There is going to be some amount of debate regarding dark matter in the next few weeks, so I’ve written an article on one of the best ways to go looking for new signs of dark matter out in space.
The reason we are almost entirely convinced that the universe has lots of matter that doesn’t shine is that we can see many signs of its gravitational effects — for instance, its effect on the motions of stars within galaxies, its ability to bend light a la Einstein, etc. It’s almost certain that most of a galaxy is dark matter. And over the years we’ve convinced ourselves this dark matter almost certainly can’t be made from any type of particle that we already know about.
But to learn more about what it is, we need to find signs of some of its non-gravitational effects, if it has any. One possibility is that dark matter particles, if and when they collide, might annihilate into ordinary known particles. If those known particles are photons, we might be able to detect them. A good way to look for them would be to point a suitable telescope toward the center of the Milky Way, our galaxy, which is one place where we expect dark matter particles to be especially numerous, and collisions among them to be especially common.
In the article I just finished, I explain how this can be done. One goes looking for photons from the galactic center, makes a plot of the number of photons observed at a particular energy, and looks for a bump in the plot — an exceptional number of photons with the same energy.
And the reason I’m doing this now is that there is a new paper claiming that a signal of this type may have been seen (with a claimed significance of 3.3 standard deviations, after including the look-elsewhere effect.) This is a paper by a theorist, analyzing publicly available data taken by the experimental group that operates the Fermi Large Area Telescope satellite. One should note that the record of theorists making discoveries using experimentalists’ data is very poor. Typically there are either detector-related or statistics-related issues that theorists screw up. And there are risks of bias — I am not yet sure whether the rather sophisticated analysis method used by this theorist was chosen in a blinded fashion. [For instance, did he choose his method first and then look at the data, or did he already know there was a hint of a peak in the data before he started designing his method?] So I would be skeptical of this claim for now. (And the theorist, knowing he’s out on a limb, was careful [and wise] to put the word “Tentative” in his title.) However, stranger things have happened, so I wouldn’t dismiss this claim out of hand either, at least not until the Fermi experimentalists tell us that in their opinion the theorist over-estimated the statistical significance of this particular bump. We’ll be looking forward to what they have to say.
I’ll have a few more details about this for you soon.