Physicists Have Disproved a Leading Hypothesis on Dark Matter’s Composition

April 27, 2016 | Joanne Kennell

Artist's abstract impression of matter

This is not bad news.

Dark matter, the invisible substance that makes up 27 percent of all matter in the universe, is known to exist because scientists can measure its gravitational pull on other objects in the universe. However, it has never been directly detected, despite researchers’ best efforts, because it does not interact with light.

Now, a new paper authored by 102 physicists has eliminated some of dark matter’s elusivity. Why? Because one of the top candidates for dark matter has just been quashed.

Currently, there is a lot of debate amongst scientists about what dark matter actually is, and one of the leading contenders is a class of Weakly Interacting Massive Particles, or WIMPs. But there are other possibilities as well, including axions, axion-like particles, and supersymmetric particles.

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Thanks to the Fermi Large Area Telescope (LAT), the number of possibilities has been reduced. So who got chopped? Axion-like particles.

Axions were first proposed in 1977 in an effort to resolve a problem in quantum chromodynamics — the theory of how quarks interact with one another. They were then used to develop string theory, and in doing so, scientists noticed particles showing up that looked a lot like axions, hence the name axion-like particles, or ALPS.

Shortly after, researchers realized that axions and ALPS could be great candidates for dark matter. For example, when the Big Bang created all of the light and matter in the universe, it should have also created a whole bunch of axions and ALPS (if they exist). And if they do exist, these particles should be gathered where astronomers see evidence of dark matter.

In order to solve the dark matter mystery, the 102 scientists decided to focus their attention on gamma rays. According to Science Alert, once in a while an axion or ALP should run into a bit of regular matter, sending a gamma ray into space with a specific energy — an energy that should be visible with modern telescopes like LAT.

There are currently several models that predict different numbers of ALPS in the universe — ranging from dark matter being composed of all ALPS to making up just a tiny fraction. What’s more, these models predict different amounts of gamma rays, so researchers can use these numbers and the kind of gamma rays observed to test the models of ALPS.

The study, published in the journal Physical Review Letters, simulated the galaxy NGC 1275 with and without ALPS, and then checked the results of these simulations against six years of LAT observations. What they found is that ALPS don’t predict observed gamma rays any better than the models without them.

So, ALPS are likely to be scrapped for their role in dark matter. However, scientists will still explore a few more models before ditching the ALPS hypothesis altogether.

Now, this is not bad news. It means we are one step closer to discovering what dark matter is really made of.

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