Desktop Project Part 24: A deep, dark mystery

By Phil Plait | April 18, 2012 6:52 am

[The Desktop Project is my way of clearing all the pretty pictures off my computer’s desktop, by posting one per day until they’re gone. I think this week is it – I’m almost out!]

Dark matter is funny stuff. We’ve known about its existence for many decades, and the more we look the better our evidence gets. We know it has mass, and therefore gravity, but we don’t know what it is! We do, however, know what it isn’t: normal matter of any kind, like cold gas, rogue planets, black holes, dead stars, or anything else made of protons, neutrons and the other types of particles we deal with in everyday life.

Since careful observations have shown clearly it can’t be any kind of normal matter, it therefore must be some sort of exotic flavor of matter, some kind of particle we haven’t yet seen.

One thing we’re pretty sure about it, though, is that it doesn’t interact with normal matter except through gravity. Dark matter can pass right through you and you’d never know it. But put enough of it in one spot, and its gravity will reveal its presence.

Which is why the galaxy cluster Abell 520 is such a mystery. Here’s the beauty shot:

Pretty, isn’t it? Abell 520 is a galaxy cluster about 2.4 billion light years away, and a mass of several trillion times our Sun’s — it’s made of galaxies, each with billions of stars in them. And a galaxy cluster is a collection of hundreds or even thousands of galaxies bound together by their gravity. In fact, Abell 520 is more than one cluster: it’s actually a collision between two or more clusters! As they move through space, clusters can collide, and actually quite a few of these cosmic train wrecks are known.

When clusters collide, a lot of things happen. The gas clouds in between galaxies in the two cluster slams into each other, heating up to millions of degrees and glowing in X-rays. In the picture above, that gas has been colored green so you can see it (invisible to the eye, the X-rays were detected by the Chandra Observatory). The orange glow is from stars in galaxies (as seen by the Canada-France-Hawaii and Subaru telescopes). The blue is actually a map of the dark matter made using Hubble observations. The gravity of dark matter distorts the light passing through from more distant galaxies, making it possible to map out the location of the otherwise invisible stuff (you can read about how that’s done here and here).

Since dark matter doesn’t interact with normal matter, we expect it to simply pass through the collision point, sailing on as if nothing had happened. That’s been seen in a half dozen other galaxy cluster collisions, including the Bullet Cluster — hailed as definitive proof of the existence of dark matter — as well as Abell 2744 aka Pandora’s cluster (seen here on the right), and the newly found Musketball cluster.

But Abell 520 isn’t like those others. The problem is, there’s a clear peak in the dark matter right in the middle of the cluster, not off to the sides as you might expect. It looks as if the dark matter slammed to halt in the middle of the collision instead of sailing on.

Here’s the thing: this does not mean dark matter doesn’t exist, or we’re wrong about it. The other clusters I mentioned above make it clear we do have a pretty good grip — so to speak — on the behavior of dark matter.


What these new observations almost certainly do mean is that there’s more going on here than we can easily see. For example, it’s possible there’s a lot of normal matter in the middle of the cluster that we can’t see for some reason. Or maybe it’s a geometric effect; it could be we’re seeing this collision right down the barrel, and not from the side. In that case, the dark matter did indeed keep moving, but directly toward and away from us, so it still looks like it’s in the middle.

The astronomers who made these observations have proposed several ideas about what’s going on here, in fact. The thing is, they all are either unlikely, or invoke special circumstances (like our viewing angle). That doesn’t mean they’re impossible, it’s just that as scientists, we don’t like doing that. We’d prefer not to have to invoke mitigating circumstances to explain phenomena — especially when you only have a handful of such phenomena to study — because once you start doing that, you start finding excuses to explain away every little thing you see that way. It’s far better to have a more general explanation for things!

The problem is we just don’t have enough examples of cluster collisions to know how weird Abell 520 is. It may be that when we observe more cluster collisions they’ll all behave the way we expect, and Abell 520 will be seen as the rare exception. When that happens, special pleading is more acceptable, since it’s only used rarely and when things really do look weird (but you still have to back it up with good evidence if you want folks to believe you). And when you observe something lots of time, eventually, statistically you’re bound to see something weird happen (like flipping a coin hundreds of times, and having it land on its side once).

Hopefully, as time goes on, we’ll get lots more observations of cluster collisions and be able to crack this nut. In the meantime, dark matter will do whatever it is dark matter does, and we’ll keep looking and trying to figure it out.

Image credit: NASA, ESA, CFHT, CXO, M.J. Jee (University of California, Davis), and A. Mahdavi (San Francisco State University)


Related Posts:

The Top Ten Astronomy Images of 2006 (scroll down to #4 for the Bullet Cluster description)
Galaxies swarm and light bends under dark matter’s sway
Dark matter, apparently, is midichlorians
Opening the lid on Pandora’s Cluster

CATEGORIZED UNDER: Astronomy, Pretty pictures

Comments (36)

  1. Daniel

    look how cool. Just today (or yesterday) the Resonaances blog posted a rumor Dark Matter detection. I know, astronomers know about it for a long time, but particle physicists still don’t.
    http://resonaances.blogspot.com

  2. Ryan the Biologist

    I’ve always been very skeptical of the idea that the phenomenon we call dark matter must be a new form of matter that has never been observed before. I’m not a physicist, so of course I need to temper my skepticism with a certain amount of deferrence to the experts in this field, but it just seems to me that if the galaxies are spinning too quickly at their edges to be accounted for under current models, then it would seem more likely to me that the model would need adjustment rather than postulating the existence of something entirely new that makes up the majority of the universe’s mass yet has somehow not been detected before now. I might be on board with existing particles, such as neutrinos, making up this extra mass. But claiming the existence of a particle indistinguishable from magic as the major constituent of the universe just seems like the theoretical physics equivalent of “God did it”.

    Perhaps someone who deals with this topic professionally can help me understand why new, exotic forms of matter is the preferable explanation for the galactic spinning/lensing problems.

  3. david

    If I were a dark matter guru, I don’t think I’d be sleeping well at night. The existence of a phenomenological model (MOND) that predicts so many effects so well would worry me endlessly. Yes, I understand MOND has plenty of problems too. I understand that it can’t possibly be right. But I also understand that if there weren’t something real and physical behind it, it couldn’t possibly be as successful with its predictions as it is. That strongly suggests that our understanding of gravity might not be as good as we’d like to believe, and that in turn means the foundations of DM are on very shaky ground. Anyone heavily invested in DM should be sweating bullets until they can explain how DM dynamics inevitably lead to effects that appear MONDian.

  4. Tom H. Type

    Although, current thinking states that Dark Matter does not (or only weakly) interacts with normal matter, it would seem reasonable to assume that Dark Matter does interact with other Dark Matter. So two Dark Matter clouds slamming into each other, may have an observable effect. This visible effect may be the gravitational distortion of light, for Dark Matter at higher Dark Energy states.

  5. Messier Tidy Upper

    Which is why the galaxy cluster Abell 520 is such a mystery. Here’s the beauty shot: (image) Pretty, isn’t it? Abell 520 is a galaxy cluster ..

    Yes it’s .. Hang on a second, you’re fooling us! :-o

    That’s really part of a still from the start of the Colin Baker Dr Who intro :

    http://www.youtube.com/watch?v=vMOvQCB5jU0

    Or / & :

    http://www.youtube.com/watch?v=D4_wSxMBNk4

    Plus :

    http://www.youtube.com/watch?v=awze7DTbERc&feature=related

    Isn’t it eh? Or maybe not? ;-)

    Good image. Fascinating mystery. Insufficient evidence to really say one way or t’other yet? :-)

  6. Lars

    @Ryan: I’m not one of the pros you’re asking for, but I’ll answer anyway. See the Higgs boson. Not observed (yet), but not just magic either.

    I also think dark matter has more effects than just making galaxies spin too fast, like gravitational lensing, so introducing a new particle to our model of the universe may be preferable to changing/tweaking a host of other sub-models to be able to explain all the different kinds of “dark matter phenomena” out there.

  7. Chris

    @2 davis
    I thought MOND was disproven and dark matter proven by looking at the power spectrum of the Microwave Background.

  8. Nigel Depledge

    @ David (2) –
    The trouble with MOND, aside from having no evidence to support it, is that, in every test of our theories of gravity that we have carried out, GR comes through looking good.

    GR has a far better theoretical basis than MOND, and they both match observations as far as we can tell – on the assumption, of course, within GR, that DM exists. DM is, if you like, a prediction of GR based on the observed behaviour of galaxies and galaxy clusters. As far as we can tell, GR is a good approximation to the truth.

    MOND, AFAICT, has no theoretical basis beyond being a post-hoc tweak of our gravity theories to accommodate the observations concerning galaxies and galaxy clusters.

    Logically, therefore, GR with DM has a firmer basis than MOND.

  9. Scott P.

    So if dark matter has a gravitational field, why isn’t it clumpier? Matter clumps to make planets and starts, could dark matter do so?

  10. Fizz

    A recent edition of Discover discusses the theories of Julian Barbour, which would seemingly explain a number of phenomenon without the need for dark matter. So… do we truly know if dark matter exists?

    http://discovermagazine.com/2012/mar/09-is-einsteins-greatest-work-wrong-didnt-go-far

  11. Harry

    @ Ryan the Biologist

    I may be oversimplifying a bit, but I do not think there is as big a problem as you describe. For example, particle physicists have never detected the graviton. However, it is obvious from our observations that gravity exists. If our dark matter model and/or general relativity makes predictions that are verified by experiment, we at least know we are on the right track. So far, that seems to be the case.

    As another example, the Standard Model of quantum field theory predicts that there is a Higgs field (or something resembling a Higgs field) and that it is non-zero on average. We’ve yet to conclusively “prove” that it exists, but we know from observations that the Higgs or something similar to the Higgs must exist.

    These predictions are based on the evidence at hand, which to me is a far cry from “God did it.” You can’t ever prove or disprove God did something. Scientific hypotheses, on the other hand, simply need to be tested until the predictions are verified or rejected in favor of a new model.

  12. Chris

    @6 Scott P
    Good question. Big difference between dark matter and normal matter is that normal matter can make atoms and molecules. As far as we know there is no dark atoms or dark molecules. Normal matter is held together by the electromagnetic force while dark matter doesn’t interact with electromagnetism. That’s why we can’t see dark matter. Unless there is some dark matter equivalent of light which doesn’t interact with normal matter, but so far the evidence doesn’t lean that way.

    With normal matter you have a gas cloud which begins to contract. As it contracts the gravitational potential energy is converted into kinetic energy, the gas heats up and expands. A gas cloud made of an ideal gas (particles don’t interact with each other) can’t contract. But real gas clouds do contract, otherwise we wouldn’t be here. Molecules form in the gas cloud and they can radiate away heat allowing the cloud to cool off and get smaller.

    Since dark matter can’t form molecules to radiate away heat (excess kinetic energy), the dark matter stays as clouds rather than making dark matter planets.

  13. Wzrd1

    One ponders on what happens when dark matter approaches a singularity, especially if a clump of it approaches the singularity at the core of a galaxy?

  14. Ryan the Biologist

    @Lars

    Gravitational lensing and galactic spinning are closely related phenomena, however. Basically, galaxies seem to have more mass than we’d expect given our observations and our current understanding of gravity (and any physicist will admit that gravity is one of those things we do not have a complete understanding of yet). So really, there is one flaw in the model causing multiple related phenomena, and that flaw is the added mass/gravity.

    @ Harry

    The problem with dark matter as a hypothesis is the very fact that it is not really testable the way a hypothesis should be. It sounds like there is a discrepancy between observations and the model used to predict those observations, and rather than altering the model, the term “dark matter” is used to fill the gap, with dark matter being essentially definable as exotic matter having the bizarre suite of properties that would reconcile the current model with the current observations. You could say “let’s test for THIS and find out whether the dark matter is present”, but then when the test fails, the bizarre properties that dark matter must therefore have could be expanded to include the properties needed to evade that test. Essentially, dark matter is the absence of a testable hypothesis, rather than a hypothesis unto itself.

    At least, that is as far as I understand the issue because, as I’ve said, I’m certainly no expert, and I’m forced to assume that there must be something very important and very definitive that I don’t know about dark matter that would convince me if I knew it.

  15. OneofNone

    @2. Ryan the Biologist:

    Others have already commented, so I concentrate on this.

    But claiming the existence of a particle indistinguishable from magic as the major constituent of the universe just seems like the theoretical physics equivalent of “God did it”.

    The “God did it” is a short form of “Let’s assume that, no further investigation needed/allowed”. And of course there is no prediction of any future findings.

    Any theoretical item in science is very different. First of all there is no “stop searching”, to the contrary. Much effort is done to first prove there is an effect worth investigation, and then hypotheses and theories come up. Their predictions are compared against experimental and observed results.

    In very short form for the current question, dark matter as an explanation has a big advantage about modified gravitation theories. This one idea/hypothesis explains several different effects, especially spinning galaxies and gravitational lensing. It may one day also tell us how galaxies form in the first place. Occams razor.

    Edit:
    I see you already answered, but now you changed your statement. Now you want an explanation for more matter. Before I understood you wanted a different explanation for more gravity at the borders of galaxies.
    Never mind, if dark matter does not exist, we never come up with a consistent set of properties for this kind of stuff. Check Phlogiston ;-)

  16. Ryan the Biologist

    @OneofNone:

    “Change your statement” sounds accusatory. Perhaps you mean that you misunderstood my original statement and my second statement clarified things?

    My point is, and always has been, that while current observations are definitely suggestive of additional mass/gravity in the galaxies that goes unaccounted-for in current models, dark matter does not seem to be the only (or even the simplest) explanation of that extra mass.

    You mention Occam’s Razor. Which of the following do you think makes the fewest assumptions?
    1) A new kind of previously undescribed exotic matter with a bizarre suite of properties not only exists, but also happens to make up the overwhelming majority of the mass of the observable universe.
    2) We are missing something important about how gravity works at galactic scales. Considering the current lack of any unifying theory of quantum gravity with relativistic gravity, we already know that modern physics does not fully understand gravity, so is it so crazy to think that something we are missing could account for the extra mass?

    I’m not saying MOND or TeVeS are valid alternatives, either. They’ve been shown to be false, and that’s good enough for me. But I truly feel, at least with what I know so far about this subject, that making the assumption that new kinds of exotic matter exist (and exist so ubiquitously!) is premature when we do not yet have a comprehensive understanding of how gravity works.

  17. david

    @8 Nigel and @9 Chris

    Yes, I’ve read that and various other criticisms. I understand why people think MOND is bogus, and as I mentioned before, I even agree that it IS bogus. But what most DM people don’t seem to understand and/or to publicly acknowledge (for reasons that I don’t get) is that there are phenomena such as rotation curves or the Tully-Fisher relation where the simple MONDian model gets the right answer, and that this is a HUGE problem for DM.

    If DM is the reality, then it’s got to somehow predict these phenomena. But as soon as you’ve got unseen DM whose distribution you can muck with to be consistent with something like a random rotation curve, then how is the prediction going to happen? If there are these DM halos whose distribution is largely independent of the mass we can see, then why do the halos generally seem to arrange themselves so that MOND+no DM appears to hold? To paraphrase some unknown writer “How is it that the baryonic tail is wagging the DM dog?”

    I’ve acknowledged that MOND is BOGUS. Is there _one_ DM guru that acknowledges that some of the phenomena predicted (often a-priori!) by MOND appear valid and that if DM doesn’t predict the same phenomena then it is ALSO BOGUS? Are all the DM people toiling away on this problem and I’m just not aware of it? Have any of them made any progress? Do I have an inordinate fondness for simple theories that make definite predictions and are falsifiable instead of just having an abundance of fitting parameters?

  18. Jim Johnson

    @ Messier Tidy Upper: I’m pretty sure this is a photo of an entity that attacked the Enterprise in “The Naked Time” episode of the original Star Trek series.

    Everyone else who’s commented saying in various ways, “this doesn’t mean there’s dark matter”: Dark matter is not a “new exotic form of matter”. It is “An unknown substance, force, or effect, which causes galaxies to remain more cohesive than the normal matter and energy in those galaxies could do, under our current understanding”. You can’t think of it like “invisible matter”. (Although it might be something similar, we certainly don’t know that.) It’s only called dark matter because it produces gravity (or another force or effect which effectively performs the same function as gravity) like matter does, and it isn’t directly detectable (i.e. it’s “dark”).

    Railing that “we can’t prove dark matter exists because x or y” misses the point. We know SOMETHING holds these galaxies together, and we know that nothing we have been able to detect could do that. Since currently, that’s close to all we do know about it, if you want to discredit dark matter, you must first either demonstrate that galaxies are not remaining cohesive, or you must produce the math showing how the normal matter in those galaxies is sufficient to keep them cohesive.

    Any other argument is not an argument against dark matter at all. At best, it could only constrain what dark matter is. You think there’s a math error? Maybe (although unlikely in the extreme at this point), but that wouldn’t mean there’s no dark matter. It would just mean dark matter was a math error. You think it might be caused by something we still don’t understand about gravity itself? Perhaps, but that wouldn’t mean there’s no dark matter – it would just mean dark matter is a misunderstood aspect of normal gravity. You think it’s something about the shape of space itself? OK, fine. But that’s still not an argument against dark matter; it would just mean dark matter is a function of the shape of space.

    Because dark matter is, by definition, “whatever reconciles our understanding of the actual cohesiveness of galaxies with our knowledge of the way matter & gravity behave”, if your argument doesn’t discredit the idea that galaxies remain whole, or doesn’t prove that normal matter is capable of making that happen without invoking some new-to-science information, then your argument doesn’t discredit dark matter. (BTW, if you did invoke some new-to-science information to accomplish this, that information would be dark matter. By definition.)

  19. david

    @19 Jim
    That seems a rather vacuous definition of DM, but if that’s the definition that people are really using, then no wonder there’s such confusion. I propose we rename it to something not involving the word “matter” so as to be clearer. How about “floobydust”? Galaxies don’t fly apart because of floobydust.

  20. HvP

    I understand the desire to reject the dark matter hypothesis because it sounds like a post hoc invention. But to me, modified gravity sounds even more like a post hoc invention. It’s the equivalent of Einstein’s “Cosmological Constant” – or its revamped contemporary equivalent, “dark energy.”

    It would be great to have a theory that intuitively gets to the root cause of these gravitational effects, and I personally feel that dark matter does a better job of explaining root causes than does modified gravity. As far as I can tell, the modified gravity hypothesis does nothing to posit a root cause for why gravity should be expected to behave differently in different parts of the universe. It’s that assumption that gravity behaves in an inconsistent manner that is the source of concern for me, and makes it an undesirable theory for gravity.

    At least the concept of dark matter holds gravity consistent across the universe. Of course, now we’re left with the assumption that electromagnetism can be absent in some massive particles. So it’s either fall within the camp of inconsistent gravity or inconsistent electromagnetism. As it stands, the experiments involving gravitational lensing have me reasonably leaning towards the concept that gravity dominates in some particles that don’t interact electromagnetically. And I think that’s all that can be reasonably assumed at this point, for me anyway. I’m willing to be convinced otherwise given enough evidence.

    Dark energy however… seems very much like simply a placeholder for some effect requiring a great deal more evidence from some spectacularly clever experiments yet to come.

  21. Brian Too

    DM/DE proponents still have a whole lotta ‘splainin’ to do! And precious little experimental data to back up their ideas.

    It takes real moxie to suggest that Occam’s Razor is on the side of current DM/DE ideas.

  22. Harry

    “It is not unscientific to make a guess, although many people who are not in science think it is.” ~ Richard Feynman

    Occam’s Razor would seem to side with the current understanding of general relativity, since it is so well tested.

    I think it is safe to assume none of us here are experts in cosmology. Therefore, I’m not sure our arguments really mean much of anything. As a non-expert, I imagine that the Standard Model is the “standard” for a reason. No one denies that dark energy is the greatest mystery of our current understanding of the universe. All we can do is make hypotheses and test them.

  23. Wzrd1

    Wow! THIS discussion has devolved into a flying spaghetti monster cloud argument!
    OK, here are a few things to ponder.
    Gravitationally bound objects tend to NOT expand in space-time from each other, whereas lesser gravitationally attracted objects can and do distance themselves in a manner described quite well with universal expansion.
    As that fact is still quite the mystery, can it not be considered that, rather than a magical particle, which cannot be defined or properly described, other than having gravitational attraction to “regular” matter, perhaps be a larger scalar effect on the rather ill understood thing we call gravity in a space-time environment?
    Is it possible for some form of matter to not interact with electromagnetic fields?
    If you say no to one, you are relying on faith, rather than science, which advances theories, tests the hell out of them and whatever survives reality is considered fact. Pending future refutation of said defunct fact, due to a large number of reasons that are fully documented, rigorously tested and thoroughly attacked.
    Science is not a field where a fact is immutable, it is a field where facts are distilled by rigorous testing and any fact that survives all tests remains a fact.
    OR we still are orbiting the sun, blazing a wide wake in the ether.
    Or we watch the sun orbit the Earth, where the sun blazes its coal fueled flames through the ether.
    And the Earth is only 6000 years old.
    Oops, those previously “known facts” failed the tests of science.
    For, in truth, science demands one to question and test EVERYTHING, repeatedly. Then, rinse and repeat.
    Even knighted laws, such as Newton’s laws. Which Einstein amended.

  24. Ryan the Biologist

    @HvP

    Gravity is already inconsistent at different scales. At quantum scales, we have different models for how gravity works than at planetary scales. This inconsistency is currently one of the great unsolved problems in modern physics. If this reduction in scale already causes problems for gravity, is it so outrageous to think that a great increase in scale may also be inconsistent?

    I do agree with you that there must be a reason that the Standard Model is “standard”. The idea that dark matter is made of a new and exotic form of matter actually has consensus among the physics community, which is the reason I am forced to assume that, against everything that makes sense to me, I am almost certainly wrong. That is why I initially asked for someone who deals with the topic professionally to explain to me why I am so certainly wrong; explain to me what important piece of the puzzle I must be missing that has so thoroughly convinced the physics community that dark matter is more likely than a hole in our understanding of gravity (a model that everyone already acknowledges has holes).

  25. Nigel Depledge

    Ryan the Biologist (15) said:

    The problem with dark matter as a hypothesis is the very fact that it is not really testable the way a hypothesis should be.

    Not entirely.

    Supersymmetry predicts a whole slew of yet-undiscovered particles, some of which look pretty good as candidates for DM. Neutralinos, anyone?

    Supersymmetry also predicts what these particles are most likely to decay into, and experiments at the LHC might detect these decay products. So, some of the cadidate particles for DM are indeed testable.

    It sounds like there is a discrepancy between observations and the model used to predict those observations, and rather than altering the model, the term “dark matter” is used to fill the gap, with dark matter being essentially definable as exotic matter having the bizarre suite of properties that would reconcile the current model with the current observations.

    Well, yes.

    The thing is that GR is pretty good at passing every other test we’ve thrown at it so far. So, if you have a model that passes 49 out of 50 tests (or whatever the actual numbers are), do you throw out the entire model (MOND does not – AFAICT – work within GR) or do you postulate a new type of matter that interacts only through gravity?

    You could say “let’s test for THIS and find out whether the dark matter is present”, but then when the test fails, the bizarre properties that dark matter must therefore have could be expanded to include the properties needed to evade that test.

    Well, perhaps so.

    Essentially, dark matter is the absence of a testable hypothesis, rather than a hypothesis unto itself.

    Not really. It is a prediction arising from GR and the observations of the behaviour of galaxies.

    What DM actually is (and what its properties must be) is a separate set of hypotheses. Some of these have been ruled out, but others have not. GR requires that DM exist, but does not dictate the properties of DM, except that DM must have mass. It is the observations we are making in the search for this missing mass that are constraining the properties of DM.

  26. Nigel Depledge

    David (18) said:

    But what most DM people don’t seem to understand and/or to publicly acknowledge (for reasons that I don’t get) is that there are phenomena such as rotation curves or the Tully-Fisher relation where the simple MONDian model gets the right answer, and that this is a HUGE problem for DM.

    Well, I don’t think it is surprising that MOND gets those rotation curves right, because that’s what it was tweaked to do.

    I don’t think MOND is a problem for DM. The problem for DM is that we are running out of candidates. If we get to a point where (say) the LHC is able to rule out supersymmetry as a theory, and we don’t come up with any other candidates for DM, then DM will be in serious trouble.

    Edit – unless you use the definition of DM in #19.

    One of the biggest difficulties is an experimental one – how can one devise an experiment (or observational test) to distinguish (1) gravity behaving differently at large scales from (2) there being additional mass out there that we cannot detect except through its gravitational influence on other objects that we can see?

  27. Nigel Depledge

    @ David (20) –
    Seconded!

  28. James

    I like to think that cosmic problems have elegant answers, even if the math can cause brain implosions.
    For instance, it would be convenient if the asymmetric anihilation of matter and anti-matter produced particles with complex mass; such particles would gravitationally attract normal matter, but gravitationally repel each other. Concentrations of such stuff would tend to puff up into shells around voids, gravitationally dragging sheets and streamers of matter on their swelling and jostling boundaries. That’d collapse 3 cosmic mysteries (dark matter, dark energy, the imbalanced representation of matter and anti-matter) into one; such convenience.
    Of course, it’s just vapour unless there’s some way to either detect or refute the existence of complex matter, but it’s pretty vapour.

  29. Wzrd1

    One question still remains to be answered though, is dark matter gravitationally attracted to matter? If so, one would expect galaxies to be chock full of it as well as having halos of it.

    As far as the LHC finding, say, the Higgs boson, the LHC is a toy compared to the greatest particle accelerator around-the universe! Indeed, there was a hint of something Higgs-like from cosmic radiation studies, but only one hint. Part may be due to rarity, part may be due to sparsity of detectors and studies.
    The LHC *HAS* had promising results, they’re finalizing results and making more observations, narrowing down energy levels of a potential Higgs boson at six sigma certainty.
    Which matches what I’ve long said, even if I don’t personally believe in the existence of a particular particle and its prevalence in the universe doesn’t mean one shouldn’t look. Indeed, one should look MORE.
    While the math looks right for the “requirement” of dark matter to account for the gravitation present in galaxies, that may or may not turn out to be the case.
    In the past, it was thought that the universe was filled with a fluid, which was called ether (or aether, for UK English speakers).
    Newton went unchallenged for gravitation, until Einstein tweaked the formula a bit.
    I can’t say what tomorrow will bring, save that it’ll be cool!
    For knowing something new IS cool!

  30. Nigel Depledge

    Ryan the biologist (25) said:

    Gravity is already inconsistent at different scales. At quantum scales, we have different models for how gravity works than at planetary scales. This inconsistency is currently one of the great unsolved problems in modern physics. If this reduction in scale already causes problems for gravity, is it so outrageous to think that a great increase in scale may also be inconsistent?

    Eh?

    I was under the impression that the first hurdle for any quantum theory of gravity was that it had to generate the same predictions about large-scale phenomena as GR.

    The difficulty between GR and QM is not so much in the behaviour of gravity as in the nature of space. GR says it is smooth and contiguous, whereas QM says it is quantised, discontiguous, and fuzzy.

  31. david

    @27 Nigel
    The problem for DM that I see is that MOND has only _one_ free parameter, so I wouldn’t exactly call it “tweaked”. The fact that you can take a huge pile of galaxies, look at the visible matter in them, and then from what you see plus ONE additional number get all the rotation curves right means that there is some fundamental underlying simplicity in galactic behavior. With DM, you could certainly calculate a suitable halo for each galaxity to make the rotation curve come out right, but that’s fitting, not predicting. DM is NOT predicting the real, physical, measured simplicity in these galaxies. Indeed, if DM (as currently understood) is true, then it would most naturally predict that the rotation curves and the visible matter should have basically NOTHING to do with each other. The fact that the two are so tightly correlated that they can be tied together with one fixed number is a huge failure of this natural prediction of DM. So a crucial question for DM is how the the apparent simplicity of galaxies can be explained. It’s highly unsatisfying to hear DM people saying “Well, maybe galaxies really are simple, but we can certainly fit them individually.” They seem to mostly just omit the “simple” part and not see how it’s an issue at all.

    The fact that they’re running out of candidates is a separate issue, but that one doesn’t really bother me as much; that’s an area where the understanding of physics seems messier, so it’s harder to say that DM has a real problem there.

    To me as an outsider, it seems like the DM camp is (rightly) proud of its successes at cosmological scales and (wrongly) just ignoring its apparent utter failure at galactic scales. I’d like to see them at least acknowledge the issue and appear to be putting some effort into explaining galactic simplicity, not waving it away with a casual “galaxies can be fit”.

  32. Wzrd1

    Nigel, my understanding is, GR gravitation is effective and highly accurate at macro scales and astronomic scales. QM is effective at micro scales and unnoticeable at macro scales. In short, GR is an approximation, where the quantum effects are too small to contribute substantially to the observation.
    As an example, in GR, virtual particles are ignored, as they’d not have significant effects at the macro scale. One would use QM for micro scale functions, such as nuclear reactions, where GR only “scratches the surface”.
    GR would tell me energy output and inputs in very general terms in nuclear reactions, but to make such efficient and predictable (such as in making a nuclear chain reaction), one would end up using QM. GR would be useless to compute the necessary pressures and energy input to accomplish fusion, but QM would be effective at it.
    QM would be useless for calculating the mass of the moon (OK, more overkill by far), but plain old Newtonian physics are more than adequate to do so.

    Unless we’re somehow expanding quanta and quantum distances to galactic distances, which my review of the math does not show.

    Now, one thing I’d LOVE to learn is, WHY is a neutron stable when bound in a nucleus, but when unbound, it is unstable.

  33. Nigel Depledge

    David (32) said:

    The problem for DM that I see is that MOND has only _one_ free parameter, so I wouldn’t exactly call it “tweaked”. The fact that you can take a huge pile of galaxies, look at the visible matter in them, and then from what you see plus ONE additional number get all the rotation curves right means that there is some fundamental underlying simplicity in galactic behavior.

    So, MOND is a numerical model that works, but we have no idea why it works.

    With DM, you could certainly calculate a suitable halo for each galaxity to make the rotation curve come out right, but that’s fitting, not predicting. DM is NOT predicting the real, physical, measured simplicity in these galaxies.

    OK, I’ll grant you this. But you missed out the word “yet”.

    However, if DM turns out to be real, then once we know its properties and figure it into the Big Bang theory, then it will give us predictions about galactic behaviour, and we’ll know why.

    Indeed, if DM (as currently understood) is true, then it would most naturally predict that the rotation curves and the visible matter should have basically NOTHING to do with each other.

    Not so. DM may be a key ingredient in galaxy formation, in which case it may be ultimately responsible for the condensation of gas clouds into stars.

    The fact that the two are so tightly correlated that they can be tied together with one fixed number is a huge failure of this natural prediction of DM.

    I think you are jumping the gun. Once we know what DM actually is, then we can expect to get some predictions out of the theories that incorporate it. Until then, we are guessing. Expecting predictions from current ideas about DM is not reasonable.

    So a crucial question for DM is how the the apparent simplicity of galaxies can be explained. It’s highly unsatisfying to hear DM people saying “Well, maybe galaxies really are simple, but we can certainly fit them individually.” They seem to mostly just omit the “simple” part and not see how it’s an issue at all.

    I grant you that the simplicity of MOND is attractive, but its lack of a physical explanation is repellant.

    The fact that they’re running out of candidates is a separate issue, but that one doesn’t really bother me as much; that’s an area where the understanding of physics seems messier, so it’s harder to say that DM has a real problem there.

    As I note above, I think you are judging the concept of DM prematurely.

    To me as an outsider, it seems like the DM camp is (rightly) proud of its successes at cosmological scales and (wrongly) just ignoring its apparent utter failure at galactic scales. I’d like to see them at least acknowledge the issue and appear to be putting some effort into explaining galactic simplicity, not waving it away with a casual “galaxies can be fit”.

    Maybe they are leaving it until later because those theories need more detail about the properties of DM?

  34. Nigel Depledge

    Wzrd1 (33) said:

    Unless we’re somehow expanding quanta and quantum distances to galactic distances, which my review of the math does not show.

    IIUC, some of the attempts to come up with the replacement for GR are a range of quantised gravity theories (things like loop quantum gravity, for instance). As theories of gravity, they have to work at large scales despite being quantum theories.

    IIUC (again), the biggest problem with these theories is that there are so many of them and we don’t have any way to experimentally distinguish them. Yet.

    Now, one thing I’d LOVE to learn is, WHY is a neutron stable when bound in a nucleus, but when unbound, it is unstable.

    That’d be the Gluon Fairy.

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