Something is amiss in our Sun. Or, rather, something is amiss in our theories of what the Sun is and how it behaves—theories that are known collectively as the standard solar model. This model, which is in part based on spectroscopic observations of the Sun's photosphere (the layer that radiates light), offers powerful predictions about the temperature, density, and chemical makeup of our local solar furnace.
However, more recent observations of the Sun's internal pressure waves reveal a major discrepancy: the churning, circulating convection zone of the Sun should feature more heavy elements by about 10 percent. We see too much helium and hydrogen.
According to a new paper from astrophysicists at Durham University, this missing material could be explained by the presence of a certain variety of dark matter known as weakly interacting asymmetric dark matter. This is a version of the elusive material featuring a lower proportion of dark antimatter. (The balance between dark and regular matter being the asymmetric thing.) This keeps dark matter/antimatter collisions in check enough to allow for the stuff to hang around in the Sun for long periods.
Moreover, unlike many theorized dark matter forms, this one is allowed to interact with regular matter through transfers of momentum as dark matter particles collide with regular matter particles. This would allow dark matter to help shuttle heat from the deeper guts of the Sun to the surface. This, the physicists argue, could explain the discrepancy between spectroscopic observations and helioseismic (pressure/acoustic waves) observations.
Assuming a dark matter particle mass of around 3 GeV—which is extremely light compared to the 100 GeV expected for a dark matter WIMP—these momentum-interacting particles would a perfect fit, explaining the observational mismatch.
There's probably a zoo of different possible particles that would give this interaction.
"There's probably a zoo of different possible particles that would give this interaction, but it's not clear yet whether any of those would really work when you work out the details," Aaron Vincent, the lead investigator behind the new study, told Physics World. "We're very close to finding out whether this really is an indication of dark matter or whether we have stumbled upon something that mathematically looks like dark matter but is actually something more subtle going on in the Sun."
The theorized particles are indeed very light, but, as Vincent and his team note in their paper, they're within the range of detectability using direct dark matter detection methods and/or collider-based searches. Indeed, the final answer may come from forthcoming efforts at the Large Hadron Collider via the Super Cryogenic Dark Matter Search experiment. This is still a very new problem.
Image: Aaron Vincent/Physics World
It's also a problem that extends far deeper than just our own Sun. Asymmetric dark matter, which has become a focus only over the past few years, may offer an answer to the universe's biggest mystery: Why does it exist at all? If matter and antimatter abundances in the universe were as symmetric as we might expect (as creating matter results in the creation of equal amounts of antimatter), it would have annihilated itself out of existence before it could even blink.
Dark matter that interacts differently with matter and antimatter may (help) explain the leftover shred of "normal" matter that we today know as existence itself.
The paper, "A possible indication of momentum-dependent asymmetric dark matter in the Sun," can be viewed in open-access preprint form at arXiv.