Yesterday, Columbia professor Brian Greene was on Colbert’s show to talk about gravitational waves. Greene gives some nice explanations for laymen (with graphics!) about gravitational waves in general and about LIGO. He even brings out a Michelson interferometer to demonstrate how the LIGO setup works (though not with gravitational waves). Since Greene is a theorist, I would assume that someone else had to set up the interferometer for it to actually show some sensible results. You can find the video on Youtube here.
The ANTARES neutrino telescope has a new result looking for “secluded” dark matter, where dark matter annihilation is mediated through some new mediator that then decays into Standard Model particles. They claim that this can explain the high energy bump in the positron/electron ratio and can also still be a thermal relic from the Big Bang.
They look at several different channels, including one where the mediator actually lives long enough to reach Earth and decay in the atmosphere, and others where neutrinos in the final state are measured.
For this model, the result is actually stronger than direct detection experiments for spin-dependent interactions and is stronger at very high masses in the spin-independent channel. While this result isn’t particularly groundbreaking, the paper mentions that it is the first search of this kind for this type of dark matter, and I think the model, which I hadn’t heard much of previously, sounds quite interesting.
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.
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.
Since this year is the hundredth anniversary of Einstein’s theory of general relativity, various websites have been publishing articles on Einstein. The New York Times has one from earlier this week on the history of general relativity that you should read if you’re interested in that sort of thing.
BBC Earth has a new overview of dark matter written for laypeople. It has a bunch of nice pictures and figures and goes over a lot of the basic concepts like early evidence from Zwicky, the WIMP hypothesis, problems with MOND models, etc.
The New York Times has a long article on the “Event Horizon Telescope,” a project using a number of large telescope facilities to try to find conclusive evidence for the existence of the black hole believed to be at the center of the Milky Way. The article talks a bit about the science goals but also a lot on the history of such work and the people who do it. There are also some really nice images in the article and a couple related articles. Finding better evidence for black holes would be hugely important because it might let astronomers test some of the predictions of general relativity.
The BBC has an interesting recent post on what happens when you fall into a black hole. It mostly gives a rundown of various ideas about what happens (we can’t really test those ideas for obvious reasons). I don’t really know enough about general relativity to comment on the accuracy of the post, but I think they are correct when they say that if you fall into a large black hole, you don’t really notice anything, Nothing particularly special seems to happen at the event horizon to the object passing across the horizon. The more interesting idea here is that the black hole in some sense splits reality into two parts that can’t interact with one another (except perhaps through some quantum gravity effects).
The ANTARES experiment has a new preprint out setting a limit on the annihilation of dark matter in the center of the Milky Way. ANTARES is an underwater neutrino telescope located in the Mediterranean off the coast of France. It uses photomultiplier tubes placed underwater to look for Cherenkov light from particles created when neutrinos interact in seawater. The basic idea sounds similar to various cosmic ray experiments like IceCube (also looking for neutrinos) and Auger (looking for ultra high energy cosmic rays of more traditional types like protons). Rather than attempt to build an enormous detector, they just place sensors in a fairly uniform natural medium like water. This is a great way to get an enormous fiducial volume, although it is obviously harder to control than a fully man-made detector system.
Since ANTARES looks for neutrinos, the dark matter signal they look for is dark matter annihilation to a neutrino-antineutrino pair. Since WIMP dark matter is assumed to be quite heavy, each neutrino gets a momentum (and energy) equal to that of a dark matter particle. Since the neutrinos are coming out of the center of our own galaxy (about 7 kiloparsecs away), things like redshifting due to the expansion of the universe are irrelevant. The main signal from WIMP annihilation directly to neutrinos would be a nice peak around the WIMP mass. They also look for WIMP annihilation to other states that eventually lead to neutrinos, such as bottom pairs and tau pairs, but the spectra for these are not quite as obvious as for the neutrino case. There are plenty of non-dark matter neutrinos, so the analysis would have to rely on finding a signal on top of a nontrivial background. In the end, they get the best current limit from a neutrino telescope, although not really the best overall.