It's a mystery that dates back at least 15 years: The makeup of the Sun looks totally different depending on how exactly scientists study it. In fact, the amount of heavier elements in the sun can appear to vary by as much as 20 to 30 percent.

A new theory proposed by a team of astrophysicists, including Pat Scott at Imperial College London, explains that the cause could be related to a patch of dark matter at the Sun's core. Scott, who calls himself a "astroparticle phenomenologist," explains the strange-sounding idea to PM.

PM: Can you outline the basics of this solar mystery?

PS: Basically, we measure different amounts of elements if we look at the sun using two different tools. The first tool is spectroscopy, in which we study the various wavelengths of light that reach us from the sun. The second tool is helioseismology, in which we look at how sound waves travel through the sun, because the propagation of those waves tells us a lot about the sun's composition.

"We just want to understand how the sun works"

If you compare measurements from these two tools, you find that you have a 20 to 30 percent mismatch in the reported abundance of heavy elements—meaning anything that's not hydrogen or helium. That's a fairly big mismeasure, and one that we started to notice around 10 to 15 years ago when the techniques with which we do spectroscopy improved.

Why does it matter?

We just want to understand how the sun works. If we can't understand the makeup of our sun, we can't really hope to understand its behavior, nor can we hope to understand the structure of other stars.

Also, in astrophysics the chemical composition of the sun is a fundamental reference point that nearly everything gets measured against—whether we're trying to understand how planets form around other stars, how different populations of stars form elsewhere in the galaxy, or even the process of galaxy formation. Even in cosmology and parts of astrophysics, it all comes back to our sun.

How have people tried to explain the problem?

Over the last decade, people have tried checking and testing a whole range of possibilities as to why we might be getting these different numbers. For example, research groups have tried to see if there was something wrong with the models of the sun's interior that we use. Other research has been focused on integrating certain types of special, hard-to-measure particles into our theories, or testing the idea that the sun gobbled up a massive light-element rich object early in its life, or even calculating in additional pressure waves caused by gravity (not to be confused with gravitational waves) in our models of the sun. But none of this research has offered up a working solution.

Your research group just suggested that dark matter could be root of the problem. Why dark matter?

Dark matter is something that we know exists in our universe. We've detected it through gravitational measurements, and we know that unlike normal matter, it doesn't interact with light (and it may not even interact with normal matter except via gravity). We've developed a theory and model based on dark matter that explains—with a high degree of accuracy—why we have this mismeasurement issue.

"We already know that the sun is sitting in a halo of dark matter"

Now, other people have looked at dark matter as a possible modifier of the solar structure before. But we looked at a different kind of dark matter. Ours is dark matter that can weakly interact with normal matter—and importantly, can exchange its momentum with the matter it collides with. And through those interactions, this dark matter transfers energy from the core of the sun out toward its edges.

So how does your theory work in action?

We already know from gravitational measurements that the sun is sitting in a halo of dark matter, and that dark matter is passing through the Sun and the Earth all the time. In theory, this dark matter will sometimes collide with material in the sun. That collision can cause a dark matter particle to lose enough energy that it can no longer escape the sun's gravitational pull, and so will pass through the sun again and again until it has finally settled down into the core. Here the dark matter will continue colliding with the material around it in the center of the sun.

The core is the hottest part of the sun, and the heat from those collisions will basically force the dark matter to travel out in the outer regions of the sun, where it deposits that heat in further collisions. Effectively, the dark matter transports heat and makes the natural heat transfer from the center of the sun outward a little bit more efficient.

This fixes our measurement issue, because as the sun's heat is spread out via those dark matter collisions, the speed at which sound waves can travel through different parts of the sun is altered. Including this dark matter interaction changes the heavy elements measurements we get from the helioseismology tool we talked about earlier, and aligns both our measurements.

Even though the idea makes sense, including dark matter makes your idea sound a bit eccentric. Is this explanation as odd as it first appears to be?

Not at all. We're really drawing on a lot of relatively mainstream ideas. We already know that dark matter exists, and that it needs to be some new type of particle. Here we're just using the sun as a great opportunity to screen for one of the ideas of what it could look like.

"If this type of dark matter actually exists, it should turn up at the LHC in the next five years"

If there's anything unusual or groundbreaking about our work, it's not the overarching ideas, but the novel and fairly technical background calculations we had to make.

Will you be able to prove or disprove this idea?

Absolutely. The interactions that dark matter has with normal matter—at least the ones that this theory requires—gives some fairly distinctive signatures that could show up in either particle colliders or direct detection experiments (experiments that involve a big vat of something inert nestled far underground and watched carefully).

The strength of our dark matter interaction is very close to what could be picked up in the next run of CERN's Large Hadron Collider, which starts next month. So if this specific type of dark matter actually exists, it should turn up at the LHC in the next five years or so. And that's around the same timeline we'd expect to see the dark matter—if it's to be found—in a pair of direct detection experiments at SNOLAB in Sudbury, Ontario.

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