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.
Now that rumors have been flying around for a few weeks, LIGO has announced that there will be an update on their latest gravitational wave results on Thursday at 10:30 am. This kind of thing isn’t very common, so it does sound quite likely that they will announce that they have found something. A discovery of gravitational waves would be one of the most important physics results of the last few decades.
The rumor from earlier this year that LIGO has found gravitational waves has returned. This time, there’s a theorist who claims that a paper will be released by Nature in less than a week, so we won’t have to wait long to see if this particular rumor is true. The claim is that LIGO has found definitive evidence of a black hole merger, which would be very exciting. Measurable gravitational waves are expected to be generated when two very large objects orbit one another at a close distance, with the waves bleeding off energy and causing the orbital radius to decrease until the objects merge. An interferometry experiment like LIGO would then see a clear oscillating signal from the orbits of these two objects. Gravitational waves are one of the most important predictions of general relativity that we could measure.
Speaking of underground physics, Gizmodo has an article today on Snolab, with pictures of some of the facilities and a few detectors. Snolab is the deep underground lab in Sudbury, Ontario where a number of particle physics experiments are operating or are planned, such as SNO (and SNO+) and quite a few dark matter searches.