
  • Redmond Washington.

    Ok, mod me as troll. I deserve it.
  • We Impress Me (Score:3, Interesting)

    by TheLazySci-FiAuthor ( 1089561 ) <thelazyscifiauthor@gmail.com> on Thursday May 10, 2007 @06:32PM (#19075905) Homepage Journal
    Is it just me, or are humans getting better and better at science as time progresses?

    I mean, it seems likely that this would be the case, naturally. Nonetheless, it still strikes me.

    We predict dark matter exists, then we show it exists. It seems pretty much assured that we will even find out what it is made of. This discovery further cements this feeling in my mind.

    We figure there is a chemical of inheritance, we find DNA. We know there is a genome, we sequence it.

    Everything seems to be a big puzzle, and we seem to be getting faster and more accurate with putting these puzzles together.

    I feel fully confident in speculating, for instance, that we will solve the gene therapy issues in mere years. That we will have household humanoid robots by 2020 for under $50,000US. That we will enhance ourselves dramatically genetically and technologically by the end of the century.

    Has science always been this inexorable in it's progress?
    • Re:We Impress Me (Score:5, Insightful)

      by rts008 ( 812749 ) on Thursday May 10, 2007 @06:45PM (#19076065) Journal
      "Has science always been this inexorable in it's progress?"

      I don't beleive so. My take on it:

      Timely communication over wide areas has started the 'inexonerable progress'. Telegraph, railroads, telephones, 2-way radio,and now the internet have boosted progress dramatically as each were implemented.

      I may be wrong, but the concept you seem to be looking for is 'singularity'. It's happening quicker as time goes- like a snowball rolling downhill, it may not reach the bottom of the hill (true singularity), but it's headed that way.
    • by iknownuttin ( 1099999 ) on Thursday May 10, 2007 @06:45PM (#19076069)
      That we will have household humanoid robots by 2020 for under $50,000US. That we will enhance ourselves dramatically genetically and technologically by the end of the century.

      It could happen. But if we piss off those robots and the genetically engineered humans, they may band together and start an extermination program of us humans. Then we'd have to flee the planet in a fleet of ships while the robots pursue us. Of course, with the genetically engineered humans, they'll look like us and they'd be used as spies. Of course, there may be a comuter scientist who falls in love with one of them and helps the robots take us out. Then he'll go insane and start imagining his robot lover.

      I don't know if we really want to go there.

      • by tloh ( 451585 )
        Don't be ridiculous!

        The most logical thing for them to do is to assimilate us all.

        • by Sj0 ( 472011 )
          There is nothing logical about making biological tissue an integral part of perfect machine life. Biological material requires huge amounts of energy to maintain, doesn't function as efficiently or as cleanly as a well designed machine, has a pitiful effective lifetime, and failure of one component WILL destroy the rest of the unit. Machine material is effectively immortal, can run on as much or as little energy as is available, is effectively immortal, and can be designed so no single failure will destroy
          • But humans could be enhanced with machine parts to serve a perfect immortal machine...
          • by tloh ( 451585 )

            I was just goofing off when I posted that lame borg joke. However, I think you're suffering from a severe lack of perspective. No machine yet created by humans have as of yet approached the elegence, efficiency, or versitility of what evolution has shaped over more than a billion years. Almost every living thing is capable of some degree of self-repair. For how much of our modern hardware can you say the same? Man has been designing tools and machines since our cavemen ancestors began forming abstract

            • by Sj0 ( 472011 )
              No machine yet created by humans have as of yet approached the elegence, efficiency, or versitility of what evolution has shaped over more than a billion years.

              We're talking about machine life, so this point is moot.

              Almost every living thing is capable of some degree of self-repair. For how much of our modern hardware can you say the same?

              If the problem of machine life has been solved, there's no good reason to assume a seperate self-repair mechanism wouldn't be possible.

              Man has been designing tools and m
              • by tloh ( 451585 )
                Well, now your post makes sense. But I still think you're misguided. machine life. robotic intelligence. Show me? If we're going to consider this from the realm of fantasy or speculative fiction, I can conjure up equally amazing notions but with super-intelligent pink dragons. Perhaps you've thought a lot about the perfect machine life. And that's fine - I admire imaginative people. But what is to say that your conception represents the peak of existential perfection? I suppose you can think whatev
                • by Sj0 ( 472011 )
                  Be fair; We WERE talking about comparisons to The Borg, from a universe where there were a number of examples of pure machine life.

                  Though you're right, it IS, since we're talking fictional things all around, like arguing that superman could beat the flash (He could. What is the flash going to do? Run away?)
                  • by tloh ( 451585 )
                    Hmmm.... do you think the borg can assimilate superhuman abilities? What would be the machine version of super abilities? (The ability to fly and super strength relative to animal life forms are pretty mundane for a machine to achieve.)

                    But I suppose you're right. What I'm having a hard time wraping my head around is trying to judge as a creature of flesh and blood what the most desirable qualities are for a machine life form that doesn't have any basis in reality yet. Am I supposed ask "If I were a mac
    • Re:We Impress Me (Score:5, Informative)

      by smilindog2000 ( 907665 ) <bill@billrocks.org> on Thursday May 10, 2007 @06:58PM (#19076223) Homepage
      There's an incredibly boring book, "Guns, Germs, and Steel", which I've mostly read (it's easier than reading the Old Testament!). The basic question posed in the first paragraph is "Why did Europe dominate the world?" He goes into fairly convincing arguments for why we are advancing faster and faster... technology feeds on itself in a positive feedback loop. He discounts the importance of the giants, like Newton, and focuses on the size of populations, the ease of communication of ideas and domesticated plants and animals between them. Technology is advancing at an unstoppable pace. The way it's going, it seems likely we'll either use it to kill ourselves, or birth a new race that we design... either biological through genetic manipulation, or electronic, or perhaps a combination of both.
      • Re: (Score:3, Interesting)

        it's easier than reading the Old Testament

        Well, that's a ringing endorsement for a book if I've ever heard one ;)

        I feel the same about our progress being both wonderful and dangerous. I am reading Asimovs' robot novels right now, and in a forward he made a deeply profound observation. Let me google it for accuracy...

        "Even as a youngster, I could not bring myself to believe that if knowledge presented danger, the solution was ignorance. To me, it always seemed that the solution had to be wisdom."

        I wonder if

      • by radtea ( 464814 )
        He goes into fairly convincing arguments for why we are advancing faster and faster...

        But we aren't. Consider:

        My grandmother was born in 1884 and died in 1980. By the time she was my age (early forties) she had seen heavier than air flight used in warfare and commerce, the end of the age of sail, the invention, commercialisation and massive popularisation of radio, massive urban electrification, and the coming of the mass-produced automobile, just to name a few of the bigger changes. Oh yeah, and votes f
        • I feel that way sometimes. How cool would it have been to see the birth of powered flight? In 1907 (according to some spam I got), California had only 1.5 million people, and the average life expectancy was only 47 years old. But... I think we've had amazing advances, just not the sort you see flying overhead. We've built the Internet, and Moore's Law has held for nearly 50 years. My cell phone has more computational power than existed in the world in 1950. I was alive to see the first man set foot on
      • The book is only boring becuase Diamond makes his basic point in the first chapter, and then repeats the same damn point as the explanation for every cultural disparity that he cites as distinguishing between people from ancestral central Eurasia and, for example, ancestral Mexico.

        I'll ruin it for you: the point is that there were significantly more square miles of easily travelled arable land in the same climate zone accessible to people on the eurasian landmass than there was available to the Incas and Ma
        • Hey! I'm only 2/3rds of the way through this damned book! Don't spoil the ending :-)
        • by Fred_A ( 10934 )
          Damn, and I was so sure that it was the butler that had done it, now you tell me it's nutrition ? You could at least have put up a spoiler warning.
        • You forgot the "germs" part. Having more domesticated animals and living among them exposed Europeans to more diseases and killed off lots of them(e.g. Bubonic plague), leaving the resistant survivors. Europeans arrived in the Americas - bringing their various animals along - exposing the native Americans to the diseases which they have not built up resistance to, resulting in widespread death without even having to use much of their advanced technology to defeat them.

          And the central "confrontation" illu
    • by vlad_petric ( 94134 ) on Thursday May 10, 2007 @07:01PM (#19076253) Homepage
      We find no luminiferous aether.

      Not all scientific predictions are made equal.

      • by Lijemo ( 740145 ) on Thursday May 10, 2007 @08:17PM (#19077081)

        "We find no luminiferous aether.
        Not all scientific predictions are made equal."

        that was a very useful prediction.

        We predicted luminous aether: it was a logical theory. We had good reason to believe that light was a wave, we had no reason to imagine that a wave could exist without a physical medium (air, water, etc.)

        It was a falsifiable theory.

        For a long time people tried to prove it, but measurements weren't sensitive enough. Finally, a sensitive enough experiment was developed, and it found-- nothing!

        This was far more useful than if they had found something.

        On discovering that the theory was wrong, they didn't try to argue that it was really still correct. They puzzled about what it could mean: how can a wave exist without a substance to wave through?

        Many incredibly significant scientific advances of the next few decades came out of this enigma. If there had been no luniniferous aether theory, there would have been no enigma, and perhaps many of these discoveries would not yet have come about.

        The usefulness of a theory is not in whether it's correct or not. The usefulness of a theory comes from what you learn while trying to discover whether or not it is correct.

        • On discovering that the theory was wrong, they didn't try to argue that it was really still correct.

          Oh yes they did. The Sagnac effect even seemed to prove that it existed, as he claimed.
          But there was still huge value in the findings. You can only really explain the absence of an ether with general relativity.
    • $50,000 will buy you ONE chocolate-flavored corn syrup bar; unless you pay with Ameros then its only $5.
    • by Ihlosi ( 895663 )
      Is it just me, or are humans getting better and better at science as time progresses?

      No, we are just able to built on the foundations that have been laid over last couple of thousand years.

      Personally, I believe that we're slowly getting worse at science, relying more and more on technology than the raw power of our brains. If you look at how much the scientists in the 19th and early 20th century accomplished with how little technology (or how much mathematicians accomplished in even earlier centuries), it

    • Has science always been this inexorable in it's progress?

      It really depends on what you consider progress, and in what timeframes you're looking.

      During the 1650-1700 period a LOT of 'new science' was thought out, by such people as Newton, Leibniz, Spinoza, Huygens. Not all of it well grounded, not all of it useful, but that was a time where a lot of new thoughts were 'floating around' and being proven and disproven on an almost daily basis. These were people that set out to 'know everything' and in the proce

    • Parent says: "We predict dark matter exists, then we show it exists."

      Err. No. We did *not* predict dark matter. We were not expecting dark matter or anything like it when the Zwicky first saw that there had to be some "more" matter in the galaxies to explain the observed rotational curves. He probably first said: "Gee, well, that looks funny!" Zwicky probably said something a lot better actually, as he was known for his, often rude, mannerisms.

      The astonishing discoveries in science come when humans

    • by Krellan ( 107440 )

      Is it just me, or are humans getting better and better at science as time progresses? ...

      Everything seems to be a big puzzle, and we seem to be getting faster and more accurate with putting these puzzles together. ...

      Has science always been this inexorable in it's progress?

      Yes. The rate of change is increasing. Each new invention makes it slightly easier to invent the next invention, and so on. It's exponential, one of those scary J-curves. Nobody knows where it will all end up spiking upward to, but many people have thought of this before.

      As somebody else pointed out, the phrase you're looking for is "technological singularity".

      Google this for many fun hours reading. Read some papers by Ray Kurzweil. Buy a book or two by Vernor Vinge.

      http://en.wikipedia.org/wiki/Techn [wikipedia.org]

  • More info (Score:5, Informative)

    by IWannaBeAnAC ( 653701 ) on Thursday May 10, 2007 @06:44PM (#19076059)

    I was about to write a comment panning this submission, because apparantly a one-paragraph press release - that doesn't give much room for an intelligent discussion - was the only information on this discovery. But I did find an abstract for a talk given at the American Astronomical Society Meeting 209, which was held in January this year.

    Authors: Jee, Myungkook J.; Ford, H. C.; Illingworth, G. D.; White, R. L.; Broadhurst, T. J.; Coe, D. A.; Meurer, G. R.; van der Wel, A.; ACS Science Team We present a comprehensive mass reconstruction of the z = 0.4 rich galaxy cluster CL0024+17 from Advanced Camera for Surveys data, unifying both strongand weak-lensing constraints. The weak-lensing signal from a dense distribution of background galaxies ( 120 per arcmin^2) across the cluster enables the derivation of a high-resolution parameter-free mass map. The strongly-lensed objects tightly constrain the mass structure of the cluster inner region on an absolute scale, breaking the mass-sheet degeneracy. The mass reconstruction of CL0024+17 obtained in such a way is remarkable. It reveals a ring-like dark matter substructure at r 75" surrounding a soft, dense core at r<50". We interpret this peculiar sub-structure as the result of a high-speed line-of-sight collision of two massive clusters 1-2 Gyr ago. Such an event is also indicated by the cluster bimodal velocity distribution. Our numerical simulation with purely collisionless particles demonstrates that such density ripples can arise by radially expanding, decelerating particles that originally comprised the pre-collision cores. ACS was developed under NASA contract NAS5-32865, and this research was supported by NASA grant NAG5-7697.

    Unfortunately I can't find the paper itself. So there is slightly more info, but not much :-(

    • Re: (Score:2, Informative)

      by jmtpi ( 17834 )
      From the ApJ website:
      Discovery of a Ring-Like Dark Matter Structure in the Core of the Galaxy Cluster CL0024+17
      M. J. Jee, H. C. Ford, G. D. Illingworth, R. L. White, T. J. Broadhurst, D. A. Coe, G. R. Meurer, A. van der Wel, N. Benitez, J. P. Blakeslee, R. J. Bouwens, L. D. Bradley, R. Demarco, N. L. Homeier, A. R. Martel, And S. Mei
      Received: 06 Sep 2006
      Accepted: 02 Mar 2007
      Dr. Myungkook Jee, Department of Physics and Astronomy, John Hopkins University, 3400 North Charles Street, Baltimore, MD 21218-2686, U
    • The mass reconstruction of CL0024+17 obtained in such a way is remarkable. It reveals a ring-like dark matter substructure at r 75" surrounding a soft, dense core at r

      MMMMM! Sounds like a delicious chewy center!

  • Typical (Score:4, Insightful)

    by malsdavis ( 542216 ) on Thursday May 10, 2007 @06:58PM (#19076233)
    Why does the title read "Hubble Space Telescope Detects Ring of Dark Matter" when - as the first line of the summary states -, the HST actually only " may have finally found dark matter".

    "Has found" and "may have found" are very different things. I "may have" the lotto ticket which is going to win me millions of dollars in Saturday's draw; on the other hand, I may not. To pre-emptively state a conclusion before it has been made is foolish and extremely unscientific and simply not an accurate description.
  • ... to be too excited about this? I mean, scientists characterizes the behavior and name the thingie 'dark matter'. So even when they can conclusively say this ring-thing is made of such and such, but who knows how many type of 'dark matter' there really are.
  • by Burz ( 138833 ) on Thursday May 10, 2007 @07:16PM (#19076445) Homepage Journal
    ...last year: astronomers could see in the aftermath of two colliding galactic clusters. [sciam.com]

    The visible matter's momentum through space was impeded at quite a different rate than dark matter. This left four distinct zones of gravitational lensing, but only TWO were associated with visible matter. The other two were dark matter halos that had been separated from each galactic cluster.
    • by spun ( 1352 )
      Yes, but in that instance, there were two blobs of matter that slowed due to mutual interaction, gas pressure and whatnot, while the two associated blobs of dark matter shot off ahead. In this, it's a ring of dark matter.
    • That link is interesting, but baked right into their assumptions is the existence of Dark Matter. They assume the mass ratio of plasma to solid stuff is high. This is probably based on the assumption that Dark Matter holds a normal galaxy together. Every time I've read about why dark matter is required to hold a galaxy together, it comes down to the galactic rotation curve "problem". I've said it many times before, using Keplers laws to say what the rotation curve should be is invalid, yet that's what may p
      • Re: (Score:3, Insightful)

        by Abcd1234 ( 188840 )
        So where's your published paper where you provide an alternative theory to explain galatic rotation?
        • by gr8_phk ( 621180 )

          So where's your published paper where you provide an alternative theory to explain galatic rotation?

          The theory is Newtons law of gravitation - applied correctly.

          I think my post above explains the problem well enough. I have better things to do than try to publish a paper for a bunch of so-called experts that think keplers laws can be used to model a whole galaxy. OTOH, every time I make one of my rant posts here, someone always makes the point you just did - put up or shut up ;-) And yes, that is a vali

          • Yes, you do. Without evidence, all you are is a likely uneducated armchair quarterback telling us that all those highly educated astrophysicists out there just got it wrong (and in a, frankly, *incredibly* obvious way... I highly doubt your argument hasn't been put forth already, and subsequently dismissed), despite mountains of evidence, simulations, and experiments... none of which you seem to have.
  • "It seems dark-matter is nature's sex drug. It's like a perverted trail mix of penguin estrogen, penguine Viagra and Spanish penguin fly." - Paul, the space hippy
  • by CokeJunky ( 51666 ) on Thursday May 10, 2007 @08:35PM (#19077269)
    I had been under the impression that 'dark' matter was simply regular matter that we needed to exist to balance some equations, but that we couldn't see. Wouldn't this simply reduce the amount of dark matter by making it observable?

    Or is my impression that dark matter is stuff we can't see wrong? Is it actually supposed to be some exotic substance (with comic-book like powers)?
    • by TMB ( 70166 ) on Thursday May 10, 2007 @09:09PM (#19077497)
      Part of the confusion is that there are 2 separate concepts that both go by the name "dark matter".

      Dark matter in the broad sense is matter that we detect gravitationally but can't observe directly through any interaction with light (and if this measurement is from gravitational lensing, which I suspect, then it certainly falls into this catagory). We infer that it exists because the motions of stars and gas in galaxies, galaxies and hot gas in galaxy clusters, and the universe as a whole all act as though they are acting in the gravitational field produced by much more mass than what we can directly detect.

      Some fraction of this dark matter is normal ("baryonic") matter that just happens to be very difficult to detect due to its temperature and density... for example, a lot of it is diffuse gas at ~100000K, which is too cool to emit X-rays but too hot to emit much line radiation.

      However, from Big Bang nucleosynthesis calculations, we can estimate how much baryonic matter there is in the universe because the relative fractions of different isotopes of H, He, Li and Be are quite sensitive to the total amount of baryonic matter. And the total amount of matter required to account for the dynamics of the universe is about 6 times higher than the amount of baryons that Big Bang nucleosynthesis measurements indicate.

      Therefore, there must be non-baryonic dark matter too, made of exotic particles (or neutrinos, but there most likely aren't enough of those either). This is also sometimes just called "dark matter", which is confusing.

      Interestingly, galaxy clusters, like the one studied here, have most of their baryonic matter in the form of hot X-ray gas that is detectable... the density of baryonic matter we can detect within a galaxy cluster is about what you'd expect given the BBN calculations. So any dark matter in a galaxy cluster should be non-baryonic dark matter, which is why measurements like this are exciting.

      • It is a rare event that a reasonable question receives such an excellent answer on /.

        Thank you.
  • With all the jokes on this thread, I'm wondering: Is humor relative, or is it a quantum phenomenon?
  • Some 18thC astronomers claimed to have seen Vulcan [schoolsobs...ory.org.uk] too.


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