Last week I attended the Eighth Harvard-Smithsonian Conference on Theoretical Astrophysics, entitled “Debates on the Nature of Dark Matter”, which brought together leading figures in astronomy, astrophysics, cosmology and particle physics. Although there wasn’t much that was particularly new, it was a very useful conference for taking stock of where we are. I thought I’d bring you a few selected highlights that particularly caught my eye.
Does Dark Matter Exist, and In What Form?
First, the question of whether dark matter even exists was nicely explored by Stacy McGaugh, from the University of Maryland, who gave a talk on the possibility that the laws of gravitation and/or motion are altered, along the lines suggested by Mordechai Milgrom. The theoretical approach of Milgrom is usually called “MOND”: MOdified Newtonian Dynamics; it involves an ad hoc change in the laws of motion when objects exceed a certain rate of acceleration. McGaugh, who is an astronomer, gave a careful illustration of how MOND à la Milgrom explains some odd features of galaxies. This involves careful examination of how stars rotate around the centers of galaxies of all sizes, from the very large to the very small. He emphasized that the hypothesis that galaxies are made mostly of dark matter, and embedded within a large clump of the stuff, does not easily explain these features (though interestingly professor Alyson Brooks of Rutgers University mentioned that her simulations of galaxy formation, which assume the presence of dark matter, do actually reproduce one of these features.) He also showed that, by contrast, MOND doesn’t do well at all with certain cosmological data, especially in properties of the cosmic microwave background. It was nice to see the evidence for and against MOND laid out in a single, well-delivered talk; it’s important to look at this question in an even-handed way, so that both the weaknesses and strengths of the dark matter hypothesis are clear.
But in his Wednesday evening talk, the famous astrophysicist and cosmologist Jim Peebles pointed out just how broad is the cosmological evidence for dark matter, from a wide variety of independent sources. He views the basic notion of dark matter as an idea that is not just widely accepted, but has been established by data. But he also suggested, alluding to the many puzzles about galaxies (some of which were mentioned in McGaugh’s talk) that we should view our understanding of galaxies and how they form and evolve as still quite poor, with room for significant surprises.
Unfortunately, not that much time was spent during the conference on the possibility that dark matter is in the form of waves in a type of field called an axion. And almost nothing was said about the possibility that dark matter is made from other types of objects, such as primordial black holes. But I guess one can’t cover everything in a 3½ day conference…
So Many Hints, So Little Time
A lot of the presentations, discussion and debate concerned the hints of dark matter that have been coming and going like clouds on a windy day.
Anyone who still believes, after the limits from XENON100 and the devastating blow from LUX, that the hints of dark matter seen by DAMA/LIBRA, COGENT, CDMS, CRESST, etc. might be due to something real now also has to confront the non-observation of any such hint by Super-CDMS — an upgrade of the CDMS detector. While not as powerful as LUX for large-mass particles, Super-CDMS is both more powerful and more robust for detecting dark-matter particles with masses in the 2 – 10 GeV range. LUX and XENON100, which use liquid xenon as a target, are very different in their technology from the experiments that see hints. These all use solid targets made mostly from smaller atomic nuclei than xenon, and this could allow one to dream that dark matter is of a sort that would show up in experiments such as DAMA more easily than in xenon-based experiments. But Super-CDMS uses a germanium target, and germanium nuclei are quite a bit lighter than xenon nuclei; so this argument is no longer so persuasive. Yes, it is still perhaps possible to imagine dark matter that would give hints at DAMA/LIBRA or COGENT and escape both the xenon experiments and Super-CDMS; but it is looking increasingly contrived.
Meanwhile, the possibility of dark matter annihilating in the center of the galaxy and resulting in unexpected numbers of positrons is suggested by an unexpected number of positrons, relative to electrons, at energies of 10 – 300 GeV. This effect was first noticed by the PAMELA experiment, and then confirmed by FERMI, after which it was carefully measured by AMS. But is this effect from dark matter, from pulsars [for a definition, see below], from supernova remnants, or just simply from cosmic rays? There were three very different talks on this question, with very different viewpoints. Personally, I wouldn’t bet on dark matter here…
Yet another hint of dark matter — an excess of photons of energy of about 130 GeV or so from the center of the galaxy, seen in the data from the Fermi satellite — appears to be going away, despite having reached 5 standard deviations of significance by someone’s measure. Among those saying so is Christoph Weniger, the researcher who first identified this excess a couple of years ago. It isn’t clear yet whether the excess was a statistical fluke or a detector problem — the significance of the signal has gone down both because of a reanalysis by Fermi of their data and because there have been no events at 130 GeV for many months. Either way, it seems to be gradually disappearing as more data is collected, and while its cause may become clear in a few years, all we can say for now is “R.I.P”.
Hint Du Jour Number 1
But two other hints of dark matter are alive and … well, not dead yet. The first involves an excess of photons (i.e., more photons are observed than expected) in the 1 – 10 GeV range, originating from a region of the galaxy that extends outward a very substantial distance from the center of the galaxy. These photons, first identified by Goodenough and Hooper here, and most recently explored here, may be a signal of dark matter particles annihilating. (Specifically, when they annihilate they may produce a quark and an antiquark, which in the end produce multiple hadrons, some of which can decay to photons.)
However, these photons might also be due to something astrophysical. So far, the only known astrophysical candidate is a horde of millisecond pulsars. What are those? A neutron star is a giant balls of neutrons, about 20 kilometers across, that is a remnant from a Type II supernova explosion. A pulsar is a neutron star that has a strong magnetic field and sends beams of electromagnetic radiation into space as it spins; since these beams are only detectable to us when they point in our direction, we see them as pulsing, hence the name “pulsar”. A millisecond pulsar spins 1000 times a second or so (wow!). These objects can emit very-high-energy photons, as energetic as a few GeV, out into space.
The excess of photons have an energy dependence that looks a bit different from pulsars, but the peak of the distribution occurs at about 2 GeV and drops off around 10 GeV, which is just what pulsar photons would be expected to do. That makes one immediately suspicious that pulsars are responsible. That said, the wide distribution in space of these unseen pulsars, and the number of pulsars required, are perhaps surprising. But pulsar experts tell me that so little is known about pulsars in that region that it would be hard to argue strongly against that possibility.
How can the two possibilities (pulsars vs. dark matter) be distinguished? If this same signal is seen in the dwarf galaxies that surround our galaxy, which should have very few stars but lots of dark matter, it might convince us that the signal is from dark matter and not from pulsars. Conversely, if it isn’t seen, it is more likely from pulsars. Maybe. Well, this hint isn’t likely to go away soon, nor is it likely to be diagnosed convincingly anytime soon, so we’ll hear more about it in future.
The Hint Du Jour Number 2
The second hint is from an excess of X-rays with an energy of just about 3500 eV; these have been identified in various galaxies and clusters of galaxies. (I wrote an incomplete article about it here; I still owe you a better one.) These X-rays are most popularly suggested to be from dark matter in the form of decaying sterile neutrinos (i.e. neutrino-like particles that are affected by none of the known non-gravitational forces, unlike ordinary neutrinos which are affected by the weak nuclear force.) However, as I emphasized to you from the beginning, the same signal could also arise from other types of dark matter. For example, if dark matter particles are, like atoms, able to be “excited” (i.e. able to absorb energy and become slightly heavier but unstable versions of themselves), then when two dark matter particles bump into one another, one might become excited, and later decay back to its original state while emitting an X-ray photon.
Alternatively, perhaps this excess is due to an unknown atomic effect, and has nothing to do with dark matter at all. Again, seeing this excess in dwarf galaxies would be potentially crucial in deciding the issue. Compared to galaxies the size of the Milky Way, dwarf galaxies have much more dark matter than hot gas, so these X-rays would be seen there only if they are due to dark matter, and not due to atomic physics.
Since either or both of these hints du jour might have to do with astrophysics, not dark matter, we are, yet again, in a waiting game — waiting for more data, and more convincing data, that might allow one or both of these sources of photons to be fully explained.
Looking for Dark Matter In New Ways
Finally, one of the most interesting things I learned at the conference was explained by professor Rouven Essig of Stony Brook University. He and his team has been working on the problem of detecting dark matter particles much lighter than those that XENON and LUX and Super-CDMS are designed to detect — as much as 100 to 1000 times lighter. XENON-, LUX-, COGENT- and CMDS-type experiments are best at detecting particles with mass of 5 – 1000 GeV/c², by picking up signals from the scattering of a dark matter particle off an atomic nucleus. But there’s a workable strategy, Essig argued, for picking up the scattering of a dark matter particle off an electron in an atom. Detecting dark matter particles with masses down to 10 MeV/c² could soon be possible in XENON/LUX-type detectors, and sensitivity to 1 MeV/c² might someday be possible in CDMS-type (i.e. solid-state) detectors. This opens a new opportunity to look for dark matter in a form that previously would have slipped through our nets.
The Debates Will Continues
Though dark matter continues to elude our attempts to identify it, many experiments are continuing to seek this mysterious stuff, using a wide variety of methods. We can expect more hints of progress (most of them false alarms) over the coming decade. But can we expect an actual discovery? Only the heavens know.