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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: 2, Interesting) by Anonymous Coward on Wednesday June 04 2014, @07:48AM
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).
(Score: 1) by boristhespider on Thursday June 05 2014, @07:57AM
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.