Matt Strassler [April 18, 2012]
It’s not easy to see dark matter, which makes up most of the matter in the universe. It’s dark. And yet, there is one way that dark matter might, in a sense, shine.
How? If dark matter is made from particles that are their own anti-particles (as is true for photons, Z particles, and [assuming they exist] Higgs particles, and perhaps neutrinos), then it is possible that two dark matter particles might encounter each other and annihilate (just as an electron and a positron can annihilate, or two photons can annihilate) and turn into something else that we can potentially detect, such as two photons, or indeed any other known particle and its anti-particle. Whether this is an effect that we could hope to observe depends on a lot of things that we don’t know… but there’s no harm in looking for it, and good reason to try.
How would we hope to find it?
First, we may want to look toward the center of our galaxy, the Milky Way. Just as the most likely place to see an automobile accident is in heavy rush-hour traffic, the place where collisions of dark matter particles are most likely to occur would be wherever the density of dark matter is highest. And that density is largest in the centers of galaxies. The reason (see Figure 1) is that galaxies of stars form in and around large clumps of dark matter — indeed, most of the mass of the Milky Way galaxy is dark matter, distributed in some fashion that is very roughly a sphere, though with a detailed structure that is unknown and possibly very complicated. The stars, and the big clouds of atoms out of which they form, form a rotating disk with spiral arms, sitting within that big sphere, with a ball of stars (the “bulge”) at its heart. The stars in the disk and bulge are presumably centered on the highest concentration of dark matter. So collisions, and consequent annihilations to particles that we can potentially detect, may be occurring near the center of our galaxy, and for this reason we might want to design scientific instruments that can look in that direction, seeking a hint that these annihilations are taking place.
Unfortunately, hints are not so easily obtained, because there aren’t many types of known particles that, if produced in dark matter annihilation near the center of the galaxy, can travel from there to Earth. The only particles that live long enough to reach the Earth are electrons, anti-electrons (positrons), protons, anti-protons, some other stable atomic nuclei (such as helium), neutrinos, anti-neutrinos and photons. But neutrinos (and anti-neutrinos) are extremely difficult to detect, while almost all of the others are electrically charged, so their paths bend and loop in the galaxy’s magnetic field, causing most of them never to reach Earth at all and assuring that we can’t tell, if they make it here, whether they came from the galactic center or not. That leaves photons as the only particles that both can travel straight from the region of the galactic center to Earth and can be easily detected.
So a good hint of dark matter annihilation could come from an unusual class of high-energy photons that are streaming from the galactic center but not from anywhere (or almost anywhere) else; see Figure 2.
However, there’s still a big challenge for that strategy. There are a lot of unusual astronomical objects at the galactic center, and they make high-energy photons also. How can we tell the difference between photons that come from dark matter annihilation and photons that are coming from some kind of unknown class of stellar processes that might be more common at the center of the galaxy than elsewhere?
The answer is that it isn’t easy, except in one special case. If dark matter particles (which have some definite mass, let’s call it M) can sometimes annihilate to two and only two photons, then both of those photons will have motion-energy equal (to a very, very good approximation) to the mass-energy Mc2 of the dark matter particles. The reason is very simple. It is the same as described in this article on particle/anti-particle annihilation, and as seen in Figure 3.
If a particle and anti-particle are (nearly) at rest, then the energy of each is (almost) entirely mass-energy and (nearly) equal to Mc2. Both have momentum (nearly) zero. Energy and momentum are conserved, so the total energy is (nearly) 2Mc2 before the annihilation and after it too. When the particle and anti-particle annihilate to a different particle and anti-particle, both the new particle and new anti-particle will have energy (nearly) equal to Mc2. In general, this will be a mix of mass-energy and motion-energy. In the specific case in which the final particle and anti-particle are photons, which have no mass and consequently no mass-energy, all of the energy will be in motion-energy.
Now we don’t know what the mass M of the dark matter particles is, and we don’t know therefore what the energies of the resulting photons will be. But since, just as every electron has the same mass and every proton has the same mass, every dark matter particle has the same mass M, every single dark matter annihilation will produce two photons of energy just about equal to Mc2. And that means that if we measure, with a special purpose telescope, the high-energy photons coming from the region near the center of the galaxy, and we make a plot of the number of photons that we detect with a given energy, we should expect astrophysical processes to generate lots of photons at lots of different energies, forming a smooth background, but the dark matter processes will add a bunch of photons that all have the same energy — a bump sticking up above that background. See Figure 4. It’s almost impossible to imagine any astronomical object, such as a bizarre star, that would be simple enough to generate a bump of this sort, so a signal in the form of a narrow bump would be a smoking gun for pairs of dark matter particles annihilating.
This gives us a very powerful way to look for dark matter. It won’t work if dark matter particles aren’t their own anti-particles and can’t annihilate at all. It won’t work if dark matter particles don’t often make photons when they annihilate. But it might work. And so there are efforts ongoing, most notably using the Fermi Large Area Telescope, a satellite experiment, which is out in space now, measuring photons coming from all across the sky, including those coming from the galactic center.