Is dark matter sterile? That’s one idea recently presented at the National Astronomy Meeting of the Royal Astronomical Society this week.

Dark matter is the enigmatic material that seems to dominate our universe. From its gravitational effects it seems to be about five times more common than the regular matter that makes up stars, planets, you and me. But since it doesn’t interact with light it has been difficult to observe directly. This has led some to propose modified gravity models to eliminate the need for dark matter, but this approach has been unsuccessful so far. Another approach is to propose some new kind of matter that could be dark matter, such as WIMPs or axions. But what if dark matter is a new variety of a known particle. This is where sterile neutrinos come into the game.

A neutrinos are a product of radioactive decay, and only interact with matter through the weak force. There are three known varieties of neutrinos, but because of their small mass and high speeds they cannot explain the effects of dark matter. But neutrinos have the unusual property that when they are produced they always spin in the same direction relative to their motion. This spin-motion connection is known as helicity, and in general can be right handed or left handed. If you imagine your thumb pointing in the direction of a particle’s motion, with your curled fingers representing the direction of the spin, then you can see which is which. Neutrinos are always formed with a left handed helicity.

Sterile neutrinos would have a right-handed helicity. Because of this they wouldn’t interact with regular matter (hence sterile). Depending on the model, they could theoretically have much higher masses than regular neutrinos, which could explain dark matter. In the recent presentation at NAM2015, a team looked at computer simulations of traditional cold dark matter models and sterile neutrino models. What they found is that sterile neutrinos seemed to be a slightly better match to observed galaxies.

While this is interesting, the sterile neutrino model is not without its problems. For example, there is cosmological evidence that points to the universe only having three neutrino varieties. Also, cold dark matter models remain the best match on large cosmic scales.

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In earlier posts about dark matter, I’ve written about how neutrinos would seem to be a good candidate, but there simply aren’t enough of them to account for all of dark matter. As far as we can tell, there are three types (flavors) of neutrinos, and we know the upper limit of their mass from the distribution of galaxies in the universe. So the three known neutrino flavors can’t be the solution to dark matter.

But there has been speculation that a fourth type of neutrino could exist, known as a sterile neutrino. Sterile neutrinos wouldn’t interact through the weak nuclear force the way regular neutrinos do, but instead only interact with things gravitationally (hence sterile). If these neutrinos had a much greater mass than regular neutrinos, then it could be an answer to the dark matter problem. But the catch is that their gravitational interactions would have interacted gravitationally with matter in the early universe, and this would affect the fluctuation patterns in the cosmic microwave background. So in principle we should see their effect in CMB fluctuations.

Previously the WMAP observations of the CMB were inconclusive. The data agreed with the 3-neutrino model, but the effect of a sterile neutrino couldn’t be excluded. But the latest Planck results now confirm that there is no fourth neutrino. The  standard model of particle physics remains successful. This is good news for the standard model, but it eliminates one more dark matter candidate from the list.

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The nature of dark matter is one of the greatest mysteries in modern astrophysics.  Based upon observations of stellar motions within a galaxy, we know that regular matter makes up only a fraction of a galaxy’s mass.  The rest consists of a “dark” matter that doesn’t interact strongly with light. We can see that dark matter exists through galactic collisions like the Bullet cluster.  Through its gravitational effect we can map the distribution of dark matter in the universe.  So we know quite a bit about dark matter, but we don’t know what dark matter actually is.

Actually that isn’t quite true.  We do know of one type of dark matter, and we’ve even detected this dark matter in experiments on Earth.  This particular type of dark matter is more commonly known as neutrinos. We normally don’t think of neutrinos as dark matter, but neutrinos have mass and they don’t interact strongly with light. Because their mass is very small, they move at speeds close to the speed of light, so they are a type of dark matter known as hot dark matter.

So why is dark matter still a great mystery?  Because neutrinos cannot be the only type of dark matter there is.  To begin with, they have such a low mass that it isn’t enough to account for all the dark matter effects we observe. Also, the distribution of galaxies in the universe doesn’t match the effects of hot dark matter.  Instead, dark matter must mostly consist of slow moving “cold” dark matter.  That’s where the mystery lies, because we don’t know what this cold dark matter is.

But the solution still might be neutrinos.  Not the type of neutrinos we’ve already discovered, but another type known as sterile neutrinos.

The helicity of neutrinos and anti-neutrinos. Credit: Universe Review

It turns out that neutrinos have an interesting property. When they are produced in particle interactions or nuclear decay they always spin in the same direction relative to their motion.  This spin-motion connection is known as helicity, and in general can be right handed or left handed. If you imagine your thumb pointing in the direction of a particle’s motion, with your curled fingers representing the direction of the spin, then you can see which is which.  Neutrinos are always formed with a left handed helicity.

Back when we thought neutrinos didn’t have any mass, it was thought that all neutrinos must be left handed. This is because massless particles move at the speed of light, so they couldn’t change helicity.  But since we now know that neutrinos do have mass, it is possible for certain interactions to change the helicity of a neutrino.  So there are likely right handed neutrinos.  Regular neutrinos interact with particles through the electroweak force. Since the electroweak interaction always produces left handed neutrinos, it must be the case that right handed neutrinos don’t have an electroweak interaction.  Thus, right handed neutrinos can only interact via gravity.  For this reason, right handed neutrinos are known as sterile neutrinos.

We don’t know why the electroweak force only interacts with left handed neutrinos, but one possibility is that right handed neutrinos are somehow much more massive.  If that is the case, then they could be heavy enough to be cold dark matter.  There are some hints that dark matter consists of sterile neutrinos, but they are just hints and there are other hints that other types of particles are dark matter.  Right now we just aren’t sure.

But it might just be that the dark matter particles we’ve known all along might be the solution we’ve been searching for all these years.

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