The Fermi Gamma-ray Space Telescope
What if the most popular hypothesis is wrong? Plenty of fringier theories exist.
As we’ve recently discussed at length, dark matter is likely to be a WIMP: a weakly interacting massive particle. Weakly interacting doesn’t mean no interactions, though, and there’s always the chance that dark matter particles will collide with something else. Since dark matter is also the most common matter in the Universe, there’s a good chance that the "something else" will be another dark matter particle. And if the collision results in the destruction of dark matter particles, it should produce a spray of things we can see, like energetic particles and photons.
On its own, these collisions will be too rare to detect. But summed over large regions of the sky, we might be able to detect the collective output of many collisions. This has led to a number of astronomical dark matter searches, some of which have claimed to observe puzzling excesses of high-energy photons. Yet for each one of these results, there have been other researchers who have suggested that the champagne bottles reserved for the discovery must be quietly put back to chill longer.
Why so many on-again, off-again discoveries? A review published in PNAS explains why looking at high-energy photons has been so difficult, and it describes what our prospects are for making one of these discoveries stick.
Signal to noise
Dark matter annihilations are expected to produce photons. While the precise energy of these photons will depend on the mass of the dark matter particles, the "massive" part of the WIMP acronym suggests that they will be quite energetic—in the gamma ray region of the spectrum. Gamma rays are very difficult to image with. To observe them directly, you need to put the observatory in space, or the gamma rays will interact with the atmosphere first (it’s possible in some circumstances to track these interactions fro imaging purposes, though). Then there’s the fact that they’ll pass through most matter, including mirrors. As a result, gamma ray telescopes tend to look more like long, reflective funnels that gradually concentrate the photons.
Despite the difficulties, a number of gamma ray observatories have been put into space, and these have given us a clearer picture of the most energetic events in the Universe. Lots of things, it turns out, produce gamma rays, from dying stars to supermassive black holes. In fact, the Universe has a diffuse gamma ray glow; superimposed on top of it are areas with a large number of sources, like our galactic core.
It’s that background that’s made spotting the ones from dark matter annihilations so hard. As Caltech’s Jennifer Siegal-Gaskins writes in her review, any signal from dark matter will be swimming in a sea of competing gamma ray sources.
Finding it, then, will require a process similar to the one used by particle physicists: figure out what all the other sources produce, then look for an excess signal above that. But the relative youth of gamma ray astronomy means that we’re still coming to grips with all those other sources. So, it’s one thing to say that objects called millisecond pulsars produce gamma rays (which they do). It’s another thing to try to estimate how many of these there are and where in the galaxy they’re likely to be located.
The best researchers can do right now is produce models that try to start from a few empirical measurements and theoretical considerations, and use those to estimate what the background looks like. And, while this has been done in the case of the potential discoveries that have been announced, there’s always been someone to point out potential issues with the models. With the gamma ray background uncertain, it’s not clear whether any signal that’s been observed represents any sort of excess.
Adding to subtract
Siegal-Gaskins argues that the real research focus for identifying dark matter signals no longer should be on looking for the signals—it needs to be on characterizing the background. This involves getting a better grip on the objects out there that produce gamma rays. What these objects are depends a bit on where you’re looking.
If you’re looking locally, most of the dark matter’s going to be in the bulge near our galactic center, so that’s where most of the collisions will be. Aside from a few point sources of gamma rays (like matter near the Milky Way’s supermassive black hole and a known supernova remnant), the majority of the gamma rays come from two sources: cosmic rays hitting clouds of gas and millisecond pulsars. Siegal-Gaskins cites estimates that as few as 1,000 extra millisecond pulsars could account for the potential dark matter signals that have been reported so far.
Getting a better grip on both of these populations won’t be easy, given we’re still not even sure about the source of many cosmic rays. But it should be possible to do population surveys of pulsars and use those to get a better estimate of their contribution to the background. Siegal-Gaskins is particularly intrigued by one of the findings announced based on data from the Fermi gamma-ray telescope. It used data from areas of our galaxy where we think the contribution from millisecond pulsars should be small.
Outside of the galactic core, things open up quite a bit. Individual sources still a concern, so millisecond pulsars in our galaxy matter, as do objects called blazars. But there are also entire galaxies that are likely to contain lots of individual sources, like star-forming galaxies and radio galaxies. Plus, active galaxies, powered by their supermassive black holes, also contribute.
Here again, the more observations, the better. Siegal-Gaskins notes that a recent measurement allowed a new estimate of how large the blazar population is likely to be. That estimate indicated that, at most, they could only be contributing less than 20 percent of the whole-sky gamma ray background. That knowledge, in turn, improved some observational constraints on a number of dark matter models.
This may all sound a bit depressing, but Siegal-Gaskins appears to be an optimist. With continued observations performed using existing instruments and a number of new instruments coming on line, she seems to think we can start narrowing down the uncertainties. This won’t guarantee a dark matter signal will emerge, but it will make the odds of seeing one a lot better. And perhaps more importantly, it might limit the arguments over the background models that have kept us from accepting any of the results we do have.
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