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Space Science

Galaxy Sans Dark Matter 92

Posted by samzenpus from the nothing-extra dept.
ChromaticDragon writes "Astronomers have crunched some numbers on a galaxy to discover that its rotation can be fully explained by the gravity of the observable matter — in effect, this galaxy seems to lack dark matter. This shouldn't come as a total surprise given that one of the stronger observations of Dark Matter was the Bullet Cluster where supposedly a good deal of Dark Matter and good old fashion regular matter had separated."
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Galaxy Sans Dark Matter

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  • Awesome (Score:1, Troll)

    by ShawnCplus ( 1083617 )
    Yeah.. guess what guys! We just discovered this awesome stuff called Dark Matter! Really? Yeah... Can we see it? um.... it doesn't exist in this galaxy.

    • Re: (Score:2, Insightful)

      by Dunbal ( 464142 )
      The trend in physics for the past 30 years has been to invent a new particle to explain any discrepancies with current theory anyway. This is no surprise.

      • Re: (Score:3, Interesting)

        by Jugalator ( 259273 )
        Only that dark matter isn't necessarily a new particle or has to be a product of fantasy. That's among the debated things. All it seems to be is some sort of particle with a mass that doesn't reflect light or emitting much else that can be detected, making it dark. So some theories have said it could be neutrinos, or maybe neutralinos [wikipedia.org], that are actually predicted to exist by modern theories.
        • Dark matter being that which is mostly having the definition that it is undefined, with the expectation that it is something new or something known with an unproven ( debated ) property of gravity, such as neutrinos.
          Its not necessarily wholly comprised of something unseen, and could be in fact made up of many different particle types having gravity/mass/weight. That is, assuming its not a gross mistake.
          • Dark matter is simply the mass of the error margin

            "Our theories predict that there is some quantity of matter in that galaxy, but we only measured a fraction of it. Either we're wrong in the estimate of mass, or in the theory that predicts the behaviour of galaxies."

            This is called Common Sense. Discrepancy == incomplete theory or wrong calculations.
            • Dark matter is simply the mass of the error margin

              Umm, no, it's not, and if you believe that, you haven't been keeping up with the latest discoveries. The Bullet Cluster results demonstrably show that *something* is there, different from regular matter, but exerting a gravitational force. What it is, we don't know, but it's presence is undeniable.
    • Not only can you not see the dark matter (and it appears to be devoid), but now, you can not find the link to it.
    • I know you were making a joke, but the the "this galaxy" that is being referred to isn't the Milky Way, it's NGC 4736.

  • A good link (Score:5, Informative)

    by The Empiricist ( 854346 ) on Thursday February 07, 2008 @12:07AM (#22329774)
    I'm not sure if it is the story the submitter was trying to link to, but this [newscientist.com] article seems to cover the subject.

  • Back of the Galaxy (Score:5, Funny)

    by chill ( 34294 ) on Thursday February 07, 2008 @12:11AM (#22329802) Journal
    All the Dark Matter is there, it just was told to move to the back of the galaxy.
    • That actually made me laugh out loud, and I'm only awake reading this because I am in too much pain to sleep tonight. So thanks, I really needed that laugh.

  • Another argument for variability of "constants" (Score:4, Interesting)

    by Ungrounded Lightning ( 62228 ) on Thursday February 07, 2008 @12:18AM (#22329856) Journal
    No (or negligible) dark matter in our galaxy, eh?

    When we're looking farther away, we're looking back in time, too. So perhaps the observations could be explained by "constants" of physics (notably the gravitational constant) varying with the age of the universe, rather than by the gravitational pull of some otherwise-unobservable dark matter.

    Let's see if "dark matter" is "more dense" the farther away we look... B-)

    • Re:Another argument for variability of "constants" (Score:4, Informative)

      by arotenbe ( 1203922 ) on Thursday February 07, 2008 @12:28AM (#22329906) Journal

      No (or negligible) dark matter in our galaxy, eh?
      That's what I thought when I first read this story, too. The summary is confusing:

      Astronomers have crunched some numbers on a galaxy to discover that its rotation can be fully explained by the gravity of the observable matter -- in effect, this galaxy seems to lack dark matter.
      Here, "this galaxy" refers not to the Milky Way but to the other galaxy mentioned in the first part of the sentence.

      • Re: (Score:1, Redundant)

        by mrxak ( 727974 )
        Definitely an interesting idea. Dark matter always sort of bugged me. But if it doesn't really exist, that means we can't use gravitic weapons (a la Schlock Mercenary) to talk to dark matter aliens!
    • 'Let's see if "dark matter" is "more dense"' I dunno, usually blond matter is more dense...
    • But dark matter seem to be unevenly distributed around various objects and not as if it was a general "constant" factor acting. It's not just theories, but observations on gravitaional lensing too.

    • Re: (Score:2, Funny)

      by Remusti ( 1131423 )
      Your galaxy? Are you posting from NGC 4736, or just visiting Earth for a while?

      Either way, I'm sure you can see that Earth isn't worth invading, right? Right?

      • Your galaxy? Are you posting from NGC 4736, or just visiting Earth for a while?

        Sometimes I feel like I'm from there. Most of the people I meet are really odd. B-)

        Either way, I'm sure you can see that Earth isn't worth invading, right? Right?

        Been there, done that. All I got was a damned teeshirt. But the artichokes were very tasty - especially in a Mongolian firepot with squirrel broth and seasoned with a bit of thulium and phenol - and the little thorns on the ends of the leaves make great antenna scratchers.

    • Re: (Score:2, Interesting)

      by Pikoro ( 844299 )
      I was always curious. If looking farther away is looking back in time, the conceivably, we should be able to find the milky way just by looking for it. It's not stationary, but not moving nearly the speed of light. Could there be a light "echo" somewhere out there and we're really looking at ourselves through a temporal colored lens?

      This post brought to you by beer.

      • I've actually thought about this, too.

        We've never sent a probe outside the solar system (yet), so what is to say that the interstellar space, or even the Heliopause itself, is not distorting the flow of energy/matter/time/space? What's to say that once Vayager I passes outside the Heliopause, we don't suddenly start receiving extremely redshifted transmissions from Andromeda that just happen to be exactly what Voyager I was sending? We don't know that our experimental laws of physics hold constant outside

      • Light moves faster than our galaxy; a double image is only possible if we were moving faster than light, or there was some kind of diffraction effect on an object far away.

        However, the movement does cause a red or blue shift, which is what describes the phenomenon you're looking for.
  • This is really interesting after reading a recent /. story about dark matter, and getting some more information from another poster. If dark matter was needed to explain why galaxies look a certain way, but this one can exist without it, what does that say about dark matter? Even if dark matter is real, there's obviously something missing if we can't explain this.

    Nature, you have once again awed me with your incredible weirdness.

    • Re: (Score:2, Interesting)

      by Vectronic ( 1221470 )
      I'm certainly no expert on the subject, but as far as I am aware Dark matter is matter that fills the equasion when the visible matter doesn't compute...

      If a galaxy without dark matter computes, and a galaxy with visible matter with the addition of dark matter computes...

      Whats the difference? wouldnt a galaxy made up of entirely dark matter be equal to a galaxy of entirely visible matter?

      If you have 3 fish, and two of them are transparent zebrafish, and one is a normal opaque one... they are still 3 fish...

      • Re: (Score:3, Interesting)

        by tirerim ( 1108567 )
        Sure. But most of the galaxies we have observed seem to be made up of a mixture of dark matter and visible matter, given their gravitational characteristics. Finding a galaxy with no matter isn't any sort of physical impossibility, but it's surprising because it's not the norm. If most galaxies contain dark matter, then something unusual must have happened to this one for it not to contain dark matter, and that's interesting; beyond that, the fact that such a thing even could happen may give us insights

      • Re: (Score:1)

        by rprins ( 1083641 )
        If you actually read the article...

        "The current picture is that galaxies form inside of dark matter halos," Diemand told New Scientist. The dark matter's gravity attracts ordinary gas, which can then coagulate into stars.
        "It is unclear how one would form a galaxy without a dark halo, or how one could remove the halo without destroying the galaxy," says Diemand. "A galaxy without dark matter really does not fit into our current understanding of cosmology and galaxy formation."

        There, apparantly dark matter makes a difference.

    • Re: (Score:2, Interesting)

      by EvanED ( 569694 )
      Nature, you have once again awed me with your incredible weirdness.

      Every time I read a story like this, I can't help but think of the following quote from Hitchhiker's:

      There is a theory which states that if ever anyone discovers exactly what the Universe is for and why it is here, it will instantly disappear and be replaced by something even more bizarrely inexplicable.

      There is another theory which states that this has already happened.

      I sometimes wonder if perhaps there is a God, and he is changing the rul

    • Re: (Score:2)

      by 2names ( 531755 )
      If dark matter was needed to explain why galaxies look a certain way, but this one can exist without it, what does that say about dark matter?

      Perhaps the researchers were lacking in gray matter?

  • Impact on gravity theories (Score:4, Insightful)

    by Dice ( 109560 ) on Thursday February 07, 2008 @12:36AM (#22329956)
    This is another nail in the "dark matter can be solved with a modified theory of gravity" coffin. If we can find a galaxy composed of stars whose observed motion is entirely explained by the mass of those stars and known theories of gravity (Newton, Einstein) that's a serious blow to theories like MOND [wikipedia.org].
    • You seem to have some clue as to what you are talking about so maybe you could help me out with something that has me puzzled for a while now.

      How do astronomers measure the rotation of a galaxy as a whole?

      • Re:Impact on gravity theories (Score:5, Informative)

        by syousef ( 465911 ) on Thursday February 07, 2008 @01:30AM (#22330284) Journal
        I'm not the GP but can answer your question.

        You measure the dopler shift of the stars on each side of the galaxy. Waves from stars travelling towards you compress, waves travelling away from you so it helps if galaxy is seen more edge on than top or bottom towards us.

        (Of course it won't be perfectly edge on so you have to calculate the component that is edge on to work out actual speeds around the galactic center. The less edge on the more accurate you can be because the component that's edge on is larger). ...which leads to how do you measure doppler shifts.

        One way is to look at the spectral lines of light in a star (ie split the light through a prism or diffraction grating). Chemicals that make up the star's surface absorb at precisely known wavelengths. It's actually really easy to do some calculation once you know what wavelength these lines have shifted to. (I did it when I did my astronomy masters. It's basic algebra andsimple equations). The difficult part is building equipment that can measure spectra so accurately. In the early days they'd be literally measuring the difference between wavelengths on glass plates.

        http://aether.lbl.gov/www/science/galrotcurve.html [lbl.gov]
        "To make a rotation curve one calculates the rotational velocity of stars along the length of a galaxy by measuring their Doppler shifts, and then plots this quantity versus their respective distance away from the galactic center."

      • Re: (Score:2, Informative)

        by NeoSkink ( 737843 )
        When we say "Galaxy Rotation Speed" we mean a measure of velocity as a function of distance from the center. To get this, you just measure the speed of stars and measure how far they are from the center. As to how you get a stars velocity, well you look at the red shift relative to the galaxy, that if the whole galaxy is moving away it will have a redshift, but stars rotating away from us in the galaxy will have a higher redshift on average, and stars rotating towards us have a lower redshift.

        At least that
      • They measure the rotation curves; which is the variation in velocity (measured via redshift, using spectroscopy) with radius of the galaxy. The wikipedia entry for rotation curve [wikipedia.org] is a good summary (you may also be interested in the entries for redshift and spectroscopy).
    • That’d be more like “serious suck,” wouldn’t it?

      Er... & isn’t MOND just like MONO [wikipedia.org] only it O-D’ed?

      I suspect that all of the dark matter is still there, it’s just much better adapted to the background than the Caucasian [reference.com] matter. Happens often with some Zambian flatmates & myself.
    • I thought MOND is pretty much dead after the Bullet Cluster X-ray/lensing observation.

      • Re: (Score:2)

        by Dice ( 109560 )
        I seem to recall some posts in the aftermath of that announcement that put forth models which explained the observed phenomena within the structures of some of the modified gravity theories. I don't know if this was MOND specifically or some of the other competing theories.

        I think that this particular observation is a stronger point against the modified gravity theories than the previous lensing observation, though, since it is precisely the absence of the original dark matter problem which is being observ
    • Yeah, I'm kinda bothered by the comments that seem to think this means the opposite -- that this observation weakens the evidence for our current theories.

    • Re: (Score:1, Flamebait)

      by Dr. Tom ( 23206 )
      MOND is KAKA. General Relativity is already the best theory. Using Newton's equations to describe anything other than a high-school physics experiment is a waste of time. If you use GR to model the rotation of a galaxy you get the observed rotation; dark matter is epicycles, created because Newton's equations don't work. The original calculations of galactic rotation used Newton's equations, and attempted to fix the problem by postulating dark matter. WRONG. WRONG. WRONG.

      • I'm going to respond to this because even though I completely disagree with you, I also disagree with your post being marked flamebait (although adding some evidence to support your point would've been useful). I'm going to presume you're referencing the claims of Cooperstock & Tieu [lanl.gov]. Unfortunately, their model ultimately requires an unphysical mass distribution (Vogt [arxiv.org] or Korzynski [arxiv.org]). It is a good point that for a long time people didn't do full GR simulations, but the end result is just that you need a

  • From TFA:

    "So for now, it seems that some of our missing mass is missing."

    If by this, they mean "holy shit, our physics no longer work," then they would probably be correct.
  • Killing two birds with one stone - dark matter decays to dark energy, with a half-life of (multiple millions of years here).
  • Im with the string theory people and what we are seeing is an "effect" of multidimensional space.

    • Re: (Score:2, Insightful)

      by nagora ( 177841 )
      Im with the string theory people and what we are seeing is an "effect" of multidimensional space.

      The only thing String Theory people see are grant cheques. ST must surely be the least successful scientific theory of all time in terms of the effort put in versus the results got out.

      TWW

  • OK, it's now more than twenty years since my astrophysics degree, but the fundamental principle of cosmology was that, at a large scale, the universe looks the same everywhere. Now we are being told it does not and, in fact, we live in a special corner?
    • You missed out a word. "Statistically", it's the same everywhere. If it was literally the same everywhere, the whole universe would be a thin gas with a uniform density of a few particles per cubic meter. There are going to be deviations from the norm, and this galaxy is one of them. It may have experienced some unusual event that stripped it of its dark matter after formation.

      What I find odd is that the galaxy is otherwise so unremarkable. If it is rotating differently from other galaxies, wouldn't we see

      • Re: (Score:2)

        by 00_NOP ( 559413 )
        Err, no. I was right. I didn't say it was "the same everywhere", I said "looks the same everywhere" - meaning it looks like clumps of matter in galaxies.

        What I hadn't done, because the link was broken, was RTFA and so therefore I thought this was about the Milky Way. Having read the New Scientist piece I now realise we are not being set up as privileged observers, but at the cosmological level this still wrankles because, as you say, the galaxy looks otherwise normal.
  • No Reapers in the Milky Way, then.

  • History of Dark Matter (Score:4, Interesting)

    by foxpaws ( 28518 ) <foxpaws@neptunes ... m ['che' in gap]> on Thursday February 07, 2008 @09:47AM (#22332608) Homepage
    I thought I could contribute a bit to the discussion by giving some background on why the theory of dark matter came about.

    Vera Rubin's work on galaxy rotation rates is still pretty compelling evidence for dark matter... OR, at least, it shows us that all galaxies do not behave they way we think they should, to be more accurate. People much smarter than my own self have decided that "dark matter" or some sort of mass/force/something that does not emit light or radio waves, etc. (which is why we never noticed it before) must be responsible.

    When we look at a solar system like ours, we see that the farther a planet is from the sun, the slower it travels. Not only does it have a much longer way to go, but it doesn't - and according to what we understand, shouldn't - travel as fast.

    Vera Rubins decided to check a whole galaxy. What she found did not hold with our understanding. The solar systems, stars and other observable matter near the outside were traveling faster than expected.

    Vera Rubin's work, combined with the discovery that the univers appears to be expanding at an accelerating rate, rather than slowing down, kind of kicked off the whole dark matter/dark energy thing.

    • Re: (Score:1, Troll)

      by Dr. Tom ( 23206 )
      Dark matter is the same as epicycles. It's total garbage. The original calculations of galactic rotation used _Newton's_ equations! That is why they came up with answers at odds with observation. They completely ignored General Relativity, the accepted law of gravitation. This has been pointed out many times. Dark matter doesn't exist.

      • Re: (Score:2, Informative)

        by Anonymous Coward

        Dark matter is the same as epicycles. It's total garbage.

        Dark matter is not the same as epicycles; it makes specific testable predictions concerning a number of independent phenomena. The key point is that it simultaneously accounts for all kinds of diverse observations including galactic rotation curves, galactic cluster behavior, large scale structure formation, anisotropies in the cosmic background radiation, etc. Lesser theories such as modified gravity can explain maybe ONE of those at one time; to explain any of the others you have to introduce extra ad-

        • Re: (Score:1)

          by foxpaws ( 28518 )
          One of the best arguments FOR dark matter is gravitational lensing, and without General Relativity, the lensing could not so convincingly show us gravitational fields caused by something that is "dark" to us.

          I also agree that the difference between Newtononian and General Realtivity calculations don't explain away the hundred or so galaxies we see rotating too fast by an factor of 10. (Ref: Michio Kaku)

          To agree with AC above, I concur that "they" (meaning physicists and cosmologists, I assume) aren't ignora
  • at the going rate, we probably have this all wrong, and its either ridiculously simple, or so insanely much more complicated that we're going to have a collective infarction when we finally get a grasp on how it really works.
  • If the universe was full of life what would it look like? Just roll the idea around for a while. Look at what current SF writers are thinking about, that we may soon be starting on our own primitive Dyson Spheres http://en.wikipedia.org/wiki/Dyson_sphere [wikipedia.org]. That we may eventually use all the energy from our sun and give of no visible radiation. It might seems impossible now but imagine we are going to work on the problem for a million years. If the universe was full of intelligence, I think it would look

    • Re: (Score:1)

      by Anonymous Coward
      This has been suggested before, and it's a fun consideration to ponder, but in reality, a Dyson sphere would radiate strongly in infrared. All the energy coming out of the sun has to go somewhere or you're just continually heating things up, so the Dyson sphere would have to be visible to the Spitzer space telescope, shining as brightly as a star but at longer wavelengths.

      Also, for Dyson spheres to explain the magnitude of the dark matter effect, there would have to be about 6 times as many of them as th
  • There have been a couple of observations that suggested that in galactic colisions or near colisions, that the dark matter can go on, and the visible matter can change course. Visible matter reacts with the gas streams, the dark matter doesn't seem to. I saw a report a couple of weeks ago of a dark matter galactic halo without a galaxy. (Found by the Einstein lensing effect.) We now have a galaxy without a dark matter halo. Looks like we have all the pieces of the puzzle, now someone just needs to put them
    • I think it doesn't want to react with any other matter and it is possibly repelled by other similar matter.

      Think of all the matter being sucked into black holes. It is approaching the speed of light as it approaches the event horizon. So it is like it is in the hugest particle accelerator possible, smashing into other particles and being ripped apart by space distortion. As matter gets ripped into the most fundamental particles at those speeds doesn't the uncertainty principal mean that it could suddenly
  • Let's not forget Occam's Razor. Complicated theories are usually a precursor to more elegant ones.

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