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Space Science

Dark Matter Measurements 246

Posted by michael from the laden-or-unladen dept.
ksp0704 writes: "According to this article at space.com, scientists have finally measured the approximately 90% of the universe we can't see (the dark matter)." I'm sure it will continue to be a topic of debate for years, but two independent measurements agreeing is a good sign.
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Dark Matter Measurements More

Dark Matter Measurements

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  • "According to this article at space.com, scientists have finally measured the approximately 90% of the universe we can't see (the dark matter)."

    Pardon me, but we can apparently only see certain bits & pieces of the universe, right? So, how the f*ck is it that we know exactly how much ELSE there is out there? Isn't this like saying "I have 3 Weezer albums, and I just figured out the names of the songs on their 4th, 5th, and 6th albums (even though I have NO IDEA how many more albums they'll make) and I now know the name of every Weezer song.

    • As I understand it, they can estimate how much matter there *should* be, based on what we know of the universe and of physics, and also based on the matter was can see now. They aren't just pulling the numbers out of thin air.

      Always remember: Weezer ain't rocket science! :)

      • Re:Uhhhh (Score:3, Interesting)

        by crashnbur ( 127738 )
        "They aren't just pulling the numbers out of thin air."

        Heh. What's the difference, if it's all just theoretical anyway? I mean, really, how is the amount of matter in the universe ever going to mean anything more to us than simply a numerical value?

        Of course, one can assume that, by knowing the ammounts of normal matter compared to dark matter as they change, scientist could predict approximately when the universe would collapse on itself. You know, if the big bang theory has any truth to it. Of course, that prediction wouldn't mean much to us either, as our sun will likely die out long before the universe itself will.

        • Re:Uhhhh (Score:3, Interesting)

          by CtrlPhreak ( 226872 )
          When the whole universe collapses because of the reversal of the big bang energy by gravity, then you may care.

          One of the biggest debates IMHO, is whether the gravitational pull of the universe can overcome the expanding motion of the universe. This expansion caused by the big bang was theorized to be decreasing and gravity would eventually overcome it, thus pulling the entire universe back together in the same manner of pre-big bang time. It could also be said that this would cause the universe to be a periodic function of explode, expand, contract, explode... The problem with this is that there is not anywhere near enough matter in the universe to create a gravitational pull strong enough to overcome the big bang energy. There is also not enough visible matter to explain many gravitational effects scientists perceive. Thus, dark matter was theorized to explain these phenomenons. However, it could never be measured. This could go a long way to supporting various theories about the universe and it's workings.
          • To achieve escape velocity from the universe itself is a mind-boggling concept. Okay, assume that it can be done (or has been done). Where does the matter go? Does it simply spread and cool? If so, what happens when black holes get hold of it? Recompression!

            What happens when black holes get hold of other black holes? Does the bigger suck in the smaller? Do they combine into a super black hole? Are they the force behind my theory that the universe will recompress itself for another bang?

            • I wrote a little manifesto I'm just gonna call "The Manual for Preservers" for now. Anyway, the manifesto was about mankind and other species getting together and hording black holes together and maintaining a galaxy for us to live in together regardless of the expansion of the universe. I'm no physicist, but the only potential hole I've found in the idea is this:

              Does the minimal distance between fundamental particles expand?

              If it does, then nothing can stop expansion short of sucking in matter from another universe. Heh, real likely.

              If it does not, then the plan could still work. Other than the difficulty in carrying it out, I haven't seen any other reasons why the theory itself is invalid.

              Let me know whatcha think,
              -l
        • How about I speculate; we discover that dark matter exists, and that it outnumbers regular matter 9:1

          Knowing it exists, we can start trying to find, create, and control it.

          N years later we have dark constructs.

          Without even having the theories about the universe showing there is dark matter, we don't find it or about it's properties, capabilities, and uses.
    • I believe this is more along the lines of measuring the circumference of the earth without a direct measurement, deriving it's value from other measured quantities.

      The basis of dark matter is that, assuming the law of gravity is true, the universe is too light to explain certain phenomena. What we can see isn't enough mass, so there must be some dark matter out there we can't see to account for measured effects.
    • Let me guess, you didn't read the article, and therefore have no clue how such measurements are made, and rushed your reply in so that you could nab a first post? Here, let me help you out:
      According to the leading theory, an enormous nuclear explosion called the Big Bang happened 13 billion to 15 billion years ago. From it, the universe appeared in an instant, but as a billion-degree mess of neutrons, protons and electrons. The explosion was so energetic that nothing could come together close enough, for long enough, to form atoms. But the universe expanded and cooled so rapidly that within three minutes protons and neutrons bonded in twos and fours, and formed all the atomic nuclei in the universe. This Big Bang Nucleosynthesis determined how much normal matter would ever exist.

      Just how much matter that was can be estimated from observing the most recently formed stars and galaxies, because they are fueled by the hydrogen atoms formed from those original nuclei of twos.

      Fields explained that young stars, like our Sun, are just now fusing that original hydrogen into helium whereas older stars fuse helium into oxygen and iron. Because the hydrogen fuel has not been converted, scientists are able to measure the proportion of original normal matter to dark matter.

    • The reason is that Galaxies are screwed up (Score:5, Insightful)

      by Alien54 ( 180860 ) on Wednesday October 31, 2001 @12:10AM (#2501185) Journal
      So, how the f*ck is it that we know exactly how much ELSE there is out there?

      They look at the galaxies, and estimate how many stars and stuff there is in the galaxy. Any rotating galaxy. And They figure out how fast the galaxy is rotating.

      They notice a problem. For any rotating galaxy there is not enough star stuff to hold the galaxy together. The spiral arms should never be there.

      The star stuff in the galaxies do not have have enough gravity to hold galaxies together. Galaxcies should not exist at all. Stars should be all flying about because that is how weak the gravity is.

      Just how much too weak? The Star stuff has one tenth the gravity needed to do the job. so something has to be doing the other 90%.

      That is what the dark matter is. It is a term to label what the other 90% is. The don't know what it is yet. but they are working on it.

  • Why its important (Score:1, Informative)

    by Anonymous Coward
    What makes the size of dark matter so important is that it will decide whether the universe continues to expand outward from the big bang, or eventually begins to shrink back to a singularity. Considering the universe started as a singularity, having it end that way may be indicative of another universe after this one. If everything continues to spread out, entropy will come out the clear winner.

  • Okay, I just read the article, and I'm down with it. There is nothing illogical about it, assuming their methods actually work, and I have no evidence that they don't. But, after re-reading the section on the "Creation of normal matter", I have one complex question that I hope can spark a bit of discussion:
    What caused the big bang? How was it initiated? What were the bounds of the "universe" as it were before the big bang?
    And how on earth (pun intended) did we get here from all of that? :-)
    • What caused the big bang? How was it initiated? What were the bounds of the "universe" as it were before the big bang?

      Stephen Hawking touched on this a bit in one of his books. The questions are largely irrelevant because the laws of physics would have been completely different "before" the Big Bang. Much like they theoretically break down completely in a singularity.

      I honestly can't explain it better than that. I suggest you pick up a copy of "A Brief History of Time", which is a pretty good lay-person's book on astrophysics and quantum mechanics.
    • Why does the big bang have to have a cause? The idea of a chain of events, each causing the next in the sequence, is a bit passé these days. If you ask for a cause for the first event you quickly lead to an infinite regress. What's the problem with there being a first event without cause? I hope you don't think that because most events have causes they all do. That's a bit like thinking that all integers are non-zero because most of them are.
      • "I hope you don't think that because most events have causes they all do. That's a bit like thinking that all integers are non-zero because most of them are."

        I was going to argue against that, but then it hit me: that's a pretty deep statement. I'm not touching it. Well said, my friend.

        However, I do believe that things must have a cause. If it has no cause, then what is its reason for having no cause? Everything has a reason, even if it is a loophole. God either does or does not exist, but there is a reason for it. The big bang theory either is or is not true, but there is a reason for it.

        I mean, com'on, things are event-driven or object-oriented, and without objects, one must only assume that an event triggered what was to follow - the big bang. Something was there to explode, and something had to cause it. Did one of the tiny dark matter particles spark up the wrong way and set it off?

        I guess we can apply the same reasoning to the inception of the big bang as to the reasoning for what we think will happen after the great universal black hole sucks it back in: We don't know, we can't know, we won't know, so why worry about it?

        Because it's damn fun!

        • 42. What was the question again?
        • ...one must only assume that an event triggered what was to follow - the big bang. Something was there to explode, and something had to cause it.

          Once you manage to unlearn that, you'll be on your way towards understanding relativity.

          You're still assuming that spacetime has linearly measured dimensions with regularly spaced tickmarks everywhere. It doesn't [nasa.gov].

        • Everything has a reason, even if it is a loophole. God either does or does not exist, but there is a reason for it. The big bang theory either is or is not true, but there is a reason for it.

          Here's the loophole in your argument.

          According to Godel's incompleteness theorem, in every nontrivial logical system there exist statements which are either simultaneously true and false (such systems are generally frowned on) or are impossible to prove either true or false.

          Mathematics is one of the later. Thus there are statements in mathematics which can be written down but never proved true or false (no easily explained examples exist). It's possible that such a property can be correct, in that it does hold in all possible cases without being able to prove that it does, and of course we can't actually test all cases to know that way.

          By extention it doesn't follow that there is neccesarily a reason for the big bang being true or not true. There doesn't strictly have to be any explanation for why it is the way it is.

          Of course this is a somewhat silly argument because there probably is a good deal that can be explained about the big bang, and much of science rests on inference and not proof (in the mathematical sense), but it is interesting that even in mathematics there are things for which there can be no reason (ie. proof).
          • According to Godel's incompleteness theorem, in every nontrivial logical system there exist statements which are either simultaneously true and false

            Does that include Godel's theorem?

        • look at my other post

          here [slashdot.org]

          Did one of the tiny dark matter particles spark up the wrong way and set it off?

          You're neglecting to think of other dimensions. This universe could have simply been created by a "black hole" in another dimension, thus requiring nothing "in this universe" to have set it off.
    • That's actually a fantastically good question. Unfortuantely, it's also one that we have no equipment to answer right now. Since we have no measurements of time before our universe (time didn't exist, at least not our time), we can't really apply the scientific method to that question. Maybe someday, we'll figure out a way of constraining theories about the "meta-universe" in which our universe is embedded. But for now, it's complete speculation and essentially not science.
    • What caused the big bang? How was it initiated? What were the bounds of the "universe" as it were before the big bang?

      There was nothing before the Big Bang as it was the beginning of existance. It's sometimes a difficult answer to accept, as you've been accustomed the existance of time all your life, but you have to realize that this wasn't always the case. If you're still having trouble there are some chemicals you can use to help make it clearer [erowid.org].

      - j
      • existAnce? Hrmm.. er .. existEnce. DMT taught me a lot but it never taught me to spell.

        - j
      • That something is difficult to accept does not mean that I won't, but that it is accepted by others does not mean that it must be accepted. There is no proof for any of this - it's all largely theoretical. We take in the evidence and believe what makes sense to us. That's the beauty of it, I think. When enough reasoning comes through to discard old theories, the new ones will take over, but there will always be others to share the limelight. Still, you make a good point...
      • Somehow I doubt the ingestion of DMT will answer that question for you. It may come in one of those acid-fueled delusions where for about 5 seconds, it's all clear to you, the answer to every question you've never had an answer for unwinds to you, and you feel godlike. Then it slips away. Too bad science isn't based on hallucinatory thought.

    • Re:Creation of normal matter (Score:5, Informative)

      by efuseekay ( 138418 ) on Tuesday October 30, 2001 @11:03PM (#2500999)
      Excellent questions. Only problem is that it has no relevance to the "Creation of Normal Matter" the article is talking about.

      The "normal matter" they were talking about are baryons (electrons, protons, neutrons etc and their composites). And the "creation" they talk about is "Big Bang Nucleonsynthesis", which is when protons and neutrons and electrons and stuff fuse together to make H, He and Li. The ratio of the production rates of these stuff implies certain "wiggles" in the CMB spectrum, so gives us a gauge (with lots and lots of caveats the scientists don't tell you) to the so-called "baryonic density". (Dark matter, by definition, do not interact with baryons, so it's hard to measure them since all the tools we have are made out of baryons.)

      Big Bang Nucleonsynthesis (despite its name) can occur without a Big Bang : we just need the Universe to be Very Hot and Dense at some point.

      Your questions about the origins of Big Bang is a much deeper and harder question. While it seems a philosophical argument, it is recently being attacked by some theorists. Most of the time, they just ask the question : do we need a Big Bang that starts from a singularity? The answer, with our current observations, is a BIG NO. But then they have to figure out a better alternative that can give us a very hot and dense Early Universe (so we can have Big Bang Nucleosynthesis, which is a very very very very well observed and constrained theory : i.e. it's fucking correct.)

      • "i.e. it's fucking correct"

        I would rather suggest that it's the best we've got right now, and it will have to hold until we develop it further and either prove it correct or prove it incorrect (or wrongly accept or discard it).

        • BBN is a robust theory. It's so robust that if you tried to even modify it a bit, you'll get nonsense. Basically, it uses a very very simple idea (the Boltzmann equation, which statistically evolve a set of particles in phase-space with any arbitrary interaction terms), put it into a large computer, and churn out the results.

          (Some) People question the validity of Einstein's General Relativity, but the confidence in BBN is unshakable. The simplicity of it (you can teach it to a bunch of people in an hour), and the fact that it actually predicts to extreme accuracy what we see in the Universe (eg. it predicts 75% Hydrogen, which we see), makes it an extremely hard theory to break.

      • Your questions about the origins of Big Bang is a much deeper and harder question. While it seems a philosophical argument, it is recently being attacked by some theorists. Most of the time, they just ask the question : do we need a Big Bang that starts from a singularity? The answer, with our current observations, is a BIG NO. But then they have to figure out a better alternative that can give us a very hot and dense Early Universe (so we can have Big Bang Nucleosynthesis, which is a very very very very well observed and constrained theory : i.e. it's fucking correct.)

        It may just be that I've forgotten all my college physics out of disuse, but is there any reason the Big Bang couldn't have been preceded by a Big Crunch? That would allow for a very hot and dense universe, and would also very neatly (too neatly?) answer the question of what was there before the Big Bang.

        So what's wrong with that picture? Not arguing, just seeking enlightenment...

        • I will add here some notes in addition to the other response to the your post (the oscillating universe has been around for a while, but observations have ruled out the so-called "idling" universe : if you believe in General Relativity which most people do.)

          There are recent revival of the idea of the "big crunch before big bang universe". People have thought about these some time ago, but it ran into a problem called the "weak-energy condition". Basically, the equations of general relativity which describes the "motion" of the universe, says that once the universe goes into the "big crunch mode", it will hit a singularity which is an infinity. But this means that we do not know how to "connect" the big bang to the big crunch "continuously" (discontinuous things are non-causal and unphysical). Recent attempts by some prominent theorists try to by-pass the WEC by finding some loop-hole so that the big crunch never hits infinity. The results are kinda mixed (I don't know the exact details.)

          Of course, people are working within the confines of current known physics. There may be some unknown undiscovered processes that allows us a way out. The verdictis still open.

    • What caused the big bang? How was it initiated? What were the bounds of the "universe" as it were before the big bang?

      In short, we don't really know. Some speculate that this universe was created from another universe. By this I mean that our universe may have been created in such a way that a bubble gets blown from a wad of chewing gum. But where did that universe come from? Others speculate that our universe started when a quantum particle "came out of nowhere" and then inflated (the big bang) into our universe. Many of the people that beleive this think that our universe will eventually deflate (the big crunch) into nothing once more. At that time, humans maybe able to escape to another more younger universe by traveling through higher dimensional space.

      Some good books dealing with these subjects that I've read/currently reading are:

      - Hyperspace [amazon.com] by Michio Kaku

      - In Search of Schrondinger's Cat [amazon.com] by John Gribben

      - The Elegant Universe [amazon.com] by Brian Green

      All are very good. Hope that helps.
      • Just to play devil's advocate a little; I really didn't think that Green's book was as fantastic as it was made out to be (by some, anyway).

        He bases a lot of his statements and arguments on the assumption that our (everyday, in a physicist's day anyway) math is the "right" math for modeling the universe. By this I mean that it doesn't seem as if he's considered the possibility that some of the mathematical objects he assumes model the universe in fact do so, just because they seem to work okay for everything we've used them for before.

        I don't pretend to fully understand what the character of an "other" math would be, nor am I an expert on string theory. I'm just trying to make a statement about Green's book. Some of his arguments didn't quite come off. That isn't to say that I don't think you should read it, just that you shouldn't take it as gospel, despite his credentials.
        • I think that the only thing you can prove with our math is that it doesn't apply here.

          in the beginning there was nothing, for an infinite amount of time, so after infinity you would have 0(zero) * X (infinity) = 0 (zero)
          For me this proves that our math is at least flawed in these cases.
          my 2cts

    • What caused the big bang? How was it initiated? What were the bounds of the "universe" as it were before the big bang?

      While it hasn't quite graduated to being "accepted wisdom" yet, an increasing number of those who look at such questions are persuaded by the idea that a big bang can occur when a black hole collapses, with the rebound from the (almost) singularity creating a new space-time manifold shifted slightly away from the space-time manifold which produced that black hole.

      One site I found [mkzdk.org] has more extensive discussion of this scenario.

      how on earth (pun intended) did we get here from all of that?

      It seems like you might actually need the physice of our cosmos to be tuned to the production of carbon, oxygen, etc. by nucleosythesis of the original hydrogen and helium for the black hole-big bang production cycle to work, so the earth and indeed us are just incidental byproducts of these cosmic requirements.

      "Why?" doesn't always have an answer.

    • Re:Creation of normal matter (Score:5, Interesting)

      by dragons_flight ( 515217 ) on Wednesday October 31, 2001 @12:36AM (#2501267) Homepage
      What caused the big bang? How was it initiated? What were the bounds of the "universe" as it were before the big bang?

      Sometimes bored physicists do try to give serious thought to this. Being a physicist, I've sometimes gotten to listen to what others consider to be serious thoughts on the matter.

      Basically there are too camps, people that want the universe to be timeless and exist forever and people that want the Big Bang to be the ultimate start of things. People in the first group will given you various stories about the cyclical nature of the universe (usually expand, collapse, repeat), or some notion of universes spawning other universes, ad infinitum.

      People who believe that the Big Bang was THE START of things tend to either believe it to be uncaused, caused by God, or unknowable and irrelevant. There are a few however in this camp that try to posit explanations of what did cause the universe out of nothing. Some bring in exotic theories (such as string theory) to try and construct physical laws that can hold before, during, and after a big bang event. Of course these people also have to change the nature of a big bang away from that strictly based on general relativity (which implicitly prevents any meaningful reference to a "before" the big bang).

      One of the most interesting stories I've heard is that the fabric of space has the property of being unstable in a total absence of energy, and at any moment and any location, there is infinitesimal but non zero probablity that it will transition to a different state which has energy, which then billows out into the rest of the universe. So basically the vaccuum has certain properties that exist forever and are timeless, and the big bang has a chance of spontaneously erupting simply because it has never happened. Hence the universe, as we expereince it, has a single well defined start within a larger timeless existence.

      As absurd as this might sound, this is quite serious, and as reasonable as many other things people say about "before" the big bang.

      Ultimately though, it only transfers the problem of first cause to the "fabric of the universe" and the basic physical laws governing everything. While science may be able to tell you that something is NOT the first cause, it can never say with certainty that something IS the first cause. As far as I'm concerned, whether you choose to believe that the chain of causation goes infinitely backward or has some definably beggining, is a matter of faith.

      • So basically the vaccuum has certain properties that exist forever and are timeless

        Ah! That's exactly what I was talking about in my other post!---not necessarily a vaccuum, of course, because I was non-specific---just that there is a self-existent reason, versus, "no reason at all" which seems just false to me. The infinite chain theory would fall under this category, too, I guess...

        Anyhow, cheers,
        -l

    • I have a very perplexing question!

      I believe that there is no such thing as "random". You can have pseudo-random, but what we commonly call "random" is really "not having sufficient information to predict an outcome". Everything is a series of causes and reactions. Similarly, nothing can happen without a cause. The cup cannot fall off the table on it's own, and if the cup was on the table and is now shattered on the ground, then something happened to cause it to do that. (Before you ask, I don't particularily think that it will ever jump back up onto the table and reassemble itself, ... ever!)

      So, having established that, it means that everything that I do, everything that I will ever do has been laid out already by a complicated series of causes. If nothing happens without a cause, then everything that I do (which are all actions) will have had a cause, and thus I have no free will. I have no choice in anything that I do, it has all been decided ahead of time, just as everyone else's actions. We are all automatons.

      Obviously I can't accept that. Thus I have a paradox.

      Anyone care to contribute? I don't claim to be up to date with the latest in anything, but I came up with this problem a few months ago while talking to a friend about some of my other theories and it has been bugging me since.
      • Congratulations, you've stumbled on the ages old conflict between determinism and free will.

        Maybe you have no free will? There is a whole school of philosophy that argues free will is an illusion and we are all really automatons.

        However, determinism went out of vogue with the maturity of quantum mechanics. There is a body of evidence (called the No Hidden Variable Argument) that makes a well accepted case that quantum mechanics truly is random from the point of view of everyone inside the universe. In particular the argument goes that waves are not merely a mathematical tool for judging where a point particle might be, but in fact the wave IS the particle and it IS spread out everywhere that the wave allows, simultaneously. Thus there are no true point-particles only wave-particles.

        Randomness comes in through what is refered to as "measurement", waves are allowed to exist in multiple states spread across a significant region, but under certain conditions they collapse to a single state often having a point-like size. According to the best known quantum mechanics this process is neccesarily random to everything inside the universe, because there are NO factors that one could know that would tell you which state it will end up in among all the states allowed. How the quantum state actually decides is beside the question because nothing in the universe can ever have access to information that will tell you about the process.

        Whether God rolls dice, or there is so guiding over will, or even a deterministic mathematical function, doesn't matter because we will experience it as random. Also, it is important to note, that whatever principle governs the evolution of quantum states, it maintains the strict probablity distributions. There is no evidence that God (or whatever) ever decides to evolve all the particles in a way that would appear unrandom and not follow the probablity functions.

        This whole discussion allows free will a loop hole. If your soul or some other guiding influence exists outside the realm measurable to us and directs the evolution of certain QM states, then it can have an impact on this world. Note that the fact that there appears to be true randomness in the universe does not imply there has to be free will, after all God could just roll dice and not give you any more real control over your life. Of course QM might get replaced someday by a theory that doesn't involve unknowable processes...

        One last comment, true free will has to come from outside the chain of causation. If you want to include free will then you have to insert it somewhere where there plausibly is no connection between the cause and a specific effect, thus giving the will something to choose (which effect, ie which quantum state). There is also a whole discussion about false free will, or "good enough" free will which deals with the appearance of choice resulting from chaotic and complicated circumstances that could never be fully understood. It's chaotic, so I could never understand it well enough to accurately predict it's behavior so I might as well treat it as if it has free choice. Not exactly what you want, but it maybe ultimately all we have.
      • Okay, I agree with the concept of empiricism which you explain very well. My answer to the paradox is simple, yet difficult to comprehend:

        There are two "levels" for the concerns of this argument. The first is human thought. The second is that which exceeds human thought, that which drives the universe - this empirical nature of it all, as you explained.

        On our level, we make the choices, because we know not of our causality. On a higher level, it is all decided for us by the natural, biological, and highly complex conditions which have created all that we are and will be and will do.

        I wrote something very similar to what you wrote on my weblog [neotope.com] a couple days ago.

  • .1 releases are better. (Score:5, Funny)

    by Soko ( 17987 ) on Tuesday October 30, 2001 @10:41PM (#2500922) Homepage
    All the "normal stuff" is thought to have been made in two steps, one occurring when the universe was roughly three minutes old, and the other some 300,000 years later.

    See? Even "God" needs to get it in production, then issue a revision some time later before it's really running right. :0)

    (P.S.- My first thought was of "Dork Matter", but then I saw the StarWars DVD ad on the page. *sigh* Too easy...)

    Soko

  • Dark Matter? (Score:1, Interesting)

    by Apreche ( 239272 )
    So apparently they think that the part of the universe we can't see is dark matter or something? And they seem to be able to guess how much of it there is. I believe that the universe has always existed, and will continue to exist forever. It is also infinitely large in every direction. The only reason we can't see the rest is because the light hasn't gotten here.
    When you shine a flashlight at a wall you can see that light spreads out as you move farther away. The stars are so far away that the light does not reach the little tiny tiny earth.

    Even better. Maybe the universe wasn't always here, and it had a "creation" date. We think the universe is "expanding". Maybe it's because that light from that far away takes a certain amount of time to get here. So light from farther away places is arriving here for the first time ever. if we can figure out how many light years the farthest away thing we can see is, then we can figure out how old the universe is.

    I still think it's amazing that when you look at the stars in the sky that you are looking billions of years into the past. Those stars you see where there before dinosaurs were here, and they might not even be up there anymore.
    • My beliefs are along those lines, but there are always bits of the argument that make no sense, largely because human thought is unable to comprehend the truth behind it (which is why we don't know the answers).

      For instance, say that the universe does go on forever. The concept of infinite space is easy enough to grasp, I think, but wouldn't that mean that there is a point somewhere were matter just doesn't reach? I mean, is matter infinite too? How cuold that be possible? Unless, of course, space is directly proportional to matter in the same incomprehensible manner - space is infinite just as matter is infinite, and neither can be explained by our extremely limited scope.

      • Yes if you believe in a homogenous and isotropic Universe.

        You can construct all sorts of universe where there are "matter" only in some parts and not others. But they violates our observations (we see stuff everywhere). Of course you can say "oh we don't see the parts where there are no stuff". You are allowed to say that, but then "why bother if we never see them?". Basically, postulating that there are "empty" regions that we cannot see violates Kant's principle that theories need to be falsifiable.

      • Picture the surface of a balloon. The galaxies are dots on the balloon.

        Now inflate the balloon. Any galaxy will see all the others moving away, speed proportional to distance.

        You'll notice that since the surface of the balloon has no boundaries, you can go forever in any direction (assume the balloon is expanding so that the circumference is growing at a rate faster than the speed of light, so you can never get back to where you started).

        There's our 2-d curved space. We live in a 3-d one. It may not be curved that way, and galaxies change the local curvature (think the dots sinking the surface of the balloon)..but you get the idea. We don't need infinite matter for a boundless universe.
      • Sure.... why is it that no one ever talks about an infinite number of big bangs occuring constantly? Our big bang might just be a fairly small one compared to the one that happened last night a gazillion miles away. Our little "universe" could simply be a little puddle of matter in our little side street of the real infinite universe.
    • I think you're misunderstanding the evidence for the Big Bang. We can see that galaxies are moving away from us, in accordance with Hubble's Law. We know that they are moving away from the Doppler effect: they are reddened (or they are abiding by totally different laws of physics). This means that whatever theory you adopt, it has to account for this apparent motion. This is pretty tricky, when you think about it. It has been attempted. Hoyle and others tried to create a steady state universe, with no beginning and no end. But when the measurements started coming in, the Big Bang started winning out (in addition to the fact that the steady state model was never really liked on purely aesthetic grounds). For example, the Big Bang correctly predicted the Cosmic Microwave background. (If the universe had always existed, where would it have come from?) The Big Bang also correctly predicts the relative fraction of elements in the universe. So any theory you come up with will be going toe-to-toe with a pretty beefy theory, depsite many claims to the contrary.

      A better to way to date the universe than using the method of your second paragraph would be to simple figure out the Hubble Constant, this allowing us to figure out the current rate of expansion. Playing the problem backwards gives an age for the universe. The trouble with your method is that there is no clear way to date how old the stars you see are. Light doesn't age as it moves through the cosmos, remember.
      Now, dark matter... It's called dark matter because we cannot see it with light gathering telescopes. But we can detect it. If it were not present, galaxies would be flying apart according to understood laws of physics. Similarly, galactic clusters would disperse. So it isn't like we can't detect the dark matter. It's like extra-solar planets. We've detected most of them indirectly by watching their stars. Still, most people think they are there.
    • Hrm. Nice try, but we know the universe is expanding because of red shift. How far we see into the universe is limited to our ability to resolve the tiny amount of light from distant objects. Currently the farthest we can see relates about 6 billion years or some such. When we get a better telescope in orbit, we'll be able to see farther.

      You can have whatever creationist theories you like, but you can't contradict what we *know*.

      I still think it's amazing that when you look at the stars in the sky that you are looking billions of years into the past. Those stars you see where there before dinosaurs were here, and they might not even be up there anymore.
      That's a very humbling thought. Not enough of humanity gets put in their place by the sight of millions of stars anymore. Gives me hope.
    • I believe that the universe has always existed, and will continue to exist forever. It is also infinitely large in every direction. The only reason we can't see the rest is because the light hasn't gotten here.

      Hmm.

      The universe is infinitely large, and infinitely old.

      Since the universe is infinitely old, light from luminous objects in all parts of the universe has been radiating for infinite time.

      But you think light from distant objects "hasn't gotten here yet?"

      I wish I'd had you on my side during my last tax audit!
    • I'll go in reverse order:


      The stars you see are all within our own galaxy, which is "only" about 100,000 light years across, so all the star light is from well after the dinosaurs bit it. There is a lot of light from other galaxies that are over 65 million light years away, but it's relatively faint, so you've probably not really noticed it.


      Yes, by seeing farther away objects we can raise the lower bound for the age of the universe. Astronomers are working on that. Yes, we think the universe is expanding, and we have a lot of proof for it too, mostly the red-shift that Hubble (the man, not the telescope) noticed. And yes, maybe the universe had a creation date (you've heard of the big bang theory, right?). (How did this get modded up?)


      Okay, top paragraph: distance doesn't stop light, only matter, and there's relatively very little of it in space. Light from far away objects /will/ eventually reach us, it will just take a long time, and be very faint (as you mentioned, light from a flashlight gets dimmer, in proportion to the square of the distance to the wall). In addition, due to the expansion of the universe, that light will be red-shifted. For very distant objects, the light is shifted completely out of the visible spectrum.


      Your belief that the universe has always been here is unsupported. (Again, big bang theory.) You're belief that it is infinite in all directions is also unsupported.


      Yes, they can guess how much dark matter there is. By definition, the part of the universe we can't see is dark matter...it's called that because we can't see it. It's not any different from normal matter (as far as we know), it's just not emiting any light, so we can't see it. And by the calculations for the estimated mass of the universe, and the calculations of the total mass of all the matter we can see, yes, there must be some (up to 90% of the total mass, by the measurements in this article).


      (I ask again... why did this get modded up?)

  • There is no dark matter of the universe.

    It's all dark.

    -nb
    • Did no one else get it? Am I the only Pink floyd fan reading here? Or maybe no one had the moderator points to use. Anyway, to paraphrase a little bit closer to the original:

      "There is no dark side of the universe, really.
      Matter of fact, it is all dark."

      (Kudos to nukebuddy for thinking about it first!)
  • I read an article in Scientific American the other day that talked about a so-called "Dark Energy" that is said to make up a large part of the "mass" of the universe that we can't see. Dark energy was defined as being evenly distributed forces throughout the universe that posessed anti-gravity. That is, they repelled each other instead of attracting.

    I'm only in high school physics, maybe someone more familiar with the field could provide an explanation and how it relates to the facts presented in this article?

    • Dark matter is different from dark energy. Basically, dark matter is just that -- normally gravitating matter which does not emit sufficient EM radiation to be detected by us, and is therefore, dark. That would include such things as brown dwarfs, and massive neutrinos. Pockets of gas that are in thermal equilibrium with the microwave background would also appear "dark" to us, as they would emit radiation with the same distribution as the CMB. Dark matter need not be anything particularly "exotic" (in the colloquial sense of the word).

      Dark energy, on the other hand, is a "cosmological constant"-like force that causes the repulsive force that leads to an acceleration of the universe. The acceleration has (tentatively) been observed in surveys of high-redshift supernovae. It was initially surmised that this force could be caused by quantum mechanical vacuum fluctuations (much like the Casimir force). Unfortunately, the "cosmological constant" those fluctuations would produce is off by an astounding (IIRC) 50 orders of magnitude!
      • You're right that "dark matter" does get used in the sense of things that aren't emmitting enough light for us to see. The article in question however is using it exclusively in the sense of (non-baryonic) matter which does not interact with electromagnetic radiation and thus can never emit light. Things in this category would be exotics such as nuetrinos, WIMPs, and a variety of other things.

        For the record the bright objects we see account for about 3-6% of the needed gravity. Dark normal objects are guessed to account for 4-20%. Nuetrinos probably make up around 10%. Anything left over either has to be accounted for either by exotic dark matter or by a serious reaccounting of the above categories.

  • Nuclear? How do they figure? (Score:4, Informative)

    by CheshireCatCO ( 185193 ) on Tuesday October 30, 2001 @10:49PM (#2500949) Homepage
    According to the leading theory, an enormous nuclear explosion called the Big Bang happened 13 billion to 15 billion years ago.

    Gack. How do they figure an explosion of spacetime is nuclear? There were no nuclei to fuse or split. My cynicism is telling me that the author just though "nuclear" sounded big and bang-y.

    • It was not nuclear in origin, I would guess, but when the exploding particles began to fuse together into what are now called protons, neutrons, atoms, etc.... That is why it is referred to as a nuclear explosion.

      Of course, it had to fuse into all those other tiny particles that make up the protons and neutrons first, but why would they want to bore us by explaining all of that too? My question in relation to that: How small was this "stuff" before the explosion? Would it be fair to wonder if it had been rapidly exploding outward from a much smaller size than we could possibly imagine for much longer than we think?

      • Creating protons isn't really nuclear as it's usually thought of. And the formation of atoms didn't occur for quite some time after the Big Bang. In any event, the term 'nuclear' completely fails to encompass the nature of the Big Bang, since 'nuclear' has nothing to do with the creation of spacetime.
  • This has bothered me since I took a basic cosmology class a few years ago. If modern astrophysics says that the universe should have 900% more matter than we can actually observe, shouldn't that cause us to majorly reevaluate the equations that give us our expected mass?
    • It seems from the article that we now have 3 independent methods to calculate the mass of the universe. 2 of them agree, and the other is 10 times as big.

      Instead of saying "there may be something wrong with the theory that gives the big answer", the scientist invent 'dark matter'. Seems a bit of a cop out to me ...

      I'm just off down to the personnel department to get them to factor in my 'dark salary' :-)

      • Argument From Incredulity is no argument at all (Score:4, Insightful)

        by MillionthMonkey ( 240664 ) on Wednesday October 31, 2001 @02:01AM (#2501490)
        We have three methods to calculate the mass of the universe. Two are based on electromagnetic interactions. Those two agree. The other is based on observations of gravitational interactions. It gives a result 10X as large for the total amount of matter. Therefore 90% of the universe is made of particles that interact gravitationally but not electromagnetically. The only way to observe them is to observe their gravitational effects. Like, duh. Why is this such a difficult concept to grasp? It's an empirical observation.

        Keep in mind that if something only couples gravitationally, it's going to be extremely hard to see. You're prejudiced by your own experience with the world, which is mostly based on electromagnetism- meaning interactions with photons (real and virtual). Get rid of electrodynamics, and most concepts and phenomena you're familiar with- atomic physics, chemistry, biology, optics, materials science, friction, pressure, radiation, viscosity, resistance, reflection, transparency, iridescence, impenetrability- all this stuff goes out the window! Your ass would sink through your chair, right through the ground, until you reached the center of the earth with everything else. Don't underestimate the importance of photon-mediated interactions. Everything else is gravitation, beta decay, and the strong nuclear force. Of those three, only gravitation operates over non-microscopic distances. And it is very weak. There could be up to several tons of dark matter in the room with you right now. You would never know it's there.

        Of course, the mass could be ordinary matter that we're just not seeing. Many people like the idea of lots of Jupiter-sized objects. Lots of black holes might also work (although a black hole can feed off either kind of matter).
  • If 10 percent of all matter in the universe is made of "stardust" and the other 90 percent of the matter is made up of dark matter, we've got a pretty serious problem. There may yet be some other sort of matter out there that we don't know about yet, since the 10% of all regular matter occupies only a tiny fraction of the actual space available and dark matter is ,by the accounts in the article, clumped together into pockets. So what about all the rest of that space? Is is occupied by purple matter?
    • "Dark matter" does not mean to me that it is all that different. It simply means that we do not have any supply of it, and therefore no means to study or to detect it, and so we only know that it is there, but not of what it consists, and so on. By that reasoning, therefore, I conclude that there is only one kind of "dark matter", and that it is essentially the same stuff from which "normal" matter is derived (according to the space.com article).

      Still, it's all very confusing. I don't think it's a "serious problem", because the assumption seems to me to be that much of the dark matter has not materialized into what we recognize as matter yet. This means that the universe is young yet... When we reach the point when normal matter and dark matter are split 50/50, then my guess is that the universe will begin to shrink back toward its state before the big bang, only to explode all over again. Of course our planet will be long gone by that time (but that doesn't mean we have to go with it)... "In the beginning, there was nothing. Then it exploded."

    • Re:Hmmmm. Think on this. (Score:5, Interesting)

      by sinster ( 518986 ) <sinster@@@ballistictech...net> on Wednesday October 31, 2001 @12:50AM (#2501303) Homepage
      If bright matter truly makes 10% of the universe, then by definition the remaining 90% of the universe must be dark matter.

      The reason is that neither the terms "bright matter" nor "dark matter" specify a single type of matter. Rather, they define two values of a single common characteristic of all matter. The characteristic in question is how the matter interacts with photons. If you shine a light on something and you can see it, then it's bright matter. If you heat something up and you can see it, then it's bright matter. If you energize something then let its energy level drop and you can see it, then it's bright matter. Otherwise it's dark matter.

      Therefore we can't measure dark matter directly merely because we can't see it. All astronomical observations depend on photons. Radio. Light. X-Ray. Gamma. Just different frequencies of photons. Since dark matter neither reflects nor emits photons, astrophysicists can't observe it. Or perhaps it does emit photons, but then immediately reabsorbs them (as in the case of black holes). Either mechanism comes down to the same thing. They can observe its effects indirectly by watching, for instance, the effect that its gravity has on surrounding bright matter, but no direct observation is even theoretically possible.

      But there really aren't any theories about the nature of dark matter, because it's fundamentally impossible to observe remotely. Maybe it's some truly strange substance. Maybe its just a whole bunch of black holes. No one knows. The only reason that we know about black holes is that some brilliant physicist who'd been downing a few too many beers one night did a thought experiment about the implications of gravity's inverse square strength. So we had a theoretical phenomenon that astrophysicists could later go and look for. But that's not true of other forms of dark matter.

      All that's important is that "dark" matter is every piece of matter that isn't "bright" matter. It's still matter, and will still behave exactly the same as bright matter behaves. But it may come to be discovered that some characteristic that we thought was endemic to all matter is, in fact, only endemic to bright matter. We have no comparison yet, so we can't make that determination.

      I don't think that anyone believes that all dark matter is in the form of black holes. Who knows, maybe so. I'm certainly not an astrophysicist (though I know a number of them who are on the bleeding edge), so someone can easily have come up with some theories about all this of which I'm unaware.

      But this is my current understanding, and with the rate that astrophysics moves, I'm probably at least 5 years out of date.

      Oh, explaining this caused me to remember a theory about dark matter that I heard from my undergraduate adviser back in my college days (Dr. Douglas Lin: he was and is a big shot in the astrophysics circles). The idea is that there actually isn't any special dark matter. It's all bright matter. But some matter might be in locations where so few photons fall on them that we just never get a chance to observe that matter. For instance, it's known that all the galaxies of the universe exist on the surfaces of voids in the universe (that observation is what gave rise to superstring theory). Think of soap suds. We've got complex surfaces, where all the soap is, each surrounding a small void with no soap. Small from our perspective, but from the point of view of a technological civilization living in one of the "galaxies" within the soap film, those voids are huge. The universe has the same structure. And these voids are just monstrously huge. In the center of one of these voids, there would be very little light, because all the light sources are very far away. So you could stick a whole lot of matter there and no one would ever see it. These voids are so huge that you could easily fit 90% of the universe's mass in them and still have a very low density of matter. It's normal "bright" matter, but insufficient light reaches it for us to observe it. The problem with the theory is that if you have 90% of the universe stuck in these voids, then the voids should collapse from gravity and make the galaxy distribution homogenous. And we don't see that. Perhaps this problem has been resolved by now. I don't know. And, of course, there are other locations where matter can be hidden, where we wouldn't be able to observe it. Those voids are just a single example.

  • Yeah right... (Score:3, Funny)

    by Kasreyn ( 233624 ) on Tuesday October 30, 2001 @11:15PM (#2501029) Homepage
    Scientists can measure ALL the dark matter in the universe, but can't build a fortune-telling weight machine that can get my weight right.

    Pfft.

    -Kasreyn
  • [two coincidentally matching results] != [answer of all space and time]

    Only further testing will be able to prove that this match is not simply coincidence. They're right, this doesn't "have to be the answer", so let's not jump to conclusions from two friendly tests. (But certainly get excited for the potential! Heh.)

  • I still think that Douglas Adams was right... (Score:5, Funny)

    by Bollie ( 152363 ) on Tuesday October 30, 2001 @11:26PM (#2501061)
    Dark matter is the packing material the Universe came in...

    No theory of everything could ever be complete without allowing for this.

  • ==braces for karma hit==

    My theory, after watching Stephen Hawking's A Brief History of Time, and based on an extremely limited understanding of all this crap, is that time is circular rather than linear as we commonly depict it. At a certain point in that circle or loop or whatever. is the Big Bang. After the Big Bang, the universe expands until it runs out of steam, then it slowly starts to contract again, bringing all the matter and anti-matter together where they annhilate each other and turn into energy, which is compressed further and further into an itty-bitty little space, then BOOM, it starts all over again. So basically, in nerdspeak, I think the universe/time is an endless loop.

    But, you know, I could be wrong. It has happened once or twice...
    • We know the universe is still expanding and not shrinking again as other calculations would indicate should be the case if this were true.

      Scott
      • That doesn't mean it can't shrink later on or that their calculations regarding the time it should take for it to start were wrong. *shrug* I was just tossing it out there.
  • an enormous nuclear explosion called the Big Bang happened 13 billion to 15 billion years ago. From it, the universe appeared in an instant, but as a billion-degree mess of neutrons, protons and electrons.

    If ever Big Bang there was, it was not a giant nuclear explosion! Damn at those temperatures there are no nucleus (as they themselves state a few sentences afterwards). It is rather a very FAT release of energy, which later congregated into quarks and antiquarks, neutrinos, etc. definitly not nuclear. And what a hell is "dark matter". They state that "dark matter" congregated and formed gravity pools...

    At the time of the original release of light, dark matter had congregated in clumps, which created small fields of gravity that eventually pulled in normal matter as well.

    Dark matter does not emit radiation by definition. It thus has to have enough gravitational pull to keep all EM radiation in. That is a freakin big chunk of matter, not small gravity fields! And what do they mean normal and not normal matter... it's all the same stuff, energy. The energy is just "stored" differently.

    "The nature of these 'wiggles' is basically saying how the normal matter was responding to that crazy dark matter," explained Fields, "by amplifying the places where the extra density was."

    Errhhmmm... that is called matter falling in the black hole to make it larger and thus increase the gravity....
  • ..sabers in a scientific site? This is just another perversion from the dumbed down culture we're in. Well anyway back to the topic, I was wondering what kind of a gravitational effect light has on the universe. As we all know, there are ALOT more photons in the universe than any other (non virtual) particle. Perhaps if light, or rather the energy contained in the photons, had a gravitational effect then it may plug up the discrepancy that dark matter is supposed to explain. Just my 6.02*10^23 cents.
  • Now if they could just record its evil laugh...

  • I believe I've got some under my couch. A rather more important discovery will be, I suspect, that all that dark matter is all the socks of every culture ever to evolve clothes dryer technology. Clothes dryers create minature wormholes which teleport your socks to random points in the universe. This revelation will ultimately lead to a faster than light drive with a clothes dryer at its core. Its mission will not be to explore new planets yadda yadda. Its mission will be to retrieve all those socks.
    • Actually, whenever I wear new colored socks, I find that I end up with some dark matter between my toes.

      I haven't measured it yet, but it looks like I could account for a few grams of the Universe's missing mass from all of the socks I've broken in.
  • I call blatant matterism!

    It's not dark matter you unenlightened cretins!

    It's matter of color!

    Heathens.
  • Something very odd just happened...

    I clicked the link to space.com to read the article. When the page loaded, the big central advertisement was for "Star Wars: Phantom Menace" videos... Kinda threw me off for a moment. Perhaps I haven't recovered yet beause I'm posting this here. hmmm

  • 1) Get Jon Katz to write a book full of book reviews and post a topic about it on Slashdot.

    2) The amount of trolls accumulating around that single topic on Slashdot will be so intense that it creates a giant gravity field that sucks everything in the universe together.

    3) The universe is saved from flickering out into nothingness due to endlesss expansion! No dark matter required, yay!

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