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Space Science

"Dark Matter" Observed 209

Posted by michael from the i-was-blind-but-now-i-see dept.
An anonymous submitter writes: "The space news site Space Flight Now has an article about the first direct "observation" of so called dark matter. Galaxies appear to have more gravitation (mass) than we can currently observe. The theory of dark matter tries to explain this missing mass by the existence of massive bodies too faint to detect. These bodies include everything from dim stars to exotic particles called WIMPs. The previously dark matter, a dwarf star, was detected when it passed in front of a brighter blue star, creating a gravitational lens. It is thought that there are many more like it out there creating all that extra gravity, we just can't see them." Wired has another story; or see the European Space Agency's original article.
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"Dark Matter" Observed More

"Dark Matter" Observed

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  • I am glad to see this report, I read some where when I was in highschool that dark matter was this exotic matter that could not be seen even if you had it in your hand....that made no sence to me. I am glad to see a more sensable discription like this.
    • We are already pretty sure that most of the missing matter must be non-baryonic (i.e., it must be made of something other than protons, neutrons and electrons).

  • How long...? (Score:3, Funny)

    by Reliant-1864 ( 530256 ) <sabarokaresh@@@yahoo...ca> on Thursday December 06, 2001 @10:54AM (#2664770)
    How long until Dark Matter is banned as a circumvention device for light waves?
    • Dark matter will be declared a terrorist tool. Anyone found harboring dark matter will be found guilty of aiding and abetting terrorists.

      BTW, I've always wondered, is the matter that has been sucked into black holes considered is the total matter of the universe equation? It seems that everywhere they look, they find another black hole. Maybe there's alot of unaccounted matter out there.

  • Fate of the Universe . . . (Score:4, Interesting)

    by JJ ( 29711 ) on Thursday December 06, 2001 @10:54AM (#2664772) Homepage Journal
    The fate of the universe is held by dark matter. Without dark matter, there is insufficient gravity to bind all matter together forever. If there is enough dark matter, with its attendant gravity, then eventually the universe will collapse back onto itself. Probably the end result of that would be another Big Bang.
    What a pair of choices.
    • both cases suck though case either way we are all gonna die......the wimper out theory is a bit more distressing however because who wants to theink of time just ending? a recolaps seems more positive.....yes I know I will be long dead but you think about this kind of stuff when you are a complete dork :-)


      • Is the theory which says, at any given moment the universe can simply destroy itself, and while the chances are 1 in a billion or something really high and unlikely, the possibility is there for ALL matter in the universe to cease to exsist.

        Time wont end, just our lives. Even if matter no longer there in this form, its energy will still be there
        • true but it will be so far dispersed that the universe will approach absolute 0 if not reach it and time stops. the only thing that will happen is the stray quantom fluctuation evey billion years or so.
          • Absolute 0? as if we somehow calculated the universe?

            We only know how much matter is in it, we dont know how much space is in it

            Time does not actually exsist, change exsists. The universe however never ends, simply changes, big bang was part of a change, and the universe will change to something else.

      • Re:Fate of the Universe . . . (Score:4, Funny)

        by archen ( 447353 ) on Thursday December 06, 2001 @11:17AM (#2664894)
        "we're all gonna die"

        I'm going to die in about 50 years (give or take 10), if you're going to die in a couple trillion, I wouldn't be that depressed, but maybe you better live it up while you can.

        Expand into nothing, compressed to a single point, eaten by a giant galactic space goat; it's all the same to me. I'd be more concerned about our sun burning out in a couple billion years myself....

    • No (Score:2)

      by HanzoSan ( 251665 )


      Dark matter is increasing, the universe is going to expand until we are so far apart that we all freeze to death.

      The universe will not collapse, that theory was proven false a long time ago. Since its proven that we are moving apart, Its safe to assume that we will move apart forever.

      Also for big bangs, Big bangs happen all the time, in fact Big bangs are happening right now.
      • just cause we are moving apart does not mean that the we will forever, what the hell kind of scientific thinking is that? they do not know how much dark matter there isin the universe, they do know how much normal matter there is.....the normal matter does not add up to create the right amount of gravitational force to keep the Universe together, however Dark matter could be there, infact, they have shown that in the galaxies they have looked at for dark matter that on average, much of the matter that in in a galaxy is dark.....if you take that and apply it to evey galaxy then you can show that we will recolaps...however, you cannot just make a jump like that so it remains that we just don't know...no to mention the dark matter in intergalactic space that could exist.

      • Re:No (Score:2)

        by Pii ( 1955 )
        The only reason it it safe to make this assumption is that the consequences of having guessed wrong will not be felt for quite some time, and in all likelyhood, humankind will not bear witness to any collapsing universe.

        That said, your assumption seems silly to me. You act as though there were no force in the universe which could counter the inertia which governs the universe's current expansion.

        I'm no astrophysicist, but I can name two off the top of my head: Friction, and Gravity.

        Space is not empty, dispite the rumors you may have heard... Every body in motion meets resistance, because there is no pure vacuum. Those particles do constitute a force, no matter how miniscule, and given enough time, they will win out, just as a rock eventually gives way to the trickle of a tiny stream.

        Also, every object currently moving outward from the center of the universe is being slowed but the sum total of all of the gravition of the objects behind it (Objects between a body and the Universal center, and objects moving in other directions from the center). Even though gravity has a rapidly diminsihing effect as distance increases, it never reaches zero. Regardless of how fast, or how far a body is, there will always be more matter (light or dark) generating gravity to slow it.

        At some point, I believe the big crunch will come again, just as I believe it came before. I think it's an endless cycle.

        • Also, every object currently moving outward from the center of the universe is being slowed but the sum total of all of the gravition of the objects behind it.

          There's no such thing as the centre of the Universe. If there was we'd be able to tell whereabouts we were because everything would be rushing away from the centre. Instead the Universe is isotropic and homogenous - ie. it looks the same in all directions and from all positions. Wherever you are in the Universe you'll see the rest of the Universe spreading away from your position.

          And anyway, whether gravity can slow down the expansion of the Universe enough depends on the amount of matter within it, which is a conserved quantity. Of course, as gravity follows an inverse square law the forces slowing down spacetime expansion get weaker over time, and we just don't know whether there's enough matter so that gravity is strong enough.

          Probably not, but who knows?

          • the center of the univers is at the center of a super sphere. the only way to view it is over time. if we can find a glow at the most far out reach...farther than the farhtes Quasar...and that glow is equal eveywhere, then that is the center of the universe.....it is kind of wierd because the center of the universe will be a point that is at the begining of the universe, but the objects that were around at the big bang, and all that energy, are now at what apears to our 3 dimentional concepts as the "outside"

            that is why it is homogenius because of the 4 dimentional nature of the universe.

            • the center of the univers is at the center of a super sphere.

              What supersphere? Do you mean a 4-d spacetime hypersphere? I still don't think that the centre of that has any physical meaning unless you ascribe it as being the initial singularity at t=0 (which probably doesn't exist).

              if we can find a glow at the most far out reach...farther than the farhtes Quasar...and that glow is equal eveywhere, then that is the center of the universe.

              Are you talking about the microwave background radiation which is everywhere? That's all that's left of the afterglow of the Big Bang, the glow itself stopped after 300,000 years when the energy density of the Universe dropped low enough so that photons stopped interacting with matter so often.

              Apart from that you make no sense. What does 4 dimensions have to do with the fact that the Universe is homogenous?

              • sorry, hyper is the correct term......and yes the back ground radiation is what I was refering to....however, 4 dimentions has a lot to do with it, in 3 dimentions the universe is homogenous but if looked at in 4 d it certainly does have a center. if the universe were to go into a big crunch we will be moving back toward the center.....my point is look at the expansion of the universe over time (4 dimentions) and you will see a center. yes it is irrelivent today, but not irrelivent to the universe and some fields of theoretical physics.

                • if the universe were to go into a big crunch we will be moving back toward the center.....

                  Not at all. Using the usual analogy think of a universe with 2 spacial and 1 time dimension, where the spatial dimensions are on the surface of a "balloon." As the universe expands (as in blowing the balloon up) things get further apart, but equally so at every point on the surface of the balloon - there is no "centre". Equally so for when it is contracting - it contracts everywhere equally and there is no centre of contraction on the surface.

                  Extrapolating from a 3-d sphere to a 4-d hypersphere, it's easy to see there doesn't have to be a centre in this case either. It's just a lot harder to visualise :)

                  • yes but that is just looking at the situation from the 2 d way....we, as higer dimentional beings than those on flatland can see the depth of the baloon and see it contract on a single point. same goes for a hypersphere.

                    • But that's nothing to do with a centre is it? The sphere may have a centre but space doesn't, and even if you contract the universe back to the centre of the sphere you can still never say that space has any particular centre. And the centre of the sphere is nothing more than the initial singularity at t=0, which isn't really a centre of anything either.

                    • ok I she what you are saying, space can get infinitly small but the distrobution of the matter will remain a constant ratio.
          • Then I propose we take Ptolemy's view... I declare Earth to be the center of the Universe.

            Seriously, even if no central point can be defined from a distance perspective (which I am perfectly willing to accept), there must still be a gravitational center - a location where the amount of matter, and the average of that matter's distance works out to be roughly equal in all directions.

            Again, even if this cannot be condensed to a single point in space/time, the effect of this central area would be the same as it relates to bodies retreating from it.

            Of course, this is speculative, and as much as I'd like to see a unified theory of everything someday, I don't know that we'll ever get there. The thing I like about the cyclic Big Bang/Big Crunch idea is that it puts us on a timer! We don't have forever to solve all of the riddles of the universe... It's more like (Forever - 1).

            • Seriously, even if no central point can be defined from a distance perspective (which I am perfectly willing to accept), there must still be a gravitational center - a location where the amount of matter, and the average of that matter's distance works out to be roughly equal in all directions.

              That's the whole point of it being isotropic and homogenous - there is no single preferred point at which you can say "this is the centre". At any point you choose there is a (approximately) uniform distribution of matter in every direction. Sure there are local irregularities (galactic superclusters for instance), but on a large enough scale this uniformity seems to hold.

              Of course, this is speculative, and as much as I'd like to see a unified theory of everything someday, I don't know that we'll ever get there. The thing I like about the cyclic Big Bang/Big Crunch idea is that it puts us on a timer! We don't have forever to solve all of the riddles of the universe... It's more like (Forever - 1).

              Heh, well the latest theory to come out of superstring research is called the Big Splat, and involves four-dimensional manifolds embedded in a five-dimensional manifold, of which two collide and start the Big Bang...

              • Re:Nope (Score:2)

                by Pii ( 1955 )
                Clearly, your knowledge and interest in this topic surpasses my own.

                You seem a smart fellow; why do you continue to spell "center" incorrectly. ;)

                • why do you continue to spell "center" incorrectly

                  Probably because he's a Brit. The spellings of many words in American English differ from that of The Queen's English. This is largely due to the anti-British sentiments held by Noah Webster: he intentionally revised the spellings of many words to differentiate the American language from it's ancestor. It helps to know that Noah Webster was a student at Yale during the Revolutionary War -- during most of his lifetime, England was the enemy of the U.S., not the close ally it is today.

                  • Thanks... It was a friendly jab at a smart fellow.

                    Since he hasn't responded to it, and many of you have, I have to believe that he interpreted it in the spirit in which it was intended, and you louts did not.

                    Subtlety is becoming a lost art...

    • >Probably the end result of that would be another Big Bang.

      so its possible that the universe has banged, expanded, collapsed in on itself and banged again multiple times already, right? (in fact, you can give yourself a serious headache by pondering the implications that this sequence of events is repeated infinitely, that there was never a 'first' time and there will never be a last).
    • I would be more inclined to say that the fate of the Universe is dependent on whether neutrinos have mass. There are far more neutrinos than any other matter predicted or known (except for photons). If the neutrino has even a tiny mass, the result is most likely a closed Universe.
    • I don't think the argument for dark matter has to do with the expansion/contraction of space-time (i.e., the universe). I think the argument for dark matter is based on gravitational models of galaxies and the idea that, according to current theory, most galaxies do not appear to have enough (visible) matter to create the gravitational force needed to hold that galaxy together. The expansion of the universe appears to be accelerating. See one article on this here. [astronomytoday.com]
    • The opposite of the Big Bang is a Gnab Gib. Read Douglas Adams!

      (Hey OmniWeb has a built-in spell checker!)

      "I'm not theoretical astrophysicist and I don't play one on Slashdot!"
    • > If there is enough dark matter, with its attendant gravity, then eventually the universe will collapse back onto itself.

      The "Big Crunch" was once thought possible, but that was before we discovered the universe is *expanding* and *accelerating*.
    • AFAIK, dark matter only accounts for about 25% of the "missing mass" of the universe. Besides normal matter and dark matter, there is also "dark energy". This is where most of the mass of the universe is supposedly locked up.

      I used to believe the universe would eventually go through a Big Crunch/Big Bang cycle again. But the recent discovery of an expansionary force acting on galaxies (ie: the universe is increasing its rate of expansion, "blowing up quicker") has been a bit of a slap in the face for that point of view. So we're back to an open universe: it is basically a big firecracker destined for heat death.

      Having a bunch of dead stars hanging around galaxies would seem to indicate a sort of "fossil history" of our galaxy. I wonder how far out these relic stars go out from the center... I mean, our system is pretty far out, but we may be the equivalent of the Earth in relation to an "Oort Cloud" of dark matter in our (somewhat bigger) galaxy.

      A friend of a friend, who is doing post-grad work in Physics at Clown College, has just switched majors from particle physics to cosmology, which is a pretty big switch. I think he's smart: astronomy and cosmology are going to be the next Big Science soon enough.

    • At least in the sense you are talking about, the fate of the universe is already sealed. The result of interest was discovered about four years ago now, but I'm too lazy to look up the exact reference.

      It turns out that you can calculate the absolute brightness of a type Ia supernova from its light curve (how quickly it fades away). By measuring the apparent brightness of a bunch of these events at large distances, we can find their physical distance. By measuring the redshift of the light arriving here, we can find out how long the light has been traveling (sorta; general relativity makes it slightly more complicated to explain what I'm talking about here. Look up "comoving distances" for yourself.).

      The thing is, up until then everyone assumed that the universe is expanding but slowing down. Not so! Turns out, it is accelerating. We know from GR that only a vacuum energy density could produce this effect, and that is a constant per *physical* volume, while everything else in the universe spreads out with the increasing size of the comoving volume. As a result, the amount of vacuum energy can only increase, barring some kind of phase transition. Therefore, the acceleration can only increase with time.

      So the answer is -- no, the universe will never collapse back on itself, but will expand forever at an ever increasing rate. The only thing that could change this would be a vacuum decay event, which would unfortunately probably destroy all matter in the universe.

  • Peek-a-boo (Score:3, Funny)

    by DaoudaW ( 533025 ) on Thursday December 06, 2001 @10:59AM (#2664798)
    The result greatly strengthens the argument that a large fraction of the 'normal' Dark Matter in and around our Galaxy exists in the form of MACHOs and that this Dark Matter is not as dark as previously believed!

    Does anyone else have the feeling we are just playing peek-a-boo.

    "Hey, its dark in here. Where did everyone go?"
    "Ummm, move your hands!?"
    "Oh, there they are. That was really weird!"

    You've just got to love cosmology...
  • The reason we can't see the dark matter is due to the proliferation of soft light in the Universe, as was depicted in the X-files episode: Soft Light. ;)

  • Um, if it's a star it can't be dark matter.... (Score:4, Interesting)

    by LMCBoy ( 185365 ) on Thursday December 06, 2001 @11:02AM (#2664823) Homepage Journal
    The observed object is a dwarf star. It is luminous. This article should have been titled "Confirmation that one of the MACHO objects is not Dark Matter".
    • I think the term "dark matter" does not necessarily apply only to non-luminous matter. I think it is used to refer to any unobserved matter that can account for the apparent gravity we see in galaxies. MACHOs have been a candidate for dark matter for a while, because they are mostly failed stars that do not emit light (at least not enough for us to see), though they do interact with the EM field. Other candidates for dark matter are indeed non-luminous, even non EM interacting (WIMPS-weakly interacting massive particles--that only interact with the weak nuclear force and gravity, but not EM so they can't be "seen" using light).
      • Pretty good point, but there's a big difference between a failed star (or brown dwarf) and a low-mass star like this object. We have a pretty good idea how many low-mass stars there are in the Galaxy, from the statistics of stars in our neighborhood. We already account for the presence of such objects when we compute the mass of known objects in our Galaxy. This object is in the "known" column of the Galactic Census; it isn't missing mass.

        Now, if you want to discuss the uncertainty of the low-mass stellar mass function, and say that it's possible that there are lots more of these dim stars than we currently estimate, that's a different story (although no one would recommend making this argument based on the observation of a single object). However, there can't be so many of these little guys as to solve the missing mass problem.
      • The term "dark matter" has wound up being overloaded in astrophysical discussions, because it has been used to name the solution to a number of different problems.

        First, people noticed that we cannot observe enough luminous matter to either produce a flat universe, or account for the dymanical behavior of large-scale systems. This was long assumed to consist of halos of cold gas, dust, brown dwarfs, etc.

        However, cosmological considerations (especially primordial nucleosynthesis) rules out this scenerio, because we can use the deuterium mass fraction to calculate the ratio of photons to baryons in the early universe. We know how many photons there are (per comoving volume, as usual), and it turns out that there are only enough baryons to account for about 4% of the density needed to produce a flat universe. Since the universe is not noticably non-flat, we can assume there is "a lot" of non-baryonic matter out there, in axions, massive neutrinos, or something more exotic. This stuff is called non-baryonic dark matter, unsurprisingly, and often gets confused with the other kind.

        Finally, in the last five years or so we have received a couple of cool new data points: the angular size of the first harmonic mode of perturbations in the cosmic microwave background, and the distance scale to various redshifts, as seen using type Ia supernovae. The CMB data tells us that the universe really is flat, to high accuracy; otherwise, the perturbations -- we know how big they should be after all -- would be "lensed" by the curvature of spacetime. The supernovae data tells us that -- BIG surprise! -- the universe's expansion is accelerating, not slowing down at all. This implies that there is actually more vacuum energy than matter and energy combined. Best guess, the universe is roughly 70% vacuum energy, 30% matter. For some bizarre reason, people have been calling this the "dark energy" lately. Thus, even more confusion about what you mean when you say "dark matter".
    • the term dark matter refers to matter which we can not see...that does not mean it is not normal matter, just that we can not detect that it is there.
      • Right, that was my point. Since we can see this object (see image in article), it can't strictly be called dark matter. OK, that's pretty specious, but see my other posts in this thread for a longer explanation.

        Executive Summary: we already knew objects like this existed, and we think we know how many there are in the milky way, so it can't really be part of the solution to the missing mass problem.
        • dark matter = exotic matter just as much as UFO = aliens

          it is just a way to classify something that is yet unexplainable of unobservable either because of lack of technology or lack of power. I am willing to bet that about 50% of the dark matter out there is just planets and moons and asteroids that we can not see or have not looked for yet.
          • It's true that calling it "Dark Matter" is just a label for something we know next to nothing about; I think the name itself embodies that ignorance nicely. In fact, we really only know a couple of things about DM: it makes up most of the universe, and most of it can't be made of baryons.

            I'm already arguing this with an AC in another thread, but we already know that not more than 25% of the DM can be "normal" stuff (and it's probably a lot lower than that). That's stated explicitly in the article.
  • I remember an episode of Dr. Who back about 15 years ago that featured 'Dark Light'. There was a briefcase type container which housed some 'dark light' and people were trying to steal it because it was so precious.

    So we got Dark Matter, and Dark Fibre ;) so what's this Dark Light stuff about?

    Dr. Who is my reference for all things scientific! It's all true isn't it?
  • It moves space around the ship!!

    Of course dark matter exists. It's pooped out by that little monster, Nibbler on Futurama. It powers starships ppl!!!
  • Correct me if I'm wrong, but the article said MACHOS were just really large clumps of normal mass that don't emit any light, so when they say they've found MACHOS, they mean they've just found stuff. You know, really big rocks and such. It's impressive that they's partially proven a theory that would explain where all the dark matter is, but it really sounds like they're trying to sound impressive when they call what they've found MACHOS. Why not just "rocks", or "stuff"?

    Mr. Spey
    • Actually, MACHOs are a specific thing, as MACHO is an acronym for MAssive Compact Halo Object. This implies that it has significant mass, is quite compact, and located in the halo of a galaxy. This says quite a bit more than "rocks".
    • Just word play, the Weakly Interacting Massive Particle theory (WIMP) was competing with the MAssive Compact Halo Object (MACHO) theory.

      They don't need to not emit ANY light, they just need to emit so little we can't normally see them. If the interstellar dust absorbs most of the light emitted by a distant dwarf star we can't see the light from the star. So we don't know there's a star there, but we can see the effects of it's gravity.
  • ...it's that stuff in the back of my refrigerator!

    which means its time to move :)

  • Galactic vs. extragalactic microlensing (Score:5, Informative)

    by KjetilK ( 186133 ) <kjetil AT kjernsmo DOT net> on Thursday December 06, 2001 @11:10AM (#2664855) Homepage Journal
    Yep, these are really interesting observations! Galactic microlensing, which is discussed in this article, is a field which is growing rapidly and has attracted a lot of interest. I look forward to seeing the lightcurves of this event.

    It was indeed Bohdan Paczynski who wrote the first paper about that specific phenomenon, if I recall correctly, the paper was titled "Microlensing on small optical depths". And indeed, he was the one who invented the term "microlensing".

    However, I'm more concerned with "extragalactic" microlensing. The funny thing is that stars in remote galaxy can cause microlensing of even more remote quasars. This was first discussed by Chang and Refsdal in an article in Nature, December 6 1979.

    The great thing about this is that in galactic microlensing, there are very few MACHOs between us and the stars, so you would have to watch a lot of stars (millions), whereas in extragalactic microlensing, there will be lots of stars, so microlensing events happen all the time. You only need to separate it from the intrinsic variations of quasar...

    Now, galactic microlensing has been a so much bigger field of study than extragalctic microlensing, we haven't really got that much attention. In part, it can be becuase galactic microlensing gives so much more solid results, but then, it is just addressing what's going on in our backyard, while the extragalactic microlensing really deals with the universe... :-)

    • What is "Galactic macrolensing"?

      • Massive objects between us and objects we're observing tend to distort light in various ways. Sometimes that distortion is a really good thing because it focuses the light for us, giving us a better picture of what we're trying to observe.

        Whole galaxies can form the basis of such lenses.

      • Re:Galactic vs. extragalactic microlensing (Score:4, Informative)

        by KjetilK ( 186133 ) <kjetil AT kjernsmo DOT net> on Thursday December 06, 2001 @12:25PM (#2665251) Homepage Journal
        Well, the term isn't really in use. Most probably, most people would think about Einstein's speculations around gravitational lensing. Einstein considered gravitational lensing, but only deflection by stellar masses, and concluded therefore that the phenomenon would most probably remain unobserved. Since "galactic microlensing" refers to unresolved images of an object lensed by things in our galaxy, one could argue "galactic macrolensing" should refer to resolved images of objects lensed by things in our galaxy, but no such object has been seen, and Einstein was probably right in that we won't see it for a long time.

        "Macrolensing", by itself, usually refers many different situations, but characterized by that several images of the object is resolved. There are a few known objects [harvard.edu]. This database includes only multiply imaged quasars, mostly lensed by a single galaxy, but you can have lensing by galaxy clusters as well.

        Actually, the question arised some controversy here among my fellow students as to whether what is known as "weak lensing" should be considered a part of macrolensing, but after consulting The Book, we figured it probably shouldn't.

  • Misconceptions (Score:2, Informative)

    by Marx_Mrvelous ( 532372 )
    I, too, used to think that 'dark matter' was some powerful, mystic thing that sucked in light like a black hole.

    My current understanding is that dark matter is just normal matter that doesn't emit light. For reference, all matter does 'suck in' light (meaning the energy is absorbed, usually given off as heat).

    So, I'm gonna go soon, and eat my dark-matter lunch :)

    • Re:Misconceptions (Score:2, Informative)

      by nerdlyone ( 539405 )
      My current understanding is that dark matter is just normal matter that doesn't emit light. For reference, all matter does 'suck in' light (meaning the energy is absorbed, usually given off as heat). WIMPs (weakly interacting massive particles) are theorized not to interact with the EM field--no photon coupling--so they (theoretically) do not suck light, or reflect it, or interact with it in any way. This is one of the other candidates for dark matter, the more exotic candidate. What this article shows is that they have found "normal" matter that can account for he apparent gravity in galaxies, "normal" meaning regular old EM interacting matter that we can see if only we shine light on it. MACHOs are such normal matter, which we can't see only because they don't emit or reflect enough EM radiation for us to detect, basically rocks in space.
      • the only thing about wimps is that I have not heard of a study that has prooven that you can make a large chunk of matter with them.....if not it realy does not matter since individual subatomic particles don't reflect light in any large amount (since only one photon can strike it at a time......so realy a wimp that can not form macro matter, like an atom, is not any more impressive than a bunch of 3 quarks flying around.
  • It doesn't matter if your dark or light!

  • damn it... (Score:2, Insightful)

    by turbine216 ( 458014 )
    My biggest problem with modern science (physics and astrophysics in particular) is this truly inane method of making "conjectural" observations...that is, assuming that and unobservable activity has been proven simply because something observable has occurred. It's an antiquated way of doing things, and it seems totally backwards. This is a good example...all this time, physicists have assumed that "dark matter" - the matter that provides a great deal of the gravitational force that holds the universe together - is "invisible" or "unobservable" or in some extreme cases "existing in a separate yet intertwined reality". Doesn't it make a LOT more sense to think that dark matter is just the stuff floating around that doesn't have any light bouncing off of it? What, just because we can't see it with our super-expensive orbiting telescopes means that it's invisible? I can COMPLETELY believe the idea that dark matter is just regular matter that isn't being illuminated or is not emitting enough radiation for us to detect! But it seems that this, the most obvious explanation, is the last one that physicists want to believe.

    I really think it's past time for these researchers to change the way they think about the universe. Stop making it so difficult on yourselves. There really CAN be very simple explanations to difficult problems. And sometimes - sorry to tell you this - you're not going to be able to determine EVERYTHING that you want to figure out. That's the way the universe works. Give it time - a LOT of time. Don't come up with unprovable theories to explain irrational phenomena. LET THEM REMAIN UNEXPLAINED UNTIL WE ARE BETTER ABLE TO OBSERVE THEM.
    • the process you have just described is called "the scientific method" by most. it is a well-established and respected system whereby scientists make hypotheses, and then perform experiments to test them. it's been working pretty damn well for the past couple of hundred years.

      the only unfortunate part is that astrophysicists can't schedule and perform their "experiments" whenever they like. they have to keep their eyes open for whenever nature decides to show off some of it's wonders.

    • Well.. neither am I. But that's what science is about. Observation, hypothesis, experimentation, etc.

      It's not BACKWARDS. It's a big universe.. so we need to have a good idea of what to look for.

      I think it was perhaps thought that, if this 'normal matter' accounted for what we saw, we would SEE MORE OF IT, because it's not hidden.

      Oh. BTW. We observe electrons, quarks, and the rest of the subatomic particles only through your so-called 'conjectural' observations. Same with some of the 'properties' of these particles.. they exist purely in a mathematical model that works for a certain set of cases; it's not complete.

      The point is that they think it's likely that, given the amount of 'missing' matter from what we have observed to date, there may be some 'exotic' reason we can't see it.

    • Re:damn it... (Score:3, Informative)

      by kaisyain ( 15013 )
      .all this time, physicists have assumed that "dark matter" - the matter that provides a great deal of the gravitational force that holds the universe together - is "invisible" or "unobservable" or in some extreme cases "existing in a separate yet intertwined reality".

      No they haven't. Let me quote from a Scientific American article on dark matter.


      Astronomers and physicists offer a variety of explanations for this dark matter. On the one hand, it could merely be ordinary material, such as ultrafaint stars, large or small black holes, cold gas, or dust scattered around the universe--all of which emit or reflect too little radiation for our instruments to detect.


      Hey, notice that part where they say a variety of explanations are offered?

      (BTW, what do you mean by "invisible" other than it doesn't have light bouncing off of it?)
    • I hear what you're saying, I too have thought, "Why the hell are they coming up with WIMPs that we have never observed to explain the fact that we just can't see all the necessary matter? Maybe we just haven't seen it because the universe is a dark place." But some particle theories (e.g., supersymmetry) allow (or even require) that particles like WIMPs exist, and if they do, they are a perfect candidate for dark matter.

      But I disagree that they need to change their method of inquiry. This very article shows that, given a bit of evidence, cosmologists are very willing to accept the mundane explanation. And this article also points out that they are in fact pursuing the evidence to support the mundane theories.

      You portray the scientists as "assuming" many things but I don't see it. The fact that they come up with wild theories does not mean that they are ignoring the obvious. It's jus that, in the absence of evidence, either the mundane or the exotic explanation could be correct. So they don't throw either theory out. I don't see them "making 'conjectural' observations...that is, assuming that an unobservable activity has been proven simply because something observable has occurred." This article certainly does not imply that, quite the opposite in my opinion.

    • Too much popular science (Score:5, Insightful)

      by epepke ( 462220 ) on Thursday December 06, 2001 @12:27PM (#2665263)

      I can COMPLETELY believe the idea that dark matter is just regular matter that isn't being illuminated or is not emitting enough radiation for us to detect! But it seems that this, the most obvious explanation, is the last one that physicists want to believe.

      I used to work in a research institute that had a lot of physicists in it, and I think most of them would prefer the mundane explanation. However, they would not rule out wild possibilities, and the minority that preferred the wild possibilities would not rule out mundane explanations.

      I think that your problem may be with the reporting of science, which I agree sucks. One thing I have learned (rather painfully) upon my transition from research science to industry is that scientists operate and think very differently from the way journalists think. The journalist tries to translate what the scientists are saying into what he and/or she thinks is the language of most people. This causes distortion, for two reasons:

      1. There is a distortion of information when it is translated into the worldview of the journalist
      2. The journalist may not be particularly good at understanding the worldview of most people, either

      I dealt with a lot of journalists during my 13 years as a research scientist, and I cannot think of a single instance where the journalist got the story even approximately right. The worldview of the journalist is simply too different from the worldview of the scientist. Very, very few scientists are gifted enough with words to provide alternate explanations, and even when they do, they are usually ignored by people who have read a lot of journalistic reviews of science and love to tell the scientists that they're wrong.

      Scientists love to toss around wild guesses and argue fiercely about them. The reason they do this is that this process stimulates imagination and the generation of hypotheses, which give hints on what to look for. The sky is just too big simply to passively look around and gather evidence that you will synthesize later. That approach might be ideal if we had an infinite number of scientists, but we don't. The next best thing is to have a diverse community of scientists, each looking for a different thing. Most may be looking for mundane explanations, but a few will be following wild hairs. This is not a bad thing, because whether the wild hairs turn out to be supported or unsupported, knowing this information reduces the number of ideas that have to be considered. Eventually, if we're lucky, a consensus eventually emerges. But, remember, this is the first observation of a class of objects, not the last.

      So, some people will be looking for A, and some will be looking for B, etc. Some of them will get evidence that confirms their guesses; some will not, but all will contribute to the sum of knowledge.

      It's a bit like doing detective work. You can't just put cameras everywhere and feed the output into a massive algorithm that solves all possible crimes. Instead, you have to follow leads, guesses, hunches, etc. The only difference in science is that a lot of scientists are doing it, and they tend to keep each other honest.

      Now, the journalist wants to make a good story, above all. The mundane does not make a good story. Neither does the concept of a working hypothesis, a guess, or a hunch. So, the journalist (or ESA public relations department or whatever) writes a dramatic story focusing on the exciting bits.

      Then, finally, when it gets to the readers, they conclude that something is an Explanation from On High, when it is really nothing of the kind. That's just what happened to it in the process of translation through the journalist.

      One thing about science that usually doesn't get around is that the scientist is always in doubt. No scientist is really, deep down, 100% sure of anything. He and/or she may be close to 100% sure, but that isn't a trivial difference; it's a vast chasm in a philosophical sense. This is a very difficult thing to learn, and some scientists forget it. The best scientists, however, do remember it, and some are articulate in describing it, such as Richard Feynman. It isn't a need that most people have to deal with at all, and so explanations tend to be ignored.

      For the notion of "dark matter," nobody is even close to 100% sure about anything. The whole need to look for dark matter is because, without it, the equations and predictions relating to the big bang look ugly and unbalanced. That may seem like the flimsiest of reasons, until you remember that radio and relativity were developed as a result of precisely that kind of aesthetic judgement of Maxwell's equations. It could all turn out to be totally wrong, which leads to another poorly understood aspect of science: the most effective evidence is that which is against an idea, not for it. However, the best way we know of to find evidence against an idea is to look for evidence for an idea. This is another psychological trick: if you are emotionally attached to an idea, you will try much harder to show it is correct, and a failure to do so means more than a failure of a casual effort. If you do unintentionally distort evidence to support your hypothesis (this happens all the time, far more than outright fraud), there is always somebody else who will poke holes in your ideas. This is good, not bad, but it's very hard to translate that into the language of most people, where auditors are the enemy, not friends.

    • Re:damn it... (Score:2, Informative)

      by Doctor Fishboy ( 120462 )
      > My biggest problem with modern science (physics
      > and astrophysics in particular) is this truly
      > inane method of making "conjectural"
      > observations...that is, assuming that and
      > unobservable activity has been proven simply
      > because something observable has occurred.

      OK. We know from the distribution of light and the measured rotational velocities of most galaxies we can see that they seem to be embedded in a large halo of gravitating mass. This has been measured and confirmed many, many times over that past 40 years.

      When you add up the total amount of emitted light from a galaxy, you can get an estimate of it's mass and that turns out to be about 10**12 solar masses, say.

      Looking and the dynamic motion of the galaxy using the Doppler shift of spectral lines from stars in the galaxy, you calculate that the required amount of mass for the galactic motion is roughly 100 TIMES the amount you count up by counting stars/gas/glowing stuff alone.

      1) Maybe this 'dark' matter is not being illuminated by stars? No - do the calculations and it turns out that this stuff would be detectable. Instead, we see nothing. So, we can rule out baryonic (protons, electrons, photons) matter. Therefore, it has mass but it doesn't interact with baryonic matter - it is only gravitationally coupled with baryonic matter.

      2) Maybe it is condensed into cool stars that we can't see? Again, no luck there. Really dim stars are hard to detect, but over the past 5 years, enough have been detected to make a guess as to whether dark matter is this form. There isn't.

      So, we still have no clear idea what dark matter is made up of, but a lot of ideas that we can test. I'll admit that it's incredible, but believe me, there's a lot of evidence for dark matter. Alternative hypotheses, such as modified long-range forces have been tried out and don't work (and no, it's a separate issue from non-zero lambda cosmologies!) so we are back into the 'small, energetic, low mass subatomic particle' game.

      What we are NOT doing is inventing dark matter, as you imply. We tend to leave it to the mystics.

      If you're interested in the more detailed reasons why, please feel free to contact me.

      mak at as arizona edu
  • as a matter of fact, it's all dark.

    The funny part is, within 90%-95%, this is really true of the entire universe.
  • Wonderful bit of observation.

    But the teams are going to need to be funded so that they can do a complete survey of a larger area of the sky, and begin to get a bound on the number of MACHOs/galaxy or /unit space. That will let us get at least a rough estimate of how much of the universe's dark matter is bound up in these MACHO's.

    Anyone know what the longer term funding situation is here? Is it NSF funded?
  • Am I correct in interpreting the phrase "Dark Matter may consist of massive compact objects (MACHOs), such as dead or dying stars (neutron stars and cool dwarf stars), black holes of various sizes or planet-sized collections of rocks and ice" as basically meaning "Dark Matter may just be ordinary shit we already know about but in this case just can't see" ?


    I mean, not that dying stars or black holes are merely ordinary, but "Dark Matter" sounded so much more mysterious.

  • The fundamental problem here in finding this stuff is that it's all pretty much black. And the basic colour of space, you see, is also black. So how are we supposed to see it?
  • Dark Matter is not as dark as previously believed!

    Does this mean that the jedi knights are winning with the fifth element weapon?

    Star Wars episode III: The Perfect Element, Source of Clones!
  • The article says that they observed the microlensing event several years back and then recently took another look at the area using Hubble. They found a faint red dwarf which probably cause the lensing of the nearby (arc second wise) main sequence star.

    So, if we can observe the 'dark' matter as being a red dwarf, it's not exactly 'dark' is it? I would assume that objects like red dwarfs, if observable, would have already been counted in the total 'bright matter' column. If not, someone is just undercounting objects that are observable using normal astronomical methods, and needs to go back and make a better estimate of how many of them are out there.

    -josh

  • Interesting Dark Matter Properties (Score:3, Interesting)

    by BadBlood ( 134525 ) on Thursday December 06, 2001 @12:23PM (#2665240)
    If in fact dark matter is matter which exists gravitationally but will not interact w/the EM spectrum, wouldn't you be able to feel it with your hands but not see it? (Provided you have a small bit of it nearby)

    Then, couldn't you somehow use this "material" for stealth purposes? Body armor making you invisible, etc. etc.

    I find it amusing that as humans, we can only detect the existence of something if we can collide EM particles w/it (photons, etc.) We should rephrase a familiar motto to be "I can interact w/EM particles, therefore I exist." :)

    • Re:Interesting Dark Matter Properties (Score:2, Interesting)

      by Anonymous Coward

      If in fact dark matter is matter which exists gravitationally but will not interact w/the EM spectrum, wouldn't you be able to feel it with your hands but not see it? (Provided you have a small bit of it nearby)


      No. If you touch an object, you don't feel it because it's interacting gravitationally with you. You feel it because its atoms are electromagnetically repelling with your atoms. If it didn't interact electromagnetically, it would pass right through you, just like neutrinos do. The mass of your body is way to small to be any hindrance as far as the gravitational interaction is concerned.


      I find it amusing that as humans, we can only detect the existence of something if we can collide EM particles w/it (photons, etc.)


      When LIGO goes online, we should be able to directly detect gravitational radiation, as opposed to just electromagnetic radiation. (Of course, we need electromagnetic radiation to read the instruments...)
  • In my kid's diaper..
    In that catbox....

    Oh wait.. that's not what you're talking about, is it?
  • Maybe the Hubble lens has a smudge...

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