If I seem a little obsessed with dark matter at the moment, it's only because there is so much interesting stuff going on right now. But I can give it up any time—really! As I reported last month, there has been a lot of excitement among astrophysicists and cosmologists because there seems to be more gamma rays than expected coming from various places, including near the center of the Milky Way and other galaxies. Unfortunately, as I also reported, it seems very difficult to absolutely rule out other possible sources for these extra gamma rays. In particular, there is the problem of unresolved sources. These could be gas clouds or other emitters that we simply haven't spotted in other observations.

The obvious solution is to simply keep looking, using other telescopes that look at the sky at other parts of the electromagnetic spectrum to rule out each and every possible source. Some new research tells us how the gamma ray signal may hold much of the evidence already, however. We just need to look closely.

Watching matter move

The idea comes down to how matter moves. The fact that dark matter doesn't really do anything but suck, gravitationally speaking, means that it doesn't really follow the cool kids around, either. When ordinary matter gets close to another bit of ordinary matter, it says hello. It does the equivalent of standing in the middle of the supermarket aisle having a long conversation about the health and happiness of both parties' electrons. Along with gravity, this meeting doesn't just cause matter to clump together, it also causes it to move together. So in our spiral galaxy, it isn't just the stars that rotate around a common center of mass. All the ordinary matter rotates around that common center of mass, too.

Dark matter isn't like that. It doesn't just ignore ordinary matter; it doesn't even like its own family. Dark matter is that guy who goes to the club to make sure that everyone knows that he doesn't dance. The only reason that dark matter is in the galaxy at all is that it's moving so slowly that it can't escape the gravitational pull of the rest of the matter around it. Indeed, without these slow-moving herds of dark matter sucking everything in, galaxies wouldn't have formed at all.

But even though it's there, dark matter watches the galactic dance from the sidelines. While the stars and gas clouds all move gracefully round each other on the dance floor, dark matter just hangs out, looking sulky.

Are you looking my way?

Lazily drifting dark matter gives itself away when it accidentally runs into something and self destructs in a burst of light: a gamma ray. Relative to the galactic center, dark matter is basically standing still, but we are flying around the galactic center of mass. Depending on where we look, we should see some component of the sun's speed imprinted in the light emitted by dark-matter annihilation as a Doppler shift—the light we observe should appear either slightly redder or bluer than would be the case if the sun and dark matter were moving together. For ordinary matter, on the other hand, both the sun and the observed matter are orbiting, so the Doppler shift has a different character.

If you do the calculations, as the authors of the new paper did, you'll find that dark matter is distinguishable from ordinary matter. As you scan your detector across the galaxy, the Doppler shift for dark matter moves nicely from a blue shift (we are approaching this part of the galaxy) to red (parts of the galaxy we are running away from). For ordinary matter, the shift is different. For a start, it is generally the opposite: where dark matter is blue shifted, ordinary matter is red shifted. It also has a different spatial dependence. Instead of the maximum Doppler shift occurring at the edges of the galaxy, it peaks midway between the edge and the center (I'm using edge loosely here; the scan is in galactic longitude from negative 45 degrees to positive 45 degrees).

The only place where the two signals are indistinguishable is right at the center of the galaxy where they are both zero. But the center of the galaxy is occupied by a gigantic black hole that obscures any dark matter signal anyway.

That’s a mighty fine spectrometer you have there

So much for calculation. The real question is: can these differences be spotted in real measurement data? The answer, at the moment, is no. The maximum speed difference between the sun and dark matter is around 200km/s. The energy of the gamma rays is about 3,500 electron volts (the light we see is between 1.5 and 3.5eV). That means the Doppler shift that must be measured is on the order of 1eV, which is really challenging, as it will take a very good spectrometer to see the shift. Present detectors can't do this.

But help is on the way. The Astro-H satellite will be launched later this year, and it has a spectrometer on board that will be able to determine the energy of gamma rays with a precision of about 1.7eV. This is just barely adequate to measure the Doppler broadening of the gamma ray line. Certainly, it's close enough to make it worth the effort.

Physical Review Letters, 2016, DOI: 10.1103/PhysRevLett.116.031301