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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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  • Re:No dark matter ? (Score:5, Informative)

    by Pi_0's don't shower (741216) <ethan AT isp DOT northwestern DOT edu> on Monday February 09, 2004 @05:21PM (#8230154) Homepage Journal
    It's more than that. If Dark Matter doesn't exist, we will be forced to re-examine more than just our current picture of the universe. Galactic Rotation curves, velocity dispersions of galaxy clusters, the flatness of the universe implied by the CMB, type Ia supernovae data, as well as other distance indicators, all imply that the parameter "Omega_mass" (the mass density of the universe divided by the critical density) is about 0.3. If there is no "dark matter", we don't know how to explain this number. Baryons, i.e. stars, planets, gas, etc., make up only an "Omega" of 0.044 +/- 0.009. This constraint is from Big Bang Nucleosynthesis and is very strong. Although there are plenty of open questions about dark matter, it seems to me (just an astrophysics grad student) that there is an overwhelming amount of evidence for not only dark matter, but the model of "cold" dark matter as well. None of the alternatives can explain even half of what Dark Matter can, including modifying gravity. Plus, Dark Matter is consistent with GR, the big bang, and everything else we hold dear about physics and astronomy, whereas other theories don't. Just my two cents... Ethan
  • 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.)

  • Re:Brief History... (Score:5, Informative)

    by nathanh (1214) on Monday February 09, 2004 @05:23PM (#8230199) Homepage
    Most Physicists believe that God created the laws of physics ...

    You keep telling yourself that... God boy. However only 7% of scientists believe in a personal god [atheists.org].

  • by haxor.dk (463614) on Monday February 09, 2004 @05:24PM (#8230206) Homepage
    ...but theories generally aren't wrong. I think you mean a hypothesis, right ?
  • by basil montreal (714771) on Monday February 09, 2004 @05:25PM (#8230230) Homepage
    Scientists come and go, and they're the reason that so many of the above posts are making fun of this whole theory. Real evidence disputing the dark matter theory can only exist because we don't have any real evidence supporting it. All we can do is observe facts and choose what theory we use to interpret the data. We see the universe expand, we assume that something must be doing it. Hopefully we'll discover some more evidence that will refine our string theory or Higgs's boson field theory. If we don't, we will discover something else and have to figure out a new theory.
  • actually... (Score:3, Informative)

    by mike77 (519751) <mraley77.yahoo@com> on Monday February 09, 2004 @05:25PM (#8230242)
    Most Physicists believe that God created the laws of physics to be elegant...

    Actually, most Physicists don't believe in God.

  • 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.

  • by Profane Motherfucker (564659) on Monday February 09, 2004 @05:31PM (#8230330) Journal
    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.

    Well, optimism doesn't drive research, unfortunately. It's really not about preference, but science. The two are unrelated really. I don't like the fact that I can't eat buckets of spicy cheese and drink high life bottles without getting like an obsese shit, but it's fact.

    I think you'll have to come to terms with reality.
  • 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.
  • by Anonymous Coward on Monday February 09, 2004 @05:34PM (#8230386)
    A theory is not a fact -- any theory can be wrong. For example, Newton's theories of motion were shown by Einstein to be inaccurate at high velocities.

    From a science standpoint, an interesting theory makes predictions which can be disproven. Some scientists then try to observe the prediction to support or disprove the theory. Other scientists accept the theory is right and use the predictions while on their way to doing other stuff.
  • by crstophr (529410) on Monday February 09, 2004 @05:35PM (#8230400) Homepage
    It's so easy too illustrate: Imagine holding two small magnets next to eachother. You can feel them pulling on each other and if you get them too close, they snap together.... so that's electromagnetism Now hold two unmagnetized chunks of the same weight/material. Feel the force of gravity pulling them together? What you can't feel it? It is there, it's just THAT weak... compared to electromagnetism.
  • 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.

  • Re:Relativity (Score:3, Informative)

    by Jerf (17166) on Monday February 09, 2004 @05:43PM (#8230517) Journal
    Actually, with Einstein's relativity, doesn't Ptolemy's theories hold true? Everything is relative to a point of view?

    No, because rotational motion isn't linear motion. A given linear motion will look like any other linear motion from the correct point of view. There is no (sub-light) vantage point from which the Earth does not have a path that describes an orbit around the sun. (That orbit, the Earth, and the Sun can appear squished and time dilated, but the path the Earth follows will always be around the Sun.)

    You can define a Point Of View where your position on the surface of the Earth is the constant zero point, but it's going to be very exicting defining the rest of physics usably. Anything further then a light-(day / (pi*2)) away is traveling faster then the speed of light, for instance (appears to travel more then one light-day per day). Again, this is because rotation isn't linear motion.

    I think you could theoretically kludge physics to work that way, but what you have is a God-awful physics where the speed of light varies based on which direction you're traveling and how from the zero point you are, and in the end, it makes the exact same predictions as what we have now. There are an infinite variety of theories that make equivalent predictions to any given theory, and are thus in some sense equivalent, but as humans, we prefer the simplest for a lot of reasons. You could build a consistent Ptolemaic theory, but you wouldn't want to.
  • Re:Theory. (Score:3, Informative)

    by Yunzil (181064) on Monday February 09, 2004 @05:47PM (#8230568) Homepage
    First off, evolution isn't about speciation. It's just about change. But speciation can eventually result from change.

    But if you want examples of speciation:
    http://www.talkorigins.org/faqs/faq-speciation.htm l [talkorigins.org]
    http://www.talkorigins.org/faqs/speciation.html [talkorigins.org]
  • That's not correct. (Score:3, Informative)

    by citanon (579906) on Monday February 09, 2004 @05:48PM (#8230603)
    Relativity holds between non-accelerating frames of reference. In such frames, one can not perform a local experiment to determine one's velocity absolute velocity. One can only define a reference frame and perform an experiment to determine one's velocity relative to that frame. Hence, the concept is called relativity.

    Orbital motion results from acceleration caused by gravity. One can measure acceleration locally at any point within the influence of the gravitational field. Thus, symmetry between difference frames of reference are broken, and a natural "center" is defined as one of the points where one experiences no acceleration. In reality, you never have a true "center" in a multibodied system such as our solar system. However, the Sun is so massive that we can say, to reasonable accuracy, that the center of our solar system is approximately at the position of the sun.
  • by Charlotte (16886) on Monday February 09, 2004 @05:50PM (#8230632)
    There is only a semantical difference between a theory and a hypothesis.

    Typically a hypothesis is when you think 'may be the universe is like this', and you try and find a way to prove or disprove that argument.

    A theory is what you get when you observe reality and try and find a rule that governs that reality.

    Both processes involve creativity, supposition, observation and either confirmation or refutation.
    As such, both theories and hypothesis can be proven wrong.

    I don't think it's a good idea to start judging scientific ideas on any other basis than comparison to reality. Limiting yourself to semantics is silly.

    Sorry to be nitpickin.
  • 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 Anonymous Coward on Monday February 09, 2004 @05:54PM (#8230706)
    As quoted, "If something else is actually causing those effects, the whole theoretical edifice would come crashing down."

    They almost make it sound like a bad thing (crashing down...) from some old beard who doesn't like all this new-fangled change. If old theories are proven false and new theories are created that more accurately describe our universe, then that's a good thing.
  • by LouisvilleDebugger (414168) on Monday February 09, 2004 @05:55PM (#8230717) Journal
    As the parent points out, not everything with regard to position and motion is described by Einstein's Special Theory of Relativity. Only constant non-accelerated motion. A frame of reference which obeys this constraint is termed an "Inertial Reference Frame."

    A common misconception, with an easy to learn answer.

    This answer can then be applied to say that Einstein does not support Ptolemy, as Ptolemy's theory describes motion that does change. In rotating even at a constant rate around a fixed point, the orthogonal ("at right angles") components of the motion (e.g. East-West versus North-South) each oscillate between maximum and minimum values. That's the acceleration. The total magnitude of this acceleration may be constant, but its direction isn't. The reason for it in the case of the planets wasn't even apparent in Kepler's time. It took Newton to find laws which approximately described this effect (his laws of motion and of universal gravitation) and this model was further refined by Einstein with General Relativity published the year after the Special Theory.

    The fact is that Kepler had no more sophisticated ideas of the mechanism underlying orbital motion than did Ptolemy. Kepler is better than Ptolemy on the grounds of superficial description of the motion alone.

    This is far from an indictment of Kepler, but speaks rather to Kepler's imagination, trust to observation (notably from the perspective of history, Tycho Brahe's data), and willingness to challenge the accepted theory, which authority at the time was backed up by armies and courts of Inquisition.

    The late Richard P. Feynman's little treatise "The Character of Physical Law" (MIT Press) is the best introduction to this history, and features Feynman's extrodinarily hilarious expository style, as well as his legendary insistance on accuracy with regard to interpretations of both the various physical theories, and their canonical histories.

    I wish to God that Feynman were still with us. Goodbye, Dick, we hardly knew you.
  • by kievit (303920) on Monday February 09, 2004 @05:55PM (#8230728) Journal
    Preprint archives [arxiv.org] to the rescue: Problems with the Current Cosmological Paradigm [arxiv.org], a talk recently given by Tom Shanks. Maybe a real cosmologist can tell us how much authority Tom Shanks has in the international community, and whether his view is taken seriously or that it is sceptically set aside as yet another attempt to kick the establishment.
  • Re:Brief History... (Score:5, Informative)

    by Hadean (32319) <hadean,dragon+slashdot&gmail,com> on Monday February 09, 2004 @05:55PM (#8230731)
    How many scientists believe in God?

    Just to show another angle (as opposed to the highly doubtful statement that only 7% believe in God), I Googled and found:

    In the US, according to a survey published in Nature in 1997, four out of 10 scientists believe in God. Just over 45% said they did not believe, and 14.5% described themselves as doubters or agnostics. This ratio of believers to non-believers had not changed in 80 years. Should anybody be surprised? (from http://www.guardian.co.uk/life/feature/story/0,130 26,1034872,00.html)

    So no, it's not most scientists, as it's mostly half and half (according to Nature/Guardian).
  • Re:Brief History... (Score:2, Informative)

    by Anonymous Coward on Monday February 09, 2004 @05:56PM (#8230745)
    1. "scientist" is not identical to "physicist"

    2. There are people who say they believe in God but not in a personal god. (e.g. Deists)
  • Re:Umm....OK? (Score:4, Informative)

    by Jerf (17166) on Monday February 09, 2004 @05:58PM (#8230798) Journal
    Would someone please translate this and tell me what the hell is being said?

    "Nobody knows for sure whether Dark Matter exists or not. But there are a whole lot of independent reasons to believe it exists, all of which result in very, very similar numbers for how much Dark Matter there is. If the Dark Matter theory is wrong, it's very hard to imagine what else could possibly explain all those numbers, all at once. (It's easy enough to explain one or two of those, but that's not an improvement over current theories, that's a step back.) Dark Matter hasn't made it to 100% certainty, but other theories have a lot to explain if Dark Matter doesn't explain what we're seeing."

    I think that's about right. I Am Not A Physicist, but I follow this and generally understand math, so I think I'm at least competent to translate... ;-) (Perhaps Pi_0's don't shower would like to confirm/deny this translation?)

    (On my own, I'd note that giving how flexible geometry can be, I can easily imagine someone constructing a geometry of the universe that doesn't need dark matter that turns out to be mathematically equivalent to a universe that does have dark matter. I'd give some of the geometry-based theories some time to be vetted by real astrophysicists before assuming they provide a real alternative; they may well just encode the Dark Matter into the structure of the Universe itself, which really isn't an alternate theory, just a restatement of the original in a different form. Until someone produces some Dark Matter, or the Universe is explored to our satisfaction to determine no such matter exists, this may remain unresolvable.)
  • by Graff (532189) on Monday February 09, 2004 @06:00PM (#8230823)
    Gravity is weak in the short-range when compared to the three other known forces of nature. The four forces are gravity, electromagnetic, strong nuclear, and weak nuclear. Gravity is an attractive force between all matter and energy, electromagnetic is an attractive and repulsive force between charged particles and magnetic fields, the strong nuclear force is what holds together protons and neutrons in an atom's nucleus and the weak nuclear force is what mediates electron and positron decay.

    If you set the strength of gravity equal to 1 then the other forces have the following approximate strengths relative to gravity:
    Strong nuclear force: 10^40
    Electromagnetic force: 10^38
    Weak nuclear force: 10^15
    Gravity: 10^0

    So why is gravity so important if it is so weak? The thing about gravity is that it falls off slowly with distance and it can't be negated or blocked (as far as we know). Other forces are either extremely short-range or, in the case of the electromagnetic force, have both an attractive and repulsive component that tends to cancel out in the long-run.
  • Re:Brief History... (Score:3, Informative)

    by Hadean (32319) <hadean,dragon+slashdot&gmail,com> on Monday February 09, 2004 @06:02PM (#8230844)
    Try using a less biased source next time! Don't you think Atheists.org would try to downplay the number of God-fearing/loving scientists?

    In the US, according to a survey published in Nature in 1997, four out of 10 scientists believe in God. Just over 45% said they did not believe, and 14.5% described themselves as doubters or agnostics. This ratio of believers to non-believers had not changed in 80 years. Should anybody be surprised?

    (from http://www.guardian.co.uk/life/feature/story/0,130 26,1034872,00.html [guardian.co.uk]
  • by wass (72082) on Monday February 09, 2004 @06:02PM (#8230845)
    Sorry to let you know but making kludges is really how alot of new physics is done. Someone finds some kind of 'kludge' that models reality, then the theorists try to explain it in terms of basic laws.

    Lots of things were done this way. Specifically, Planck's attempt to correct the ultra-violet catasctrophe of black-body radiation theory by quantizing the radiation was a total kludge. The theory matched the data fairly well, which led to a flood of new inquiries, leading to Einstein's description of the 'photoelectric effect' and the birth of quantum mechanics.

    The concept of the gyromagnetic ratio, or Lande g factor, for particles was another kludge that can be adequately explained using sufficient detail of Quantum Field Theory.

    Even more macroscopic phenomenological theories, like Landau's theory of 2nd-order phase transitions expands the free energy of a physical system in terms of one or more order parameters. That's a kludge and a half, but in many cases adequately describes physical systems close to phase transition points that formal Hamiltonian interaction methods cannot get to.

    Extending on this is the Ginzberg-Landau theory using a complex order parameter for superconductors. (Remember Ginzberg just won the Nobel Prize for Physics a few months ago. Landau won it decades ago and would have won it again if he was alive). It was shown by Gor'kov that the BCS theory of superconductivity (ie, formally-applied theory involving Cooper pairs of electrons and superconducting gap) approaches the Ginzberg-Landau expansion at the critical point.

    So yes, Kludges are really used all the time in physics, and they're no black eye at all. There's two reasons we need to use these. Firstly - macroscopic systems are just so damn complex one cannot solve a 10^23 dimensional Hamiltonian, that's ridiculous. So even from basic principles complicated order can emerge.

    The second reason is that it is quite likely we don't fully know the ultimate physics basic building blocks, just a very good approximation of them. Complicated systems can reveal small perturbations from the standard model that's accepted.

  • by ajagci (737734) on Monday February 09, 2004 @06:03PM (#8230862)
    Dark matter was postulated because galaxies spin at the wrong rate for the amount of observable matter they contain [nasa.gov]. That's not some obscure thing having to do with background radiation or age of the universe, it's a pretty concrete problem that can't be easily be explained away by reinterpreting measurements.

    The existence of dark matter has also been inferred from other observations (see above link), but even if that observation doesn't hold up, the odd behavior of galaxies still remains.
  • 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 Warhaven (718215) on Monday February 09, 2004 @06:16PM (#8231059)

    ...it's wrong. Dark Energy doesn't really have anything to do with Dark Matter, or regular matter for that matter. The universe is around 30% regular matter, and 70% dark matter (plus or minus 10-20% on both ends, depending on who you're talking to), which accounts for the "missing" matter in the universe. If you sum up all the matter in the universe with what we can see, it doesn't account for all the gravity residing in the universe, which is why the Dark Matter theory was created. SOMETHING out there is generating gravity, but we can't see it, and apparently, this invisible substance is creating most of it.

    Dark Energy on the other hand is the equivalent of Einstein's Universal Constant. For some unknown reason, the universe is accelerating in its expansion (counteracting the gravity that SHOULD be causing it to slow down), instead of slowing down (and eventually ending in a Big Crunch) as we previously had thought.

    Just FYI.
  • Huh? (Score:5, Informative)

    by Ayanami Rei (621112) <rayanami@gmai l . c om> on Monday February 09, 2004 @06:20PM (#8231117) Journal
    There is no decent explaination... emergent intelligence...
    it may explain why things can be completely random at a quantum mechanical level, but balance out in larger systems...


    It's called the Central Limit Theorem and Superposition. You've got billions of identical particles (low variance), and a huge sample size at macroscopic scales, thus your mean (likelyhood of "expected" things, the precision, and thus "intelligence" in systems) will be pointy as a pin.
    I am 100% dead serious.

    Perhaps maybe your REAL question is "why are quarks so damned sticky, protons so stable, and h_bar conveniently small?" because that encapsulates the huge gap between the quantum world and the stable world we live in.

  • 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 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]
  • Re:Umm....OK? (Score:5, Informative)

    by barawn (25691) on Monday February 09, 2004 @06:39PM (#8231383) Homepage
    Well, the lack of newlines didn't help, so here's a simpler version.

    Dark matter is implied by several things:
    • Galactic rotation curves
    • The velocities of galaxies in clusters
    • Anisotropy of the Cosmic Microwave Background
    I'm leaving out Type Ia supernovae because I don't think they really imply dark matter *by themselves*.

    Galactic rotation curves: If you have an object that rotates, and you know the velocity as a function of radius, you should be able to get the density as a function of radius. This is obvious, because the velocity is coming from gravity.

    The problem: you can also get the density by assuming that light-emitting material carries the majority of the matter (stars - pretty good approximation) and then looking at the luminosity as a function of radius (how bright it is). So, in a perfect world, these two profiles would match.

    They don't. Therefore either
    • Not all of the matter is light emitting
    • Gravity doesn't work.
    Option 1 there breaks the least physics, so it's preferred. :) There are also other concerns - namely, there are some galaxies that do rotate correctly, and some that don't. So either gravity sometimes works and sometimes doesn't work, or option 1.

    Velocity dispersion in clusters : See above - just with galactic clusters, rather than galaxies. Note that fixing one of these problems would probably fix the other!

    Anisotropy of the CMB : This one's tougher to explain easily. 100,000 years after the Big Bang, the Universe was an extraordinarily uniform big fireball. Extremely uniform - because electrons hadn't cooled enough to form hydrogen yet, so it was one big hot plasma.

    When hydrogen cooled, the photons in the Universe suddenly found themselves free to move, because hydrogen can only absorb certain wavelengths, and free electrons absorb continuously. Those photons are the Cosmic Microwave Background. Their uniformity is a very good indicator that the Big Bang theory is real - at least, from 100,000 years after the Big Bang to now.

    However, matter that was in that fireball DID distort the radiation slightly - through gravity. And so we see anisotropy (nonuniformity) in the microwave background, and it looks very much like standing waves in the sky. The ratios of the strengths of certain frequencies tell us the ratio of dark energy ("lambda", the cosmological constant) to matter, AND also tell us how "flat" - i.e., how much total energy - the Universe has. It's flat. Exactly. Really really flat. It has exactly as much energy as would be needed to reverse the initial Big Bang (if it were all in matter, which it isn't). And it also tells us that dark energy is 70% of the energy content of the universe, and matter is 30%.

    Big bang nucleosynthesis . BBN basically says "you can only get this much normal matter from a big bang explosion cooling to form atoms". It's amazingly accurate so far - it gives great answers for the ratio of certain elements, for instance. But it also puts a stringent limit on the amount of normal matter, of about 5%. The CMB *also* gives this same measurement - and, amazingly! - they agree! There are in fact even OTHER measurements which give values consistent with this number - 5% - so it's hard to imagine how measurements coming from completely different areas of physics (one is standing waves in the early Universe, one is nuclear physics) could give the same answers, and both be wrong. (But Nature can be perverse...)

    So, Omega_m has to be about 30%, and Omega_b is about 5%. Plus there has to be something making stars and galaxies rotate too fast. Physicists, wanting elegance, say "two problems, one solution is a great theory."

    Basically: If dark matter doesn't exist, we've got a lot of work to do to come up with other models, and a huge amount of it would affect gravity, which we thought we were beginning to understand!

    It's very hard to imagine a form of gravity which could answer all of these problems, AND still be consistent with what we observe today.
  • by brunes69 (86786) <slashdot AT keirstead DOT org> on Monday February 09, 2004 @06:52PM (#8231566) Homepage
    Correct me if I am wrong, but I thought scientists had already created ( in sub-microscopic amounts ) anti-matter in laboratories, and measured it's presence.

    Or is this "dark matter" something different than anti-matter?

    Someone please explain, the article is high on fluff but low on details.
  • by BigBadBri (595126) on Monday February 09, 2004 @06:54PM (#8231603)
    He's got the Powerpoint presentation, with associated material, on his website here [cam.ac.uk].

    I'm reading the presentation at the moment, but my math's a bit rusty (20 years rusty, if truth be told), so I can't comment on it.

  • by aderusha (32235) on Monday February 09, 2004 @06:58PM (#8231648) Homepage
    anti matter is well known, and can in fact be created in labs (along with being created in our ionosphere all the time). dark matter is completely different stuff.
  • by schwatoo (521485) on Monday February 09, 2004 @07:03PM (#8231707)

    Rowan Atkinson did a great comedy sketch [rowanatkinson.org] based on a premise a little like that:

    Rowan Atkinson plays of course the lead role in BlackAdder and Mr. Bean.

  • by StillNeedMoreCoffee (123989) on Monday February 09, 2004 @07:08PM (#8231761)
    I think you will find that Ptolemy of Alexandria's Circles of Ptolemy was an elegant, if not emperical, result. It has since been systemitzed by Fourier and his Fourier series with can come up with an infinite series of diminishing Cosine terms that can approximate and function say like the motion of planets from another viewpoint. An equivalent if not infinite series form of the same thing. I think it took mathematics a few centuries to understand the brilliance of Ptolemy's insight. Most it seems even know have no clue what it really represents.
  • by egomaniac (105476) on Monday February 09, 2004 @07:27PM (#8231992) Homepage
    No one has bothered to even look to see if the rules by which our universe exists today are the same as a few million years ago, or a few billion years ago. How would you be able to tell that, say, the gravitational constant of the universe has been constant all along?

    You are mistaken. There have been a number of studies done to try to determine if fundamental "constants" such as the speed of light are in fact constant.

    It is, of course, very difficult to devise experiments to test such theories, but a number have been designed and performed. The phrase "no one has bothered to even look" comes up in other fields, such as paranormal research, and it is just as untrue there. Scientists would love to find evidence of (say) the gravitational constant changing, extraterrestrial organisms, or psychic power, and to suggest that they haven't even bothered to look is an insult to the field.
  • Not laughable, and still usefull, even if it is 'wrong.' It is still 'right' in most circumstances, and we know what those circumstances are. We know when to use it as a good enough approximation, and when we need to use more accurate theories.

    Theories can never be proved. We will never fully understand the universe. We may develop theories that accurately predict every phenomenon, then the next day, something new could come along and show us we were wrong.

    What is important is for scientists to fully understand that theories are always merely theories, not facts.
  • by Anonymous Coward on Monday February 09, 2004 @07:43PM (#8232182)
    Anti-religionists seem to think everything, including anything within religion, should be "provable", no matter what. The underlying problem with this mindset is that (for the most part) religion relies upon faith - so there's no need to rely upon physical evidence.

    Yes, in fact, that is indeed the underlying problem with religion. It's unprovable and relies on "faith".

    I forget who it was -- it was repeated by Douglas Hofstadter which is where I first read it -- but there is written a brilliant rebuttal of Pascal's theorm. Part of it can be found on the web, in the essay "Is God a Taoist", but it was the addendum to that, which I haven't seen on the web, which rebutted the idea that it's "safer" to believe in God than to not do so. I suggest you seek it out. I believe it was outlined in "Metamagical Themas", though it may be "The Mind'd I", I don't recall.
  • by Josh Booth (588074) <joshbooth2000@ya3.14159hoo.com minus pi> on Monday February 09, 2004 @08:03PM (#8232393)
    I believe that if you moved two protons to a distance at which the electromagnetic forces between them were the same as the gravitational forces between them, they would be about 40 lightyears apart. I don't remember if that is the right number or particles, but indeed, gravity is really weak. A comb attracting a light piece of paper is overcoming the entire gravitational force of Earth.
  • All made-up (Score:4, Informative)

    by bstadil (7110) on Monday February 09, 2004 @08:07PM (#8232415) Homepage
    Hmm... two references to the same Almighty, and one to a made-up religion

    They are all made-up, most likely by someone with Schizotypal syndrome [ffrf.org].

  • 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.
  • by pokeyburro (472024) on Monday February 09, 2004 @08:24PM (#8232604) Homepage
    As others have said, this has already happened. More specifically, the theory that "if anyone discovers just exactly what the universe is for and why we are here, that it will instantly disappear and be replaced by something even more bizarre and inexplicable" in fact implied why we were here (namely, to discover the above theory), and so it no longer holds.

    Which means the purpose of the universe and of ourselves is now ridiculously simple and inane, and furthermore, if we ever figure it out, it won't be replaced. Which is a shame.
  • by Goalie_Ca (584234) on Monday February 09, 2004 @08:49PM (#8232824)
    f = G.m1.m2/d^2

    actually that's newtonian. They use Einstein's theories for gravity now.
  • Re:Theory. (Score:2, Informative)

    by odeee (741339) on Monday February 09, 2004 @09:03PM (#8232926)
    In a sense this is a situation where the gulls have in most ways already evolved into two species.

    Perhaps you misunderstand the position of anti-evolutionists (or creationists). As a creationist, I don't disagree with variation within or without of a species (speciation); that would be intellectual suicide and I'd be foolish to hold to a belief for which scientific evidence proves to be wrong...

    But rather, creationists believe that rather than all living organisms having evolved from a single organelle, that in the beginning there was created a number of discrete organisms (kind's). Since then these organisms have changed, been bread into 'purebreads' with specific features, and in some cases speciated to become separate species that can't interbreed with their original 'kind'. So in this model, rather than believing in the upward process of evolution that pushes towards better and better species, we see the organisms in the world degrading (from a genetic point of view) to more 'refined' species. i.e. once a new species how sprung out of it's original 'kind', it no longer has the genetic information in it to climb up the genetic ladder and produce new variation.

    I hope this helps explains the creationist position as I find it is something that is often misunderstood. I know that a lot of people will argue that they simply can't accept that there was a number of organisms originally created, but this is an assumption on creationists behalf that we can't prove; just an evolutionists assumption that in the beginning there was nothing is an assumption that they can never prove. All we can do now is scientifically test our hypothesise against observable evidence to see which one seems more correct.

  • by Anonymous Coward on Monday February 09, 2004 @10:50PM (#8233705)
    In case you want to read a more scientific version of this article, it's here [lanl.gov].
    The first thing that occurs to me upon reading it is that his proposal for sweeping the CMB anisotropies under the rug, the Sunyaev-Zeldovich effect, really isn't an issue in WMAP's lower frequency bands. Even if he's right and the 94 GHz band is slightly contaminated, there remains the four other frequency bands, and those show the same spectrum of anisotropies as everyone else. (WMAP is hardly the first to measure the CMB anisotropies. Check out ACBAR, MAXIMA, CBI, COBE, BOOMERANG, DASI and others and convince me that they all got it wrong too.)

    And anyway, the SZ effect is well-understood; a couple large surveys are coming on line in the next couple years that hope to map out galaxy clusters using their SZ signatures on the CMB (APEX-SZ and the South Pole Telescope, for example). Noteably, they're all observing at much higher frequencies (220 GHz, etc) than WMAP.

    WRT the large-scale WMAP power spectrum points, the cosmic variance (uncertainty that comes from the fact that we can only observe one universe, rather than a whole ensemble of them) dominates on those scales, so as the WMAP team has pointed out, those points need to be taken with a grain of salt. You can't throw out the concordance cosmological model based on them, certainly.

    His point about SNIa evolution is well-taken, to a point, though it's been addressed in some detail by both the Perlmutter and Reiss groups.

    Essentially, Shanks's problem boils down to a complaint that we still don't understand dark energy or dark matter, and it would be nice to do away with them. He's right on both counts, but at this point there's too much evidence in their favor to discard them so readily.
  • by conan_albrecht (446296) on Tuesday February 10, 2004 @12:51AM (#8234439)
    I should know better than to argue evolution on Slashdot, but what the hey, I'll bite.

    First, my field is MIS. Yes, I've read the posts about worthless MIS profs, and I'm probably one of them. You can reference my many GNU apps I've contributed to OSS. I've contributed patches to several OSS projects, most recently Spyce. I know assembly, C, C++, currently teach Java, python, and a few others. Yes, I've programmed several genetic algorithms for use in real situations. However, I was trained in the scientific method just like other scientists. Most PhDs are very much the same as far as science goes. But no, I'm not a specialist in evolution or biology. I should have been more clear.

    I won't nitpick your post. Let me just talk about the mathematics of evolution. I may believe in God, but I am not against evolution. God and evolution are not mutually exclusive, and evolution may just be right. My post said that from a scientific perspective, evolution doesn't seem to hold weight with me.

    The human body is a base-4 computer (A,G,C,T). Take one side of DNA, and you essentially have computer code. The human genome project suggests we have about 30,000 genes. While genes is not the same as bits (it's a collection of base-4 "bits", I'll use them for the mathematics.

    To get to where we are now, we'd need at least 30,000 mutations (actually quite a bit more) that were useful enough to select over other mutations. If we assume an x percent successful mutation (quite liberal) rate, we'd need x^30,000 mutations.

    The universe is believed to be about 13 billion years old. Thats 297648000 billion seconds, or y^17. How many mutations would be required per second to get to x^30,000? Statistically, I just don't see it.

    Again, I'm not against evolution from a "God" perspective. I'm against it from a mathematics perspective. Just like any theory, it's useful because it allows us to model the world and understand at some level. I think evolution will be a great step to a more correct theory at some point.

    I just get very bothered that people (even some scientists) think evolution is "truth", when science never proclaims to find truth. It's a *theory*.

    Newton, Einstein, and others were all shown to be wrong in time, even though their theories were elegant and helped us do wonderful things (like go to the moon, fly, etc.). String theory right now is quite interesting, but it's probably not the final theory either.
  • Heaven help us! (Score:3, Informative)

    by Black Parrot (19622) on Tuesday February 10, 2004 @01:02AM (#8234517)
    To get to where we are now, we'd need at least 30,000 mutations (actually quite a bit more) that were useful enough to select over other mutations. If we assume an x percent successful mutation (quite liberal) rate, we'd need x^30,000 mutations.


    The universe is believed to be about 13 billion years old. Thats 297648000 billion seconds, or y^17. How many mutations would be required per second to get to x^30,000? Statistically, I just don't see it.
    I am speechless that a professor in any discipline would make such an idiotic mistake.

    (Let us know if someone needs to explain it to you.)

  • by IdntUnknwn (700129) on Tuesday February 10, 2004 @01:52AM (#8234763)
    Wow, that is incredibly horrid math.
    If we assume an x percent successful mutation (quite liberal) rate, we'd need x^30,000 mutations.
    Ok, so let's assume that there is only .001% chance of a useful mutation. Using your math, we would need (.001)^30,000 mutations, which is approximately...0. Wait a minute...

    The formula you're really looking for is 30,000/x.

    Now, with the correct equation, even if we assume there is only .000000001% chance of useful mutation, 300000000000000 mutations are required. Which is significantly smaller than "297648000 billion seconds."

    Please go back and revise your math. And try another approach, this one simply doesn't work.

    You're a scientist. I'm 17. :)
  • by Noren (605012) on Tuesday February 10, 2004 @02:11PM (#8239880)
    There are, of course, other [gimb.org] explanations for "God"'s behavior.
    Stan: Why would God let Kenny die, Chef? Why? Kenny's my fr-f-f-friend. Why can't God take someone else's f-f-friend?

    Chef: [sighs] Stan, sometimes God takes those closest to us, because it makes him feel better about himself. He is a very vengeful God, Stan. He's all pissed off about something we did thousands of years ago. He just can't get over it, so he doesn't care who he takes. Children, puppies, it don't matter to him, so long as it makes us sad. Do you understand?

    Stan: But then, why does God give us anything to start with?

    Chef: Well, look at it this way: if you want to make a baby cry, first you give it a lollipop. Then you take it away. If you never give it a lollipop to begin with, then it would have nothin' to cry about. That's like God, who gives us life and love and help just so that he can tear it all away and make us cry, so he can drink the sweet milk of our tears. You see, it's our tears, Stan, that give God his great power.

    [pause]
    Stan: I think I understand.

  • Re:Brief History... (Score:3, Informative)

    by Noren (605012) on Tuesday February 10, 2004 @04:04PM (#8241124)
    That statement is deceptive- though it's derived from a real study, that study didn't make that claim. The phrasing of the question and method of selection of those to be questioned are critical for such questions, so that should be included in the description. Here's where the numbers are coming from:

    In 1916, James Leuba sent a survey to 1000 scientists (500 biologists, 250 mathematicians, and 250 physicists/astronomers) drawn randomly from the appropriate sections of the 1910 edition of American Men of Science. Leuba broke his data up between all scientists and "greater" scientists, based on labels of "greater" as listed in his edition of American Men of Science.

    Section A had three options, requesting the responder to choose one:
    1. I believe in a God in intellectual and affective communication with humankind, i.e. a God to whom one may pray in the expectation of receiving an answer. By "answer" I mean more than just the subjective, psychological effect of prayer.
    2. I do not believe in God as defined above
    3. I have no definite belief regarding this question.
    (There was a B question regarding beliefs in "Personal Immortality" or afterlife I'm not going to elaborate on)
    Leuba found 41.8% of all scientists responding answered 1 (belief), 41.5% answered 2 (disbelief), and 16.7% answered 3 (doubt)
    Leuba found 27.7% of the "greater scientist" group answered 1, 52.7% answered 2, and 20.9% answered 3.

    In 1997, Edward J. Larson and Larry Witham published in Nature ("Scientists are still keeping the faith" [nature.com]) a survey of scientists intended to be similar to Leuba's- a survey of 1000 people drawn randomly from American Men and Women of Science in similar disciplinary proportions using the same question that Leuba used.

    Larson & Witham found 39.3% of the 'scientist' group answered 1 (belief), 45.3% answered 2 (disbelief), and 14.5% answered 3 (doubt)

    In 1998, Edward J. Larson and Larry Witham published in Nature ("Leading scientists still reject God" [nature.com]) a followup survey of "leading" scientists- in this case, all 517 members of the (US) National Academy of Sciences at the time were sent the survey.

    Larson & Witham found 7.0% of the NAS respondants answered 1, 72.2% answered 2, and 20.8% answered 3.

    I don't expect a particular bias either way (either of believers being less likely to respond or of nonbelievers being less likely) but it's possible. An argument could be made for either bias.

    The 7% figure of the parent post is taken from the second survey, but its description of the body being surveyed as "scientists" would be more valid to use 1997 study. Larson and Witham's estimate for the percentage of scientists(given the limits of their study) who are believers was 39.3%, not 7%. This, however, is still not the "most" claimed by the grandparent.

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