I happened upon a very readable and thought-provoking article on dark matter and thought others might find it interesting, too.
Dark matter is the commonest, most elusive stuff there is. Can we grasp this great unsolved problem in physics?
The past success of standard paradigms in theoretical physics leads us to hunt for a single generic dark matter particle -- the dark matter. Arguably, though, we have little justification for supposing that there is anything to be found at all; as the English physicist John D Barrow said in 1994: 'There is no reason that the universe should be designed for our convenience.' With that caveat in mind, it appears the possibilities are as follows. Either dark matter exists or it doesn't. If it exists, then either we can detect it or we can't. If it doesn't exist, either we can show that it doesn't exist or we can't. The observations that led astronomers to posit dark matter in the first place seem too robust to dismiss, so the most common argument for non-existence is to say there must be something wrong with our understanding of gravity -- that it must not behave as Einstein predicted. That would be a drastic change in our understanding of physics, so not many people want to go there. On the other hand, if dark matter exists and we can't detect it, that would put us in a very inconvenient position indeed.
Dark matter is the commonest, most elusive stuff there is
Dark matter is the commonest, most elusive stuff there is
Really? I thought it was hydrogen and stupidity.
I believe that according to current observations, if dark matter exists it is more prevalent by mass than hydrogen.
Whether it is more prevalent than stupidity depends on two things: whether stupidity has mass, and whether stupidity is limited to Earth. (I'll call the latter the "strong form" of extraterrestrial intelligence: that alien species are actually intelligent, as opposed to being kind of like us.)
At first glance I thought the author was accusing dark matter of subverting capitalism.
Stupidity is hardly elusive.
Well, you've got quarks. Quarks feel the strong, EM, weak, gravity force. You've got charged leptons. Charged leptons feel the EM, weak, gravity force. You've got neutrinos. Neutrinos feel the weak force (and maybe gravity). It is quite plausible that there will be something out there that feels just gravity, and quite plausible that it is stable. But hey, we really don't understand gravity at all. I mean, we have only measured gravity for large amounts of electrically neutral matter. What about gravity effect on bulk charged stuff? What about gravity effect on antimatter? What about gravity interaction with all the weird quantum numbers like isospin? So a lot to learn here, and no surprise that we don't really understand what is going on.
As an aside, dark matter is cool in itself. But what if there are Dark bosons, Dark EM, Dark chemistry? For me, as a physicist, dark matter is the most exciting particle physics out there - a completely undiscovered stable particle! Just a shame they are still mucking about with 10 tonne detectors when they should be in the business of kT detectors like the neutrino people.
I am also a physicist and this kind of talk about exotic particles drives me nuts when it is very clear that we haven't done nearly enough to account for the ordinary matter in the universe. Astronomy is still only picking the lowest hanging fruit (easiest to observe objects) and we have seen enough that we should expect a "long tail" type distribution with lots of small objects and disperse gasses that add up to significant masses over the enormous volumes of space. Positing new particles based on gravity observations is like looking at a map that has only capital cities marked on it and saying that since the combined population of these cities isn't large enough to eat the worlds food production, then there must exist a race of invisible hungry fairies!
Actually, If you think about it, there's a quite obvious candidate:
Every elementary particle comes in a left-handed and a right-handed version, except that for the neutrino we have only observed a left-handed version. Now usually you are told that there is only a left-handed neutrino. But given that all other elementary particles have right-handed versions, it would seem more natural that there are also right-handed neutrinos.
Now, what properties would a right-handed neutrino have?
Since it is a lepton, it would not participate in the strong interaction.Since it is a neutrino, it would not participate in the electromagnetic interaction.Since it is right-handed, it would not participate in the weak interaction.Since it has energy and momentum (and probably also mass), it would participate in gravitation.
In short, it has exactly the properties needed for dark matter: It participates in gravitation, but in none of the interactions we use to detect particles (so there's no way we could have detected it).
Unfortunately, unless it also has a mass orders of magnitude greater than the neutrino it would also imply a washing out of large-scale structure. Dark matter is instrumental in forming structure, from galactic to supercluster scales, and the form that structure is therefore highly sensitive to the physics of the dark matter. Heavier dark matter will be non-relativistic and will clump together from very early times, and give a large amount of clumps on smaller scales. Neutrinos, however, are so light that they stay relativistic for most of the universe's history, which means that until fairly recently they were moving at somewhere around the speed of light, which strongly limits the clustering on smaller scales. Structure can be characterised, at least in part, by the power spectrum which shows how much clustering there is on each scale, and observation is very clear that a so-called warm dark matter such as neutrinos simply cannot be the main form of dark matter.
None of that says of course that a currently unknown form of neutrino can't be *a* dark matter, simply a warm component. Hell, even normal neutrinos are a dark matter, just not very significant, and there are speculations about sterile neutrinos, or about neutrinos coupled into a dark energy whose mass grows as the universe expands (which solves some other issues to do with dark energy). Nor does it say that a partner of the neutrino in some extension to the standard model *can't* have a higher mass - the neutralino would be a reasonable candidate, after all - but unless there's a mechanism to boost the mass of your new type of neutrino it certainly wouldn't solve the whole problem.
at tedious length in the past:
* Einstein may be right (as he certainly was on Solar System scales), but we may be applying gravity very wrongly on galactic and extragalactic scales. We do not possess a mean-field theory of gravity since formulating such a thing -- a statistical mechanical version of general relativity -- is currently an unsolved problem. Since galaxies are conglomerations of more than 10^9 confined, weakly-interacting particles, it would seem obvious that we should be addressing them with mean-field theory rather than our current, apparently absurdly naive, single-body plus test mass approach. (Whether our current approach is actually naive or is actually right is, of course, unknown. The truth is probably between these.)
* Even assuming gravity does act as an effective general relativistic theory when "averaged" across billions of what are after all extremely complicated metrics, the geometry of a galaxy is not Euclidean but is instead a complicated mess closer to an axial, or cylindrical, metric. Unconvincing attempts have been made to model a spiral galaxy in this approach and have equally unconvincingly demonstrated that less dark matter is necessary than one would naively expect through applying Newtonian gravity, but it's a tough problem and a closer study might show something more interesting (or, equally, nothing interesting at all.)
* All of the above. What we see as dark matter may well be a combination of *multiple* particular sources (neutrinos, which are a known form of warm dark matter; sterile neutrinos; axinos or neutralinos or some metastable particle from beyond the standard model), that gravity is not exactly general relativistic and instead we've got something like a scalar/vector/tensor theory (as with the recent upsurge in interest in ghost-free bimetric theories, which can be phrased as scalar/vector/tensor, or in the rather less elegant TeVeS which is an... unaesthetic covariant form of MOND), that the bulk geometries should not be taken to be Euclidean and nor should we immediately assume we can "smooth" out a metric as we do in cosmology since such an operation is ill-defined, and that in principle we shouldn't even be describing these systems with a metric-based theory at all but instead a gravitothermodynamic system emerging from the statistical mechanics of spacetime curvature. Indeed, I would put a lot of money on this being true -- except that it's impossible to constrain something with so many unknowns, meaning that people will assume some dust-like cold dark matter, slap constraints on it, and then immediately sell it as physics rather than phenomenology, leaving us with a ten -year struggle to convince even other astrophysicists that they're stating it way too strongly.
For all that in principle it's wrong, data from cosmology is pretty final that what we call "dark matter" can't simply be undercounted normal matter (which we call "baryons" in cosmology, somewhat inaccurately). The thing is that the current concordance cosmological model ("Lambda CDM", for a cosmological constant and cold dark matter) simply fits too much data too well for that to be coincidental; and besides, the observed acceleration plus the almost uniform and isotropic CMB, strongly imply that at least until relatively recently in the universe's history we should be looking at a standard cosmology -- that is, homogeneous and isotropic bar for some small perturbations. Even as something of an iconoclast in cosmology I wouldn't use anything but a standard model (even just as phenomenology where dark matter is concerned) up to perhaps the first billionth year. If nothing else, ripples on the microwave background (formed in the standard model when the universe was roughly 370k years old) and in the large-scale distribution of galaxies (as observed when the universe was around 10bn years old) provide an extremely powerful test. This is because when the universe was very young, the standard model predicts that photons and protons (and electrons, of course) were extremely tightly-coupled together: an electron would condense into a proton to form hydrogen, but the photons were all vastly more energetic than necessary to instantly reionise it, leaving the universe as a perpetual seething game of billiards. Eventually the universe cooled enough that photons had less energy than the ground-state of hydrogen and at that moment the electrons could condense and the photons run free. The point is that while being buffeted by photons, the protons are *also* gravitationally attracting one-another, and tending to fall into gravitational wells, but the photon pressure acts to push them back out -- so they start oscillating. When the universe cooled enough, these oscillations suddenly stopped and the protons were free to condense, but the final wave that was ringing through the universe was still there -- loosely speaking, like you still see ripples and waves on the sand when the tide is out.
So if the standard model is right we should be able to see the imprint on the CMB, and then turn and look for it in the galaxy distribution. Given the CMB value we can *predict* it on the galaxies. And see the ripples we do: http://2.bp.blogspot.com/-3vn0KzoCBxc/UVC0NAlJe6I/AAAAAAAAAI8/bbdFpHs8JPM/s640/power+spectra.png [blogspot.com] (CMB), http://scienceblogs.com/startswithabang/files/2012/08/figure6.jpeg [scienceblogs.com] (LSS). But those are almost 10 *billion* years of evolution apart, and it turns out that the position of the ripple in that second figure is extraordinarily sensitive to the make-up of the universe, and what that position tells us is that *when viewed through the standard model* the universe has to be roughly 70% dark energy and roughly 30% stuff that clumps together.
All well and good, but the CMB plot also tells us interesting things about the composition of hte universe in the standard model. The relative heights of the peaks and how well spaced apart they are can also tell us about the dark energy, but in particular the ratio of the heights of hte first two peaks can tell us how much dark matter there is -- and that gives us about 25% dark matter, 5% baryons.
OK, also well and good, but we're still looking through a phenomenological model. True enough, but if we also look at what is known as big bang nucleosynthesis, when the atomic nuclei formed in the very early universe, we find some extremely precise predictions on the composition of the universe. It turns out that to fit observations, we can't have more than roughly 5% baryons -- this is an independent observation and an independent prediction. We're also heavily limited on the amount of dark energy (not that that's normally significant, but some odd models have an early dark energy and they have to ensure they don't violate BBN constraints), and on radiation, and magnetic fields.
We also simply have the evolution of perturbations, which are taken to be the seeds of everything in the universe. From a simple initial spectrum we can solve (within a second on modern computers) the linear theory and predict to an astonishing accuracy both the form of the CMB and of the LSS power spectra.
That the supernovae data independently predict the same composition of the universe when cross-correlated with one of the CMB or the LSS as the CMB and LSS do together I think is pretty good evidence that we're on roughly the right track. And that massive simulations of the later, non-linear universe recover at least the gross features of the observed universe (meaning down to clusters of galaxies roughly the size of our local group) is even more persuasive.
Of course, absolutely none of this says that "dark matter" is particulate, but I think it's fairly persuasive that we can trust something that acts exactly like the standard model for the early universe and that it serves as an extremely good approximation later on. And I think it's at least convincing enough to argue that most likely the problem isn't due to under-estimating normal baryonic matter; if nothing else, that would seem to violate BBN constraints which are almost universally held to be robust.
All of this is part why it's such a big problem...