It's been a roller-coaster month for many scientists. In the US, there was a government shut-down. And in the wider community, there have been a number of news article on some interesting, but inconclusive experimental finding. The goal of this post is to highlight the findings and give people links to the articles by Scientific American and New Scientist.
So, a list of some recent experimental findings:
(1) Dark Matter particles likely have rest mass between 8 keV and 14 keV
Horiuchi et al. recently published a paper that compares experimental measurements with dark matter theory suggests that the rest mass of dark matter particles is somewhere between 8 keV and 14 keV. Only in this range of values of the rest mass can the number of experimentally measured subhalo counts be predicted. (See Figure 2 and Table II from their paper below.) This appears to be strong experimental evidence against dark matter with rest mass values of MeV or GeV. I look forward to see more data collection and analysis along these lines.
(2) Speed limit found for sluggish dark matter
It appears (if the paper in the following link is correct), that there is a limit to the root mean square (RMS) velocity of dark matter. This maximum limit is 54 m/s.
By analyzing data on dark matter, it appears that they can rule out dark matter having a mass less than 1 keV. It should be pointed out that, in the recent past, virtually nobody thought that dark matter is less 1 keV. The real question is: does their recent results rule out dark matter particle near 2 keV?
There are many researchers who think that a 2 keV sterile neutrino is the best-fit for dark matter because it doesn't clump in the middle of galaxies, as would MeV or GeV dark matter. The reason that it doesn't clump up is that the de Broglie wavelength of the particle is so large that you can't confine it into regions as small as the center of a galaxy. For example, the de Broglie wavelength of a hypothetical sterile neutrino with a rest mass of 2 keV and a velocity of 54 m/s is approximately 4 mm. (Note that the deBroglie wavelength of an electron in a metal is on the order of ~ 1 nm.) Since there would be a large number of hypothetical sterile neutrinos if they were dark matter and since neutrinos are Fermi particles, it's fairly easy to see how quantum effects keep dark matter particles from collapsing intot he center of the galaxy due to their gravitational effects. (If 2 keV sterile neutrinos are the source of dark matter, then we have another excellent demonstration of quantum mechanics.)
However, here's the problem. If the 54 m/s RMS speed limit is true, then that implies that the temperature of the dark matter (if it is 2 keV in rest mass) is only 3*10^-7 K. This number is so small that it seems completely unrealistic. As such, it seems that the finding of RMS velocity of dark matter of less than 54 m/s is likely a death-blow to the idea of a keV sterile neutrino. However, I think that it's important to collect some more data before ruling out the possibility of a keV neutrino because keV neutrinos seem to be able to explain why dark matter doesn't collapse on itself in the center of galaxies.
(3) Cosmological Constant is not supported by data
It appears that a strictly constant cosmological constant is not supported by recent analysis of super-novae explosions in the early universe. The data leads to a parameter value of -1.186 whereas a strictly constant cosmological constant predicts exact -1. I'm not familiar with the research, so I don't speculate much further. However, I do want to point out that my current guess is that dark energy is not constant with time and that the rate of expansion of the universe depends on the number of 'time-irreversible' collisions involving the weak nuclear force. There is no model to back up this claim; it's just an educated guess based on the following known facts: (1) the acceleration of the universe is not constant, (2) the weak nuclear force is the only time irreversible force, (3) irreversibility seems to go away in superconducting wire and in superfluid helium when Fermi particles combine to form Bose particles (which no longer can interact via the weak nuclear force), and (4) the largest acceleration of the universe was at the beginning when there were a lot of particles that could interact via the weak nuclear force.
(4) No evidence of Majonana neutrinos in tests of Neutrinoless Double Beta Decay
Recent experiments in Russia find no evidence that neutrinos are their own anti-particle, i.e. that they are Majonana fermions. If neutrinos were their own anti-particle, then it should be possible for neutrinoless double beta decay to occur. However, there have now been a number of experiments showing that Neutrinoless Double Beta Decay, if it occurs, occurs so slow that it can rule of neutrinos from being their own anti-particles.
So, what does this mean? I'm not totally sure, but I think that this research lends credence to the idea of sterile neutrinos that act very different than conventional neutrinos. The sterile neutrinos are not just the anti-particles of conventional neutrinos. There is still a lot of interesting research to do in this area.
Also, I want to quote Andrew Grant from the Dispatches from Turtle Island blog
Also, if neutrinos are Majorana rather than Dirac, there may be as many as three CP violating phases governing neutrino oscillations and the PMNS matrix is not necessarily unitary.
If the weak nuclear force is the cause of the expansion of the universe, and since there only appears to be one time dimension, my guess is that there is only one CP violating phase in PMNS or CKM matrix. This, along with the lack of neutrinoless beta decay, seems to imply that neutrinos are Fermi-Dirac particle and not Fermi-Majorana particles.
(5) No evidence for WIMP Dark matter
The latest results from the Large Xenon Underground detector suggest that Weakly Interacting Massive Particles might not be dark matter candidates. The underground laboratory found no signs of dark matter particles in the GeV/c2 range of rest mass.
These results seem to contradict the results from (1), which found a thermal velocity greater than 54 m/s. If it weren't for the 54 m/s thermal velocity minimum, then 2 keV sterile neutrinos would seem to be the best explanation for dark matter. However, things are likely to be more complicated than we can imagine today.
(1) We are living in a time period in which there is no accepted model of the Universe. None of the following models are deemed acceptable:
String Theory, Loop-quantum gravity, Cold-Dark Matter, Warm Dark Matter, Cosmological constant, Supersymmetry, and, lastly, the Standard Model is deemed unacceptable because the Standard Model doesn't claim to explain the reason for the mass, charge, and mixing angles for particles even though it makes really good predictions given this experimental inputs.
(2) Given the inconclusiveness of the data we've collected so far, it seems like we have no choice but to continue investing in (likely expensive) experiments that can push the boundaries of our known universe.
(3) Nobody correctly predicted ten years ago the all of the experimental results from LHC, Planck, Super-K and other experiments. We are all stumbling around in the dark. All of our theories are at least partially wrong...we just don't know what parts of wrong and what parts are right for each of our token theories. All we can do is to group together to help educate the public on what is known with certainty and to argue to politicians why we need funding for the likely expensive equipment we hope to build in the future to test our theories. The reason we should make this investment is that studying particle physics and astrophysics helps us understand who we are and where we are going. Until we know this more precisely, we still can't answer the question either of these questions.