One of the things that isn't widely advertised in science is that progress often looks a bit like this video—things get done, but the results are rarely quite what you expect. A recent example of this two steps forward, one step back progress may be the study of the recently observed excess positrons coming from the center of our galaxy.
Although there are many possible astrophysical explanations, none of them were that clean or appealing, leaving one alternative attractive: dark matter. Dark matter is thought to be made up of weakly interacting massive particles, which every now and again collide and annihilate. One particular pathway for the annihilation results in positrons with about the same energy of those seen coming from the core of the galaxy. Hey, presto! thought some scientists. We may have seen dark matter decays, which then allow us to pin down dark matter. Oh and incidentally, the medal should be pinned on my left—that's your right—lapel.
But, as a recent Physical Review Paper shows, the excess may be real, but if it comes from dark matter, we have some serious cosmological problems on the horizon.
Let's take a step back. We know that dark matter exists from numerous lines of indirect and direct evidence. These observations also tell us how dark matter is distributed within galaxies. There are two key parameters here: the physical shape of the dark matter distribution and the density of dark matter. We also know from dark matter's apparent lack of visibility that it can't interact with regular matter very strongly, which allows physicists to piece together the basic physics of positron production.
The latter act was performed by the researchers who had observed excess positrons from the center of the galaxy. They found that dark matter could only explain the excess if there was some kind of resonant enhancement of the decay. Luckily, particle physics comes equipped with these resonances naturally, so you end up with a lovely if-then scenario. If the positrons come from dark matter, then we know the resonant enhancement must be about so much, and therefore dark matter must have the following properties. Very tempting conclusion.
Unfortunately, the original research did not consider whether the required enhancement would be in agreement with other data. Now, a trio of researchers from University of California in Irvine have taken a look at this. They observed that the shape of the distribution of dark matter in the galaxy NGC 720 was elliptical, but that increasingly stronger resonant enhancement resulted in nearly spherical dark matter distributions. In the end, the shape of the distribution could not be reconciled with the idea that the excess positrons came from dark matter annihilations, resonantly enhanced or not.
The other key factor is the density of dark matter. Here, the researchers found something a little more interesting. First, the density does allow for dark matter annihilations, but with only a very small resonant enhancement.
More interesting is the fact that resonant dark matter annihilations predict different dark matter densities than those obtained from astronomical models, meaning that we can look for galaxies that don't fit the models to see whether they might be accounted for by dark matter annihilations. This will then allow us to place even tighter bounds on the nature of dark matter.
So, if I had been an author on one of the original papers, I might be feeling a bit bruised right now. However, the result was good for the field—this research offers a new direction from which to approach the dark matter problem. We can now turn our attention back to the universe, and see if she happens to have another crumb or two of information for us.
Physical Review Letters, 2010, DOI: 10.1103/PhysRevLett.104.151301
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