Scientific Method / Science & Exploration

Fermi telescope detects signal that could be annihilating dark matter

Distribution and energy is what we'd expect when WIMPs collide.

The distribution of gamma rays seen by Fermi (left) and the hot spot that remains after accounting for known sources (right).
Daylan et. al.

Researchers using data obtained by the orbiting Fermi Telescope may have found the first clear, direct evidence of dark matter in our own galaxy. The signal comes in the form of an excess of gamma rays coming from an area surrounding the galactic core, and it appears to be exactly what we'd expect from a weakly interacting massive particle, or WIMP. Perhaps as significantly, however, there are no known astronomical features that can produce a signal like this.

The Universe provides plenty of evidence that dark matter exists. Everything from the behavior of galaxies to the structure of galaxy clusters indicates that there's more matter present than we can detect. We have spotted instances of gravitational lensing by matter in what appears to be largely empty space. Even the cosmic microwave background, which reveals the details of the Big Bang, indicates that most of the matter in the Universe is in the dark category.

But when it comes to identifying the particles that actually comprise dark matter, we've tended to look closer to home. Searches in the Large Hadron Collider, which could produce dark matter in its atom-smashing debris, came up empty. Direct detections of dark matter collisions with normal atoms sometimes produced promising results, only to have a different experiment shoot them down.

But another category of experiments has looked where the evidence already exists: in the skies. When the densities of dark matter are high enough, two of its particles could collide with each other, producing energetic debris. By looking at the places where we expect the highest concentrations of dark matter and seeing if there's an excess of certain types of radiation, we might be able to detect signs of these dark matter collisions.

A group of astronomers have now reanalyzed data obtained by the Fermi Gamma-ray Space Telescope to look for signs of these annihilations. A few years back, Fermi made the news for finding an excess of gamma rays that could be an indication of dark matter collisions at the Milky Way's core, but the community as a whole wasn't convinced by the analysis. In the intervening time, the Fermi team gathered more data and, perhaps more significantly, tagged each detection with an indication of how confident they are in the instrument's ability to assign a direction to its source.

The new analysis took advantage of those tags, filtering the data to eliminate anything with poor positional certainty. Mostly, the emissions traced the outlines of our galaxy as you'd expect—there are plenty of energetic events taking place in binary star systems throughout the galaxy. But once the expected distribution of those normal events was accounted for, the center of our galaxy retained a spherical area of intense gamma ray emissions.

The key feature of this hot spot of emissions is that it covers a very large area; it extends about 6,500 light years in all directions from the galaxy's central black hole. That's too far from the galactic bulge for there to be many stars present, and thus many normal source of gamma rays. So at the moment, there's no mundane process we know about that could be producing these gamma rays.

In contrast, the dense concentration of dark matter near the galactic core provides a good fit for the site of these emissions. Better yet, it behaves exactly as we expect some of the simpler options for dark matter should. "Excellent fits are found for dark matter that annihilates to bottom, strange, or charm quarks," the researchers' paper says, which means we don't have to postulate some sort of exotic decay process. And the apparent mass, 30-40 giga-electronvolts, is within the range that could easily produce the sort of behavior we see in our Universe.

Better yet, the frequency of collisions (as measured by a value called the "annihilation cross section") would produce a particle that stopped interacting with the known members of the Standard Model as things cooled down very early in the Universe's history. At that point, the amount of stuff that was "frozen out" as dark matter would be roughly the amount that we expect from the measurements of the cosmic microwave background.

So all the pieces seem to fall neatly into place, and the data is pretty compelling (the excess is present with a statistical significance of over 40 sigma). Does this mean we've finally nailed down dark matter?

Even the authors of the paper don't want to go there. "Given the frequency of such false alarms, we would be wise to apply a very high standard before concluding that any new signal is, in fact, the result of annihilating dark matter," they write. We'll have to wait for astronomers and particle physicists to mull over these new results. And even then, the radiation will only be compelling evidence; we're not likely to accept it until we can detect something on Earth. But by figuring out this many of its likely properties, the Fermi results could help us build a detector that's more likely to succeed.

The arXiv. Abstract number: 1402.6703  (About the arXiv).

Expand full story

You must to comment.

You May Also Like