The Process of Elimination

It was previously thought that the gamma rays that we see coming from a dense region of space within our Milky Way galaxy were likely caused by the collision of invisible dark matter particles. Now, new methods of statistical analysis have allowed two separate research teams to independently find that these gamma ray signals are not what we expect from dark matter.

In short, these amazingly energetic rays aren't caused by dark matter. Or at least, it doesn't seem so.

Ultimately, the teams argue that the bursts of gamma rays likely come from other astrophysical phenomena. We don't know exactly what causes them, but we do have some ideas. One candidate is something known as millisecond pulsars, which are fast-rotating stars.

The teams presented these findings in the journal Physical Review Letters. One team is a collaboration based at Princeton University and the Massachusetts Institute of Technology, and the other hails from the Netherlands.

Assistant professor Mariangela Lisanti of Princeton University said that the work seems to throw out the old model, which relied on dark matter: "Our analysis suggests that what we are seeing is evidence for a new astrophysical source of gamma rays at the center of the galaxy....This is a very complicated region of the sky and there are other astrophysical signals that could be confused with dark matter signals."

Estimation of dark matter, dark energy, and normal matter. Credit: NASA
Gamma Rays and Dark Matter

Scientists think that about 85 percent of all mass in the universe is made up of dark matter, although it has never been directly observed. Instead, scientists look for evidence of the presence of dark matter. And we have a lot of evidence.

One example of such evidence is based on a hypothesis involving dark matter particles called weakly interacting massive particles (WIMPs). According to a widely accepted model of dark matter, the collision of two WIMPs causes the annihilation of both and produces gamma rays.

When the Princeton/MIT research team studied images of gamma rays captured by NASA’s Fermi Gamma-ray Space Telescope and applied their statistical analysis method, they found that the images collected by the Fermi telescope showed that the distribution of photons among the pixels was clumpy.

By contrast, the photons should have been smoothly distributed, based on the accepted model of dark matter. This means that these gamma rays were not caused by dark matter.

It is not yet known what this new source of gamma ray is, but Lisanti points to one possibility, referring to the aforementioned very old, rapidly rotating stars known as millisecond pulsars.

But notably, the conclusions are not final (things rarely are in science. "Either we find hundreds or thousands of millisecond pulsars in the upcoming decade, shedding light on the history of the Milky Way, or we find nothing. In the latter case, a dark matter explanation for the gamma ray excess will become much more obvious," says Christoph Weniger, lead on the paper from the Netherlands. Hopefully, further studies will help us track down the final culprit.

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