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Space Science

What If Dark Matter Really Doesn't Exist? 1063

Posted by Hemos
from the what-then dept.
sonar67 writes "According to The Economist: 'It was beautiful, complex and wrong. In 150AD, Ptolemy of Alexandria published his theory of epicycles--the idea that the moon, the sun and the planets moved in circles which were moving in circles which were moving in circles around the Earth. This theory explained the motion of celestial objects to an astonishing degree of precision. It was, however, what computer programmers call a kludge: a dirty, inelegant solution. Some 1,500 years later, Johannes Kepler, a German astronomer, replaced the whole complex edifice with three simple laws. Some people think modern astronomy is based on a kludge similar to Ptolemy's. At the moment, the received wisdom is that the obvious stuff in the universe--stars, planets, gas clouds and so on--is actually only 4% of its total content. About another quarter is so-called cold, dark matter, which is made of different particles from the familiar sort of matter, and can interact with the latter only via gravity. The remaining 70% is even stranger. It is known as dark energy, and acts to push the universe apart. However, the existence of cold, dark matter and dark energy has to be inferred from their effects on the visible, familiar stuff. If something else is actually causing those effects, the whole theoretical edifice would come crashing down.'"
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What If Dark Matter Really Doesn't Exist?

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  • by garcia (6573) * on Monday February 09, 2004 @05:07PM (#8229942) Homepage
    So what if it doesn't really exist? We know very little about anything anyway. Trying to find a unified explanation via "String Theory" is spotty at best but at least it "helps".

    What's the difference if dark-matter is really just another false theory? In the long run it's not going to make a whole heck of a lot of difference.
    • by Daniel_Staal (609844) <DStaal@usa.net> on Monday February 09, 2004 @05:17PM (#8230110)
      Sure, in the long run it doesn't matter.

      That is, of course, if we keep testing it and trying to see if it is true. (Or the closest approximation of 'true' we have been able to come up with.)

      It matters now if it is not true because then we know we need a better theory. And that means we either didn't understand something we thought we understood, or that we hadn't explored our understanding fully. Either way, there is likely something else that will be affected...
      • by j-turkey (187775) on Monday February 09, 2004 @06:25PM (#8231186) Homepage
        That is, of course, if we keep testing it and trying to see if it is true. (Or the closest approximation of 'true' we have been able to come up with.)

        You're absolutely correct. If we accepted theory as fact without any repeatable testing it would be religion, not science.

        We may never fully understand the nature of our universe, and almost certainly will never understand it in our lifetimes. But the question raised in the topic is actually a fundamental one that spans far beyond dark matter to all forms of theoritical science. Many theories are based heavily upon other theories. The "root" theories (with any luck) will eventually be proven or disproven, affecting all research and theories which follow that "root".

        What is important is for scientists to fully understand the theories that they base their work upon, and knowing the risks involved. Not doing so is irresponsible, and can lead to misinformation and confusion.

        With the above in mind, it's also important to note that many theories have been disproven throughout and entire scientific disciplines have crumbled around the fall of these theories. However, from those ashes, new disciplines have arisen (the first that comes to mind is chemistry rising from the "ashes" of alchemy). I'm sure that in 100 years, many if our current ideas will be laughable, but this failure has proven fundamental to our growth (how's that for rhetoric!?)

    • by LnxAddct (679316) <sgk25@drexel.edu> on Monday February 09, 2004 @05:19PM (#8230131)
      Well actually knowledge of its existence and how much of it exists will determine whether or not the Universe eventually implodes on itself in the "Big Crunch" or whether the universe will keep expanding at the speed of light forever. So technically speaking, "in the long run" it will matter quite a bit :)
      Regards,
      Steve
    • by pla (258480) on Monday February 09, 2004 @05:26PM (#8230263) Journal
      So what if it doesn't really exist?

      You can write a "hello world" program in most programming languages in under ten lines of code.

      You could also write a program to synthesize speech to say "hello world" in an MP3, rip the MP3 to a wav file, and then write a speech-to-text engine to finally dump "hello world" to the screen.

      Same idea here. Kepler's laws reduced a nightmarish tangle of mathematics to a three line "program", if you will. Out current model of how various things in our universe interact requires a degree in cosmology to fully grasp, and a PhD to do any meaningful work in. Imagine reducing that to one chapter of a freshman-level physics or astronomy course.


      So, it matters for that reason. Unneccessary complexity slows down work in the field, and in the long run can actually prove counterproductive to the field as a whole (think about it - 1500 years wasted trying to make epicycles work).
      • by Anonymous Coward on Monday February 09, 2004 @05:44PM (#8230537)
        >> think about it - 1500 years wasted trying to make epicycles work.

        Dang, their billable hours must have krunked the project.

        Maybe if we wait another year the program will halt...?

        Too bad they were so bent on epicycles, TRON has a much cooler cycle game and it works!

        (-1, troll)
      • by tgibbs (83782) on Monday February 09, 2004 @05:46PM (#8230560)
        Same idea here. Kepler's laws reduced a nightmarish tangle of mathematics to a three line "program", if you will. Out current model of how various things in our universe interact requires a degree in cosmology to fully grasp, and a PhD to do any meaningful work in. Imagine reducing that to one chapter of a freshman-level physics or astronomy course.

        Einstein's Special and General Relativity, Maxwell's Equations, and Schrodinger's Equation are all expressed in a few lines of equations. But you need extensive math and physics training to relate them to the familiar world around us. Simple doesn't mean easy. Theoretical physicists are already busily looking for theoretical formulations in which dark matter and dark energy arise naturally, rather than as a kluge. Of course, if the original observations turn out to have been misinterpreted, they may be wasting their time.
    • by swschrad (312009) on Monday February 09, 2004 @05:32PM (#8230364) Homepage Journal
      well, let's see here. 4% of postulated matter in the universe is known to exist. 96% of postulated matter in the universe is NOT known to exist. that's a fine fudge factor to have in a test, and might explain where budget figures come from in the government :-D

      it certainly explains where a lot of my assignments come from at work, lol :-D
    • by Graff (532189) on Monday February 09, 2004 @05:45PM (#8230553)
      What's the difference if dark-matter is really just another false theory? In the long run it's not going to make a whole heck of a lot of difference.

      Actually it will make a huge difference. Just look at how Bohr's model of the atom changed chemistry and particle physics. Or how Plank's quantum theory caused a revolution in the physics community. And one of the most famous examples of an upset in scientific theory is Einstein's theory of Relativity verses the Newtonian theories most commonly held at the time.

      Each of these theories caused an almost immediate revolution in their respective fields which spread out to similar disciplines. Fast forward 20, 30, 50 years or more and a number of innovations and inventions appear which stem from these theories. If these theories had not been introduced then we would most likely not have had such an explosion in technology.

      Just because we wave our hands and say something is out there doesn't mean that we understand it or can use it. If we know the true mechanism behind dark matter and wether or not it is just "hand waving" then we can apply that knowledge to useful applications. For example, it is assumed that this dark energy exhibits a repulsive force similar to gravity but opposite to it in direction. If we truly understand how this works then we might be able to apply that knowledge toward "anti-gravity" spacecraft, etc. On the other hand if there is some other cause for the repulsion then we would need to know IT'S mechanism in order to utilize it.

      In the end, science is the quest for truth, not convenience. Just knowing that there is a certain effect is not enough. Scientists are not looking to solve the question of "what is that" but rather "why does that exist and how does it work". That is why it is important to seek out the true reasons behind the dark matter observations.
      • by EvilTwinSkippy (112490) <yoda&etoyoc,com> on Monday February 09, 2004 @05:59PM (#8230811) Homepage Journal
        And of course there are those New-Age whackjobs who think that in truth we are simply making the rules of the universe up as we go along.

        I used to think it was crazy. But then I imagioned what life would be like for a process on a Linux box. In some respects, the system never changes. In other respects, as chunks of the system are refined an upgraded, previously famliar systems take on more complex, and at times, incomprehesible behavior.

        A process would be oblivious to the universe stopping and restarting with a new kernel. (Assuming the system had a suspend-to-disk function.) You would only be able to understand the universe indirectly through it's behavior, not through reading it's source. And assuming you could read parts of the source, it is always being updated and revised.

        It the process under Linux is too strange, how about a citizen under a government. Laws are just another form of code, and they too are every changing. Some parts are like the Constitution, broad in scope and largely set in stone. Others are like legal precidents, situation specific and sometimes arbitrary.

        Ok, time for more coffee.

  • If something else is actually causing those effects, the whole theoretical edifice would come crashing down.

    As it should.

    -Colin [colingregorypalmer.net]
  • by Babbster (107076) <aaronbabb.gmail@com> on Monday February 09, 2004 @05:08PM (#8229965) Homepage
    Much like a dog staring at a shiny object, I'm fascinated by this but I don't understand it.
    • by Anonymous Coward on Monday February 09, 2004 @05:13PM (#8230045)
      I think I can help - here's a translation of the article: "Physicists are not quite sure what's going on."
    • by OECD (639690) on Monday February 09, 2004 @05:44PM (#8230547) Journal

      Even if you don't understand it, you can always find nuggets like this:

      The Newton observations are at the limits of accuracy, so a mistake could have crept in.

      The next time I've got to report on something, you can bet that my estimations will be at "the limits of accuracy."

    • by Anonymous Coward on Monday February 09, 2004 @05:51PM (#8230646)
      Ok, Dark Matter in a nutshell.

      When scientists look at the way that galaxies move through space, they see that many of them move a great deal faster (about a factor of 10) than theory predicts. Assuming that current theory is correct, the most likely explanation of these observations is that there is a great deal more matter in the universe than we can currently detect. If we can't detect it then it must be pretty much invisible across the EM spectrum, so scientists have christened it dark matter. Much effort has gone into trying to prove its existance but as as far as i'm aware there has not been too much sucess.

      As I remember from my astrophysics class (and this was some years ago so feel free to correct me) there are two main candidates for dark matter, both of which have been tediously acronymed.

      MAssive Comapact Halo Objects (or MACHOs) are basically chunks of ordinary matter, floating around in space that give off no radiation. Think brown dwarfs (stars without the necessary mass to initiate fusion). As I remember, most scientists are very sceptical that a significant amount of dark matter could be contained in MACHos.

      Weakly Interacting Massive Particles (WIMPs- gotta love that scientist humour) are the other candidates and are hypothetical particles, heavier than neutrons, that were formed in the Big Bang and have been travelling through space ever since. As their name inplies they would have almost no interactions with normal matter and so by definition would be almost impossible to detect. Again there have been attempts to prove the existance of these particles, mainly involving mine shafts and a lot of water, and again there have been no conclusive results.

      Now the significance of all this is that as you may or may not know, the universe is presently expanding and will continue to expand for some time. What will happen after that, however, is a matter of some confusion. One theory says that it will continue expanding forever (open universe) , while another says that the gravitational force of the matter in the universe will cause the expansion to stop and then a period of contraction to start, ending up with all the matter coming together in a 'big crunch'. This second theory creates what is known as a closed universe and people have postulated that the 'big crunch' is analagous to the 'big bang' that started the universe in the first place. In this way we get an infinite cycle of universes, each starting with a bang and ending with a crunch

      • by TMB (70166) on Monday February 09, 2004 @06:37PM (#8231356)
        A few minor quibbles...

        (I am an astrophysicist. I am not a cosmologist, but I do galaxy evolution... we hang out with cosmologists)

        There are quite a few pieces of evidence for dark matter:
        - internal dynamics of galaxies: when you look at how fast the outer parts of galaxies move around the central parts, you find that the amount of mass necessary is much more than what you see
        - dynamics of galaxies in clusters: when you look at how fast galaxies move around in galaxy clusters, you find the amount of mass necessary is much more than what you see
        - non-linear growth of primordial perturbations (sounds complicated, isn't really): the universe used to be almost completely smooth. now it's filled with clumps of matter like galaxies and clusters and big voids without much matter. the structures collapsed because of their mass. if there were only as much mass as you can see, there hasn't been enough time for galaxies to have collapsed

        The amazing thing about all of these measurements is that they all give you the same answer for how much mass is really out there.

        [TMB]
        • by PhilK (20847) on Monday February 09, 2004 @07:17PM (#8231857) Homepage
          It isn't really amazing that they all give the same answer, because they all make the same assumption:

          f = G.m1.m2/d^2

          What if this is only a *very* good approximation for all normal purposes, and even for things as large as the solar system (in the same way that Newtonian mechanics is good enough for all earthly based stuff).

          What if gravity doesn't quite work this way at galactic scales?

          There was a piece in New Scientist last year making this exact point, and the researcher was able to explain most effects that are otherwise explained by dark matter, by slightly changing the theory of gravity.

          Einstien did it for Newtonian Mechanics.

          The real problem I see here is that the scientific method has been largely ignored. We observe the universe, we devise theorems to explain it, we test the theorems against other observations. If the test doesn't match reality, we assume that the theorem is wrong.

          This doesn't occur with cosmology.

          We observe the universe, we make theories, and when they don't fit, we assume there must be something wrong with the universe!
  • by rafael_es_son (669255) <rafael@human-[ ] ... o ['ass' in gap]> on Monday February 09, 2004 @05:08PM (#8229971) Homepage

    Jedi don't stand a chance.

  • by visgoth (613861) on Monday February 09, 2004 @05:09PM (#8229982)
    It will be interesting to see how scientists who have staked their entire careers upon the existence of dark matter would react to the discovery that it does not in fact exist. Ideally an invalid theory is dropped, and a new, more "correct" theory is created. However, I have a feeling that a lot of people have invested too much time and effort into dark matter to let it go without some serious evidence.
    • by garcia (6573) * on Monday February 09, 2004 @05:17PM (#8230105) Homepage
      they will continue to work on it for years and years until their death just like Einstein did? A previous mover/shaker forever lost in the past by refusing to move along?
    • by vondo (303621) * on Monday February 09, 2004 @05:22PM (#8230177)
      Five years ago, every cosmologist "knew" that the universe was flat and matter supplied the critical density (in other words, no dark energy, that 70%). Conventional wisdom has completely changed with the discovery of the accelerating universe.

      If the data is there and convincing, the views will change. But any alternative theory is going to have to explain all the observables, not just the two mentioned in the artice.

      (E.g., the convincing data on dark energy comes from two independent groups studying supernovae.)

      • by Michael Woodhams (112247) on Monday February 09, 2004 @08:09PM (#8232450) Journal
        Five years ago, every cosmologist "knew" that the universe was flat and matter supplied the critical density (in other words, no dark energy, that 70%). Conventional wisdom has completely changed with the discovery of the accelerating universe.

        No they didn't. I hung out with cosmologists when doing my astrophysics PhD over 10 years ago, and they were considering various mixes of hot and cold dark matter, dark energy, open and closed universes. Flat universe and no dark energy was merely the provisionally accepted most likely solution.

        The implied existence of dark energy is revolutionary to cosmology, but it didn't catch people by surprise - they were actively looking for it.
  • Relativity (Score:5, Interesting)

    by mozumder (178398) on Monday February 09, 2004 @05:09PM (#8229984)
    Actually, with Einstein's relativity, doesn't Ptolemy's theories hold true? Everything is relative to a point of view?

    Sorry I didn't ask this question in Modern Physics's class. It was a morning class, and I was sleeping.
    • Re:Relativity (Score:4, Insightful)

      by RLW (662014) on Monday February 09, 2004 @05:23PM (#8230197)
      A theory of how things work is only as good as that theory's predictions. Ptolemy's model must have been very useful for predicting the position of celestial objects or it would have been put aside even 'longer' ago. It's only when a model is in direct conflict with observed data that it is in trouble: even if there is no formulated model that works with the new observations.

      'Dark' energy and matter will only be in serious trouble when that model no longer explains observed data.
    • No. (Score:5, Informative)

      by Rufus88 (748752) on Monday February 09, 2004 @05:34PM (#8230385)
      At the risk of feeding the trolls...

      No, Relativity (neither the Special nor General theory) says that "everything is relative". Special Relativity says that inertial motion is relative in flat spacetime (i.e. in the absence of gravity). This is another way of saying that all inertial coordinate reference frames are equivalent. (Special Relativity says more than that, namely that light propagates at the constant speed 'c' independent of the motion of its source. This is what separates Special Relativity from Galilean Relativity.) General Relativity says that *locally*, accelerated motion is equivalent to inertial motion in a gravitational field. (The "locally" part accounts for the fact that the gravitational field lines are not parallel, but converge on the gravitational source.)

      What this boils down to is that circular motion is accelerated motion, not inertial motion, and is not simply relative, and spacetime is not flat surrounding bodies that planets orbit. So no, Relativity does not validate the epicycles theory.
    • Re:Relativity (Score:4, Informative)

      by CaptainCarrot (84625) on Monday February 09, 2004 @05:37PM (#8230432)
      No. The fact that any inertial frame of reference can be regarded as equally valid does not begin with Einstein, it's fundamental to the way all physics is done. Einstein's insight was that regardless of your frame of reference, light appears to always be travelling at the same speed relative to you.

      The key word here is inertial. An orbiting body is accelerating towards the center, and is therefore not an inertial frame of reference by definition. As far as calculations on the surface of the Earth go, non-inertial effects (also present because of Earth's rotation) can generally be ignored for comparatively small times and distances without significant loss of accuracy, but on the scale of the Solar System and other systems where celestial mechanics are employed, they cannot be ignored. Epicycles introduce a new non-inertial component, and therefore can't be regarded as merely relative.

  • by AKAImBatman (238306) <<akaimbatman> <at> <gmail.com>> on Monday February 09, 2004 @05:10PM (#8229991) Homepage Journal
    ...but doesn't String Theory tend to suggest that "dark matter" isn't actually dark matter, but instead is gravitation bleeding from other universes? The same theory also explains why gravity in this universe is so weak. Because most of it bleeds of into other universes via the higher dimensions, it's weak enough for you and I to move our limbs.

    • by Zoolander (590897) on Monday February 09, 2004 @05:23PM (#8230196)
      That's what I love about physics: it's so out there that you'd think the person who just said something like that was smoking crack, if he didn't have a PhD.
      Gravity bleeding between universes...
      Who needs science fiction?
    • by bob the Martian (113876) on Monday February 09, 2004 @05:27PM (#8230273)

      There are current astrophysical models which postulate that the Universe is a hyperplane embedded in higher dimensional space, called Randall-Sundrum models. In which case gravity can propagate trasverse to this plane, hence there can be matter outside the Universe which can still interact with it. There is also the idea that this 'brane' (as it is called - nothing to do with zombies) has a small extra dimension (less than 1mm in size) so the current gravitational law needs to be changed to r^{-4} or so at very short distances.


      String theory as such tends not to comment on dark matter (could be D0-particles, could be fish) as no-one knows how to compactify it down from 10D and break supersymmetry in a useful way.

  • No friggin way? (Score:5, Insightful)

    by Bryan Gividen (739949) on Monday February 09, 2004 @05:11PM (#8230003)
    We don't understand something fully? Wow... that's about as brilliant as deciding to cut my sandwich in triangles instead of in squares.

    The truth is this. We have such a little understanding of actually governing laws that we can't begin to fathom it. However, that doesn't stop us in progression to learning. Just because this theory might not be right (and probably isn't) doesn't mean we are any less an idiotic species. We've been working on these theories for many millenia. One of them turning out to be wrong won't be a surprise... it's a probability. Without the wrong hypthosesis, we can never stumble onto the correct ones. Its Edison's, "Every time I fail, I know one more way how to NOT build it" idea.
  • by suso (153703) on Monday February 09, 2004 @05:11PM (#8230017) Homepage Journal
    ...then 99.9999999% of the world won't notice. But it will be on CNN anyways.
  • Well... (Score:5, Funny)

    by Kethinov (636034) on Monday February 09, 2004 @05:12PM (#8230027) Homepage Journal
    What If Dark Matter Really Doesn't Exist?
    Then Star Trek has a lot of episodes to rewrite...
  • by Tablizer (95088) on Monday February 09, 2004 @05:14PM (#8230068) Homepage Journal
    There seem to be growing "hints" that something is wrong with current theories about the very nature and behavior of gravity. This includes alleged dark matter that cannot be identitied, planetary space probes with slight deviations from expected sun "pull" [1], and the fact that there is no identifiable "negative" gravity while the other forces do have negative values or particles.

    [1] It was originally thought that heat generated from nuclear fuel cells was "pushing" the probes, but this was mostly ruled out because the heat lessens over time, but the pull was constant.
    • by vondo (303621) * on Monday February 09, 2004 @05:29PM (#8230292)
      There are several plausible candidates for dark matter. There are lots of suggestions from particle physics that every particle we know now has a partner. This theory is called "super symmetry" and the lightest of these particles may be stable (and many times heavier than a proton).

      This question we may actually know the answer to in a decade or so when the LHC comes online and is producing results.

      Dark energy is much weirder.

  • I hope its a kludge (Score:5, Interesting)

    by mnmn (145599) on Monday February 09, 2004 @05:17PM (#8230108) Homepage
    Science has been progressing on the basis of constantly proving theories as kludges and bringing about something newer and more real. Imagine if our currently held view was true (before Standard Model), we will never be able to travel faster than light, we'll never harness energy bigger than a hydrogen bomb, we'll never really travel far beyond the Solar system, travel back in time etc.

    Before the cannon was invented everyone thought the arrow was the greatest weapon, and few could really predict the power of "Little Boy" on Hiroshima. Quantum Mechanics has given us so much hope, of unknown and unexplainable realities, and that far more is possible than we first thought. It means the road before us is much longer, but far more interesting. I'd prefer it that way.
  • by npistentis (694431) on Monday February 09, 2004 @05:17PM (#8230118)
    Dark matter had better exist- otherwise, I've wasted a hell of a lot of money on that dark matter damage insurance I bought a couple years back...
  • by anandamide (86527) on Monday February 09, 2004 @05:17PM (#8230119)
    Then I can walk down the hall in the middle of the night without fear of stepping on my little boy's building blocks.
  • dark matter evidence (Score:4, Interesting)

    by dpa (579262) on Monday February 09, 2004 @05:22PM (#8230181) Homepage
    There is some intriguing evidence [smu.edu] of the existence of strange quark matter, a dark matter candidate, which we've recently published in the Bulletine of the Seismological Society of America [seismosoc.org]. as previously discussed on /.
  • In 100 years... (Score:5, Insightful)

    by iiioxx (610652) <iiioxx@gmail.com> on Monday February 09, 2004 @05:22PM (#8230182)
    Dark Matter will be taught to school children as the Aether [wikipedia.org] of 21st century science.
  • by peter303 (12292) on Monday February 09, 2004 @05:25PM (#8230235)
    The keynote speaker [siggraph.org] at the 2003 SIGGRAPH conference in San Diego was the British astrophysicist Anthony Lasenby. He claimed that a new kind of unified Euclidean and hyperbolic geometry could explain acceleration and deceleration in the Big Bang. He was talking at SIGGRAPH because his new unification of geometry is supposed to be more elegant for computer graphics modeling than the current homogeneous coordinates now used. He wrote a book [amazon.com] about the geometry. But I have been unable to find a paper relating to the cosmological application on the web.

    This is not the first time geometry has been used to unify and simplify physics. Previous examples are Galilean coordinates, special relativity, and general relativity.
  • M.O.N.D. (Score:5, Interesting)

    by bokmann (323771) on Monday February 09, 2004 @05:28PM (#8230289) Homepage

    Bringing this up without mentioning M.O.N.D. is irresponsible journalism. MOND (Modification of Newtonian Dymanics) is a theory that simply says that gravity 'decays' at a slightly different rate than expected over astronomical distances. The effects predicted by this theory are spot on to the observed effects that dark energy and matter try to explain.

    I googled about found this link [umd.edu], but I first read about it in New Scientist about a year ago.

  • by peter303 (12292) on Monday February 09, 2004 @05:37PM (#8230425)
    I may be showing a few gray hairs here, but revolutions in the sciences have occurred in my lifetime with scientists adapting fairly well. The first was the acceptance of Big bang in the late 1950s. Between 1927 and 1955 the Big Bang was just one of several "equally attractive alternative theories" which included the eternal-infinite universe and continuous creation of matter. The microwave background and the abundance of helium brought the big bang into the fore front.

    In the 1960s the quark unification of subatomic particle became the predominate theory. Plus quantum electrodyanamics was verfied in high energy experiments to extremely high precision.

    Also in the 1960s plate tectonics replaced an up-and-down explanation of geologic forces.

    If the evidence suggests a more powerful theory, then physicists will revise their theories again. Science does not stay attached to incorrect theories (though block-headed individuals do).
  • Pardon my naivete (Score:5, Interesting)

    by El (94934) on Monday February 09, 2004 @05:38PM (#8230451)
    I may be incredibly naive, but it has always bothered me that we insist on believing there are only 4 types of force in the Universe, each operating on widely different scales. Why can't there be other forces that operate on too large a scale or too small a scale for us to observe? Is the postulate of "dark force" effectively a theory about a fifth type of force?
    • by Dirtside (91468) on Monday February 09, 2004 @06:06PM (#8230916) Journal
      Well, a force that we could never observe, we could never test the existence of. Sure, you could postulate it, but it wouldn't help the theory at all -- you wouldn't be able to tell if your theory was right or not. You might as well say that tiny invisible demons are causing strange things to happen...
  • by renard (94190) on Monday February 09, 2004 @05:44PM (#8230527)
    I love the Economist as much as the next person, but in this case the search for "controversy" badly mischaracterizes the current state of our understanding of the universe.

    First claim: Analyses of the WMAP data on the cosmic microwave background (CMB) show correlations with galaxy clusters that indicate the official analyses of the data are wrong. I find this highly unlikely. First, the effect of the hot gas in those galaxy clusters on the CMB is well known - it is called the Sunyaev-Zeldovich effect and perturbs the spectrum in a well-known way. Second, the official fits to the WMAP data use a consensus cosmology model with about 12 free parameters to fit a dataset of more than a hundred points... beautifully. Third, the consensus cosmology itself has been built up out of a huge array of other observations (supernova distances; Big Bang nucleosynthesis; the "weighing" of galaxies and galaxy clusters; the age-dating of globular clusters), all of which were pointing to the existence of dark matter (even within our own Galaxy!) long before WMAP was even launched. Fourth, modern theories of particle physics also give us good reason to expect the existence of dark matter particles, independent of any astronomical observations whatsoever. So WMAP has simply been the final nail in the coffin, and anyone who wants to overturn dark matter and dark energy has a great deal of additional work ahead of them.

    Second claim: Measurements of the masses (actually, the luminosities and temperatures) of high-redshift galaxy clusters indicate a high fraction of baryonic mass, removing one of the justifications for positing dark matter. This finding is even more fishy-sounding. To understand this, realize that the group in question has deliberately chosen the most-distant and therefore hardest-to-study clusters to study, and adopted temperature-mass relationships that are calibrated in the local universe (and may not apply at these great distances) in order to find that their sample differs from the standard model predictions. Without even bothering to list all the ways in which they might be wrong, let me simply state that even if they are right there is a lot of independent support for the dark matter + dark energy picture that neither of these groups is addressing.

    Rather than distract yourself by trying to figure out why the carefully constructed consensus cosmology might be wrong, then, I think it is more useful to examine the remarkable ways in which it has been proven right in the last few years. Altogether it is truly a wonder of the modern world - even if it may at some point be shown inadequate to the universe we live in.

    -renard

  • by sdedeo (683762) on Monday February 09, 2004 @05:55PM (#8230729) Homepage Journal
    It is doubtful that the entire theoretical edifice of dark matter and dark energy will collapse all at once (in the way it might more reasonably have been said to have happened for the electromagnetic aether.)

    In particular, dark matter, though incredibly mysterious, is probably on firm enough ground that it will withstand a series of challenges. Galactic rotation curves and measurements of cluster temperatures both give very strong evidence for dark matter on vastly different scales; in addition, it is difficult (OK, fine: downright impossible in standard Einsteinian gravity) to get any kind of structure to form *at all* in the universe if one is only allowed to use the visible matter. The precise ratio of dark to visible is definitely up in the air; and, of course, there are competing models that modify gravity -- if these matured enough (they may already have -- I haven't kept up) to make predictions on a wider range of scales, they might work as well.

    Indeed, a lot of gravity modifications (extra dimensions, etc.) behave *phenomenologically* as if there was dark matter -- so all the effort we've put into simulating dark matter may not be in vain after all, even if Einsteinian four dimensional spacetime is not the name of the game.

    In contrast, indeed, is the exact count of the "baryons" (ordinary matter.) I would be very surprised if we were off by a factor of (lets be ultra-conservative here) five in the baryon number, which is constrained very well by big bang nucleosynthesis, whose predictions remain in the "ordinary" realm of nuclear explosions. Something we know a little about.

    The real mystery is "dark energy." There, the evidence is a lot shakier. It rests on a few pillars. There is a theoretical bias that wants the universe to be flat (so that the missing mass-energy is made up for by some dark energy component that doesn't cluster and affect our galactic rotation curves.) There are some really excellent (but difficult) measurements of universe acceleration, a signature of dark energy, from people who observe distant supernovae (these provide "standard candles" that allow you to measure distance given an apparent brightness.)

    Finally, there are the CMB measurements, which provide a similar kind of distance measurement, but are open to alternative interpretations (instead of measuring apparent brightness, they measure apparent angular size -- but it is perhaps possible, if you squeezed around, to construct a different model where the apparent angular size is squished in odd ways.)

    And then there are a host of other measurements that one might call more "marginal" (without prejudice to the people who work very hard to do them -- I aspire to be one of them.) They rely on a few more astrophysical assumptions, and perhaps would not convince the slashdot skeptic. (My profound apologies if I've missed out someone's awesome measurement.)

    One big "trouble" is that we haven't seen good evidence for a very particular signal that one would associate with the simplest model of dark energy. (This is the "low quadrupole" -- the news stories you read about finite universes are from people who, in part, are motivated by the desire to explain this low quadrupole signal by other means.) Of course, it is entirely possible to make more exotic dark energy models that don't show this signal (I've coauthored a paper on one such model), but that missing signal, gosh, damn.

    The Economist is usually good with science articles, but it really kind of missed the point on this one. Shanks et al. are not "bringing down the whole edifice"; they are pointing out what they see as a possibly problematic signal in the CMB data. This may inspire in some a little additional -- and very healthy -- skepticism about the dominant models. But it is important to mention that there really is no "dark energy mafia"; nearly any astrophysicist worth his or her salt would drop dark energy like a stone if the evidence started piling up, and many, many astrophysicists keep a hand in alternate models that don't rely on dark energy because, hey, what a scoop that would be.

  • by Rupert (28001) on Monday February 09, 2004 @06:00PM (#8230821) Homepage Journal
    At least they have observations. And astronomers in general are a genial bunch. Anyone who finds (and this is the most likely case) that there is dark matter, but not nearly enough of it, is assured of nothing more that a few years of ostracism before enough new scientists come into the field who don't have the same emotional investment in dark matter theories.

    Compare that to the potential fate of the poor wretch who disproves the Riemann Hypothesis, and undoes almost all progress in pure mathematics since the beginning of the 20th century. I know for a fact that there is a basement in Cambridge [cam.ac.uk] where this person will live out their days being forced to review unsolicited "proofs" of duplicating the cube, trisecting the angle, and squaring the circle.
  • by xihr (556141) on Monday February 09, 2004 @06:08PM (#8230937) Homepage

    Dark matter is simply a theory. If Newtonian mechanics is correct (we don't even need to worry about relativistic corrections here), and the laws of physics are the same everywhere (a fundamental principle of science), then there is a lot more matter than we can see (i.e., that is glowing). We can tell this by looking at the rotation curves of galaxies, and even the behavior of clusters of galaxies. There must be a lot of matter there that we can't see, if Newtonian mechanics is a reasonable approximation. It's called dark matter.

    Dark matter in and of itself is really not a revolutionary concept. In most wavelengths of light, for instance, you qualify as dark matter (you emit no visible light, although you do emit infrared radiation, so you're not completely dark matter). Look around your room or office. How many things emit electromagnetic radiation. Your computer and your monitor, sure. Your light fixtures and other electronic equipment either emit light or heat. But most of the stuff around you emits internal radiation. A pen is dark matter. A cup of dark matter (once its reached thermal equilibrium, of course). That book is dark matter. The concept of dark matter is not only not revolutionary and mind-blowing, it's downright mundane. Given the survey of stuff in your office/room, is it any surprise that most of the junk in the Universe doesn't emit radiation on its own?

    When we start getting into the weird realms of dark matter is when we start applying the Standard Model and find out that it doesn't seem like all that dark matter can be explained by baryonic matter (basically, protons and neutrons -- what we would normally consider matter). That's where things start getting sketchy and speculative, although we have some theories about what might be responsible. But dark matter in and of itself is simply a consequence of the mediocrity principle (that is, the laws of physics operate elsewhere just the same as they do here) and Newtonian gravitation.

    All the popular media's fascination with dark matter is only so much hoopla.

  • by fetta (141344) on Monday February 09, 2004 @06:51PM (#8231559)
    You take the facts, come up with a working theory that fits the facts. As more facts come in, you continue to test your theory against the facts. When too many anomolies show up, it's time to come up with a new theory.

    And there is nothing wrong with that. Is Newtonian physics worthless just because it couldn't explain everything? No, but we had to be willing to look for new answers when we began to see evidence that the old answers didn't work for everything we observe. It's called a paradigm shift.

    Check out Thomas Kuhn's Structure of Scientific Revolutions [amazon.com]
  • by Cyno (85911) on Monday February 09, 2004 @07:59PM (#8232346) Journal
    Let's say our limited perspective from our solar system were somehow enhanced by a telescope/sensor at a neighboring solar system. Wouldn't this give us a much more accurate map of the universe than our current narrow view? For all we know most of the matter might not be visible because something is standing between us and it. The very fabric of space gets distorted by the weak gravitational field of our small star and each and every bit of matter floating around out there and we believe that larger gravitational forces exist, like black holes. If we can't even completely understand how and why our star warps the fabric of space how can we expect to KNOW the universe and all the matter contained within it?

    We don't understand the laws of this universe. We've barely been able to explain it with simple mathematics. The universe, for all we know, might require higher mathematics than the human brain can easily comprehend. And what if there are other universes? But what do I know, I'm just one small voice in this titanic harmony-challenged choir. I'm sure one day someone with a lot of money will figure it out and tell all of us about it in an infomercial late at night on TV.
  • by Compass Man (701268) on Tuesday February 10, 2004 @12:05AM (#8234173)
    This thread misses an important point. Even though Ptolemy's theory was wrong, it was a lot closer to the truth than previous ideas like "the lights in the sky are gods with flashlights." The point is that even theories that are wrong add to our knowledge by providing a starting place for deeper inquiries.

Any sufficiently advanced technology is indistinguishable from a rigged demo. - Andy Finkel, computer guy

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