SoylentNews is people
SoylentNews
A just-released paper Researcher advances a new model for a cosmological enigma—dark matter offers a novel model of dark matter, dubbed "flavor-mixed multicomponent dark matter." From the article:
"Dark matter has not yet been detected in a lab. We infer about it from astronomical observations," said Mikhail Medvedev, professor of physics and astronomy at the University of Kansas, who has just published breakthrough research on dark matter that merited the cover of Physical Review Letters, the world's most prestigious journal of physics research.
Medvedev's theory rests on the behavior of elementary particles that have been observed or hypothesized. According to today's prevalent Standard Model theory of particle physics, elementary particles—categorized as varieties of quarks, leptons and gauge bosons—are the building blocks of an atom. The properties, or "flavors," of quarks and leptons are prone to change back and forth, because they can combine with each other in a phenomenon called flavor-mixing.
"In everyday life we've become used to the fact that each and every particle or an atom has a certain mass," Medvedev said. "A flavor-mixed particle is weird—it has several masses simultaneously—and this leads to fascinating and unusual effects."
Medvedev said that dark matter may interact with normal matter extremely weakly, which is why it hasn't been revealed already in numerous ongoing direct detection experiments around the world. So physicists have devised a working model of completely collisionless (noninteracting), cold (that is, having very low thermal velocities) dark matter with a cosmological constant (the perplexing energy density found in the void of outer space), which they term the "Lambda-CDM model."
But the model has hasn't always agreed with observational data, until Medvedev's paper solved the theory's long-standing and troublesome puzzles.
"Our results demonstrated that the flavor-mixed, two-component dark matter model resolved all the most pressing Lambda-CDM problems simultaneously," said the KU researcher.
The original of the article seems to be on the Kansas University web site. According to: Cold Dark Matter is often called Lambda-CDM:
Cold Dark Matter is an abbreviation for Lambda-Cold Dark Matter. Cold Dark Matter represents the current concordance model of Big Bang cosmology that explains cosmic microwave background observations, as well as large scale structure observations and supernovae observations of the accelerating expansion of the universe. Cold Dark Matter is the simplest known model that is in general agreement with observed phenomena.
I am fascinated by stories on cosmology but have only a passing understanding of the material; is there a physicist or cosmologist around who would like to weigh in and shed some light on the subject?
(Score: 2) by HiThere on Saturday September 06 2014, @07:14PM
I have essentially no knowledge in this area, but just based on the past history of people exploring new areas I would expect that it's a lot more complex than two components. Saying that Dark Matter is two components may well be like saying the baryonic universe is built out of Hydrogen and Helium. There are other important constituents, even though they are minor as a percentage of volume.
Javascript is what you use to allow unknown third parties to run software you have no idea about on your computer.
(Score: 1) by boristhespider on Sunday September 07 2014, @10:39AM
Absolutely. There are a variety of ways we can get the behaviour of a dark matter: particulate dark matter, scalar fields, modifications to gravity, consequences of modifications to gravity such as massive matter on branes near to us (a setup with the additional benefit of introducing an effective scalar field, the dilaton), a more careful consideration of the gravitational field of a galaxy, a more careful consideration of how average behaviours actually work in metric theories of gravity, and a more careful analysis of the paths that light travels on.
Particulate dark matter is clearly the most popular, and is usually taken to be a single species of almost collisionless, massive particle. Indeed, we already know of a particle that acts, to some degree, as a warm dark matter: neutrinos. Neutrinos have mass, and are abundant throughout the universe, and so they are a dark matter even if this is not often appreciated. That's not least because neutrinos cannot be "the" dark matter, since as I noted above they would wash out structure on small scales, and by "small" I mean "galaxy cluster". A second popular candidate is a lightest supersymmetric particle; if there is supersymmetry (which I have my doubts on, but it's still always possible) then there is a stable particle that is lower of mass than the other supersymmetric particles. Then supersymmetric particles will tend to decay into the LSP, but this cannot decay into normal matter and neither does it interact in any normal way other than gravitationally: it's a dark matter. (Candidates include the axinos and neutralinos.) But there's no a priori reason to limit ourselves to one, two, or thirty particulate dark matters.
Or we can have scalar fields. These would normally be invoked in the current universe to provide a dark energy but there's no reason at all not to employ one as a dark matter - if we're going to accept a field with an absurdly flat potential in the present universe, it's surely less of a stretch to accept one with a less finely-tuned potential that makes it virtually pressureless.
Or we can modify gravity in one way or another, with famous examples being the likes of MOND and its relativistic generalisations. Many ways of modifying gravity yield a theory that can be transferred into the "Einstein frame", in which the equations are those of normal general relativity, plus an effective matter which is typically then described as a scalar field or a combination of scalar and vector fields, so in some ways this is linked to the above. For instance, relativistic generalisations of MOND tend to be scalar/vector/tensor theories of gravity; and the differently-motivated bimetric theories are also basically scalar/vector/tensor theories. To be honest, on galaxy scales MOND is probably the most successful and simple approach to dark matter, since with a single, universal parameter it fits practically every observed galaxy - a success that standard dark matter can't reproduce. On the other hand, on cluster scales MOND is laughably pathetic, so I wouldn't take this as more than indicative that *something* is going on that looks impressively like a bottoming out of acceleration in some regimes.
Or other ways of modifying gravity emerge from attempts at string theory - and, in particular, from M theory. They've lost popularity somewhat in the last decade but for a while braneworld theories were all the range. In M theory one can have surfaces of various dimensionality strung through the 11+1d "spacetime". In particular, one can have 3 dimensional surfaces. The ends of "open" strings snap onto these surfaces and are exhibited as particles, while "closed" strings pass through, and act as gravitons. If we then have two such surfaces suspended near to one another, particles will react to each other gravitationally, but in no other way - dark matter.
On the other hand, studies of gravitational dynamics, let alone cluster dynamics, are typically done assuming effectively Newtonian physics (which is why MOND is MOND, not MOED). This assumes a flat, Euclidean spacetime -- but the spacetime around a galaxy is not Euclidean. It's a horrific mess of about 10^9 to 10^12 spacetimes, each of which individually is something near to Schwarzschild or Kerr-Newman. A better description of the spacetime around a spiral galaxy might be something more or less cylindrical, and charged, given that the galaxy sustains a magnetic field. Preliminary and unconvincing studies of this typically find that the need for dark matter can be reduced by something between 1-15%.
But this also raises the next suggestion, that we're simply trying to apply local theories (such as relativity) to large systems. We don't do that in other spheres of physics. There's a reason we study the gas in a room with temperatures and entropies and specific heats -- thermodynamics is a theory that emerges from a statistical mechanical mapping of the local physics up to a collection of around 10^23-10^24 atoms. We do this because solving the dynamics of 10^24 particles would take about the age of the universe to do properly. But as I mentioned, galaxies are collections of around 10^9-10^12 "atoms", and we're attempting to describe them as if we already know the mean field. And we most certainly do not. There are two ways of solving this, neither of which work: we can take spatial averages (a procedure riddled with problems, and, technically, impossible if there's a single gravitational lens in a galaxy), or we can take statistical averages (a procedure riddled with even more problems, not least of which is that statistical mechanics maps the local Hamiltonia of the atoms onto the free energy of the systems... and there *is* no Hamiltonian for GR; it's constrained to be zero from the start). Crude approaches to the spatial averaging ignoring this issue typically find things that operate as a dark matter (or, on cosmological scales, sometimes as a curvature.)
Or we could look a lot more carefully at the influence of gravity on the propagation of light. For dark matter I'm not aware of anyone who's managed to show much from this, but that it can be significant can be seen on cosmological scales, where one can take as a toy model of the universe a Lemaitre-Tolman-Bondi, an isotropic but spherically inhomogeneous solution. Tuning this, and not actually all that carefully, one can recover signals that look like dark energy -- but in a universe filled with nothing but dark matter. LTBs cannot be a good model for our universe because while we can mimic the *local*, supernovae, observations of dark energy, things go to pot as soon as we look further away, particularly at the CMB. But the point isn't to suggest LTBs are a good model for the universe, but rather that even very simple changes in the setup of a gravitational system can strongly bias your observations, and since every observation we make is of light we should probably understand what's happened to it along its path.
Most likely, the answer is a combination of all of the above. I've often argued against particulate solutions for dark matter, but ultimately I've got no genuine reasons to assume that it doesn't exist, and no reason to assume we don't have more than one unknown type. I also know our theory of gravity is wrong, that simple modifications to gravity work beautifully well on galactic scales, and that we don't even understand how to apply gravity on such scales, and nor do we understand how to recover a mean field, or the backreaction of individual stars as they propagate through it. The only problem is that we can't even properly approach some of these, and even if we could we'd be throwing so many parameters into the mix that we'd have absolutely no constraining power at all. But the flipside of that is that at some point in the next few years someone is going to make a lot of noise about "the" dark matter, and pinning down "the" dark matter, and it might be decades before we can demonstrate that actually it's nothing like abundant enough to be "the" dark matter and there must be other contributors - and that "the" dark matter is actually three, or six, or twelve related particles anyway...