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from the now-you-don't-see-it-and-now-you-still-don't? dept.
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.
(Score: 1) by boristhespider on Wednesday June 04 2014, @07:02PM
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...