There’s a new white paper/review that just appeared on the arXiv about the possibility of keV-scale sterile neutrino dark matter. I haven’t read through it, but this is an interesting non-WIMP possibility for dark matter. Sterile neutrinos don’t interact at all with the Standard Model except maybe through neutrino oscillations, but if they have mass, they can still be a dark matter candidate if there’s a good production mechanism. Current dark matter experiments would generally be insensitive to this kind of dark matter, since the standard signal of low energy nuclear recoils would probably be disallowed, or at least heavily suppressed, but there are apparently still some paths toward finding sterile neutrino dark matter beyond just discovering keV-scale sterile neutrinos.
The Chronicle of Higher Education has an article about PRL, which is the most prestigious physics journal in many fields. Last week’s LIGO result was published in PRL, and traffic during the announcement actually crashed the PRL website for a few hours.
US Supreme Court Justice Antonin Scalia died yesterday, which is one of the most important political stories of the year. With about a year left in Obama’s term, this potentially has a major effect on the upcoming election. Obama has already had two Supreme Court appointments, so a third would mean that Obama will have gotten to appoint three of the nine justices.
IceCube and Antares have looked for evidence of neutrino emission coincident with the gravitational wave signal seen by LIGO. They see no evidence of neutrinos being emitted by the gravitational wave source. That doesn’t mean there are no neutrinos, just that even if there are, not enough reached us to be able to see them. But, as the article points out, this means that gravitational wave and neutrino observatories can now work together to try to study rare astrophysical events.
Sean Carroll, who wrote one of the most popular textbooks on general relativity, has a new article in The Atlantic. He goes over some of the historical context and some of the ideas behind gravitational waves so that people who know almost nothing about physics can understand a little about yesterday’s announcement.
While yesterday’s big news was clearly the gravitational wave result, LUX also put its first spin-dependent WIMP interaction limits on the arXiv. In direct detection experiments, the spin-independent limit is typically stronger because the amplitudes add together in coherent nuclear scattering, leading to a dependence that scales like a polynomial factor of the atomic number. Spin-dependent interactions, typically from axial vector interactions, give rise to terms related to the individual nucleons’ spins. Nucleons tend to arrange themselves so that the spins mostly cancel, so the spin-dependent terms tend to be smaller than the spin-independent terms by a factor of approximately A2 if you assume that the fundamental couplings are the same. Because the cross section ends up with some angular momentum factors in terms, you need to know the isotopic abundances very well to get a reliable spin-dependent measurement. In this result, LUX gets the best result of any direct detection experiment for WIMP-neutron spin-dependent scattering and is about an order of magnitude behind in the WIMP-proton channel (xenon is not the best nucleus to use for spin-dependent proton interactions).
LIGO has confirmed the rumors about what they had seen and announced that they have found the signature of a merger of two black holes over a billion light years away. This event was actually very fortunate, as it happened before the main science run started but while the detectors were operating as if a regular science run was going on. The signal is good enough to tell how long ago the event happened and even how much mass the system has and how much energy was lost due to radiation.
Both LIGO sites – Louisiana and Washington – saw the signal but unfortunately, there were no other interferometry experiments operating at the time to get a third signal. Hopefully some new sites will come online in the near future so that a worldwide gravitational wave measurement network can be set up. Large neutrino detectors do something similar for supernovas so that if several detectors see a bunch of events at once, we know that a supernova will be seen in a particular part of the sky. With three sites, there would be some ability to point back at the direction of the source of the gravitational wave using timing information.
Regardless, this is a very strong signal that was seen at two sites that are several thousand miles apart. It looks quite convincing, and hopefully if we’ve already seen one event in a short run time, we’ll see a lot more as LIGO continues to run and as other experiments are built.