Dark matter cropped up in the news again recently (scientists think they might have detected something which may or may not be produced by dark matter; this happens every four or five years or so with no conclusive result so I wouldn’t hold your breath), so I thought now might be a good time to write something about it on the blog. This is particularly difficult — or easy, depending on how you look at it — since we currently know next to sod-all about dark matter and its even more mysterious counterpart, dark energy. It’s even been said that our calling it dark matter reflects more on our total and utter lack of understanding of what it actually is than any intrinsic properties on the part of the dark matter itself. Still, while we’ve never been able to directly observe dark matter (indeed, this may not ever be possible depending on what the dark matter eventually turns out to be) we have been able to infer its existence from certain odd phenomena that don’t make any sense without something that fits the rather broad description we have of it, so we can at least have an interesting discussion about that.
(Well, I say discussion. Mostly this is going to consist of me writing at you, and you hopefully taking it all in.)
There’s a word that’s going to crop up a lot in this post. That word is baryonic, which is a term used to describe the overwhelming majority of visible matter in the universe. A baryon is a particle made up of three quarks — the quark is what’s called an elementary particle, which means that it is a particle with no known substructure and which is effectively a base building block from which you construct other particles — and which is subject to the four fundamental forces: electromagnetism, strong nuclear, weak nuclear and gravity. The protons and neutrons that give atoms most of their mass are baryonic particles, and so anything made up of atoms is called baryonic matter. Literally everything we can see (and quite a few things that we can’t, except with very high-tech observing equipment) is made of baryonic matter — you, me, the Earth, stars, galaxies, the whole works. So when a scientist is throwing around the term “baryonic” in the context of dark matter, what they mean is that it’s something that behaves rather conventionally according the laws of the physics: we can see it, we can detect it, we can predict how it will behave and what properties it will have.
There are also an apparent minority of things in the universe that are non-baryonic in nature. Most familiar to us are the electrons which orbit atomic nucleii in a fuzzy cloud, which are members of a family of particles called leptons. Like quarks, leptons are also elementary particles, but with the crucial difference that — for reasons that I’m not going to go into here for the very good reason that I barely understand them — they are not subject to the strong nuclear force which binds atomic nucleii together. This means that if you have a lepton which carries no charge and is electrically neutral you wind up with something that isn’t subject to electromagnetism either; the resulting particle is called a neutrino and it is, famously, a complete bastard to detect thanks to it ignoring two of the four fundamental forces. The other two aren’t much help either; we cannot use gravitational interaction to detect neutrinos because they have next to no mass, and the weak interaction is an incredibly short range force that is only really relevant if your neutrino is in physical contact with another particle. This is why the only way we can spot neutrinos is by detecting the aftereffects of them ramming into other particles and exchanging charge, resulting in the creation of a muon/electron which moves faster than light in water (not the same thing as faster than light in a vacuum) and an associated burst of Cherenkov radiation. And even then dedicated neutrino detectors hundreds of metres underground with billions of neutrinos sleeting through them every second will spot only a few of these.
Still, despite the difficulty we have finding it we’ve seen enough of it to know that all this non-baryonic stuff tends to have next to no mass, and that the total mass of estimated non-baryonic matter inside a given galaxy is going to be negligible in comparison to all the heavy baryonic stuff like stars and planets. This is why this graph is such a big problem for modern physics.
This is a graph of the rotation curve of a typical galaxy. The way this works is that the rotation velocity of individual stars as they orbit the galactic core is dictated by the distribution of mass throughout the galaxy. With much of a typical galaxy’s visible mass concentrated within the galactic core we would expect to see a rotation curve broadly similar to that of a solar system’s planets orbiting the central star: this is described by Kepler’s third law, which says that orbital velocity will decrease exponentially as you get further away from the star, and which is the reason why Mercury has an orbital period of 88 days and Neptune has an orbital period of 165 years — not only is Neptune covering a larger distance thanks to the increased orbital radius, but it’s doing it more slowly1 to boot.
Now, the problem here is that this expected behaviour is represented on the above graph by the dashed line. What’s actually happening is represented by the red one, which is that the orbital velocity of matter within a galaxy remains constant no matter how far you travel out from the galactic core. This is patently absurd considering the distribution of visible mass within a typical galaxy, and yet it’s a relationship (or lack of one) that’s been proven to hold firm no matter how many galaxies we look at. There are only two ways that we can make this flat rotation curve fit in with our established laws of physics. One is to rewrite the laws of physics, which is what a vocal minority of physicists are attempting with Modified Newtonian Dynamics (or MOND). The other is to accept the existence of a hitherto unsuspected and almost completely invisible collection of matter within every galaxy that can distort the galactic mass distribution to the point where the flat rotation curve becomes possible. There are substantial issues with each approach, but since this is a post about dark matter — and because MOND is having difficulty getting traction in mainstream scientific consensus — I’ll be talking about the second one here.
As I said earlier, the name “dark matter” reflects both the fact that it’s next to impossible to detect conventionally as well as the almost total paucity of information we have about the nature of the dark matter itself. What little we know is inferred from the effect it has on the stuff we can see, like the galactic rotation curve. We know that it’s subject to gravity, since without its gravitational interactions with baryonic matter we wouldn’t know it was there in the first place. We know that there must be a staggering amount of it present in every galaxy in order to distort the curve away from its expected shape to the degree that it does, to the point that the baryonic matter component of a galaxy — all those stars, planets, and dust clouds — comprises only 15% of its actual mass. The other 85% is dark matter, coexisting in the same place (and possibly even in the same space) but otherwise eerily beyond our perception. And despite the idea being around for a good seventy years now, those two facts are just about all we know about the stuff.
Not that that stops scientists from speculating about what it might be, of course. The two major candidates for dark matter are referred to — and you’ll have to imagine me gritting my teeth as I say this — as MACHOs and WIMPs. MACHO stands for MAssive Compact Halo Object, and represents the idea that the galactic halo (the bit just outside the visible rim) might be full of extremely small, dense and dark objects like neutron stars and black holes. These MACHOs would be composed of regular baryonic matter, but they’d be impossible to spot directly thanks to being situated in a region of the galaxy with very little ambient radiation to make them show up, not to mention being distinctly non-luminous themselves. Unfortunately there are a couple of drawbacks to this idea: first, you’d need a truly ridiculous number of MACHOs in order to multiply the galaxy’s known mass by seven, and second… well, it’s complicated, but let’s just say that our current models of the Big Bang indicate that it created X amount of baryonic matter. Adding these MACHOs to the mix takes the amount of baryonic matter present in the universe to quite some way above X. There’s simply not enough regular matter around to create the required number of MACHOs.
So MACHOs are falling out of favour as potential candidates for dark matter. This leaves WIMPs, or Weakly Interacting Massive Particles. These are hypothetical non-baryonic particles somewhat like neutrinos — they are not subject to the strong nuclear force, which means they barely ever interact with other particles/atomic nucleii, and their absence of charge means they are also unaffected by electromagnetism. However, they do differ from neutrinos in that a WIMP — as its name implies — is a relatively heavy particle. It has to be in order to inject enough mass into a galaxy to produce that distorted rotation curve. There is no known particle in the Standard Model2 that combines this hard-to-detect nature with a heavy mass, so if scientists ever do manage to find one the Standard Model is going to need a fair bit of rewriting. They haven’t yet, despite a number of seemingly-promising signs that they might exist; this is largely because in addition to being immune to the two major forces used to detect regular particles our hypothetical WIMPs are also very, very slow, which means we can’t use the same method we use to find neutrinos. Still, you should never underestimate the ingenuity of scientists when faced with a seemingly-impossible challenge; here’s just one of the proposed detection methods cribbed from Wikipedia:
Halo WIMPs may, as they pass through the Sun, interact with solar protons and helium nuclei. Such an interaction would cause a WIMP to lose energy. The resulting slower WIMP would not have enough energy to escape the gravitational pull of the sun and thus would be “captured” by the Sun. As more and more WIMPs thermalize inside the Sun, they begin to annihilate with each other, forming a variety of particles including high-energy neutrinos. These neutrinos may then travel to the Earth to be detected in one of the many neutrino telescopes.
Which seems to be saying that if WIMPs exist we should be seeing an excess of neutrinos being emitted from the Sun. Exactly how many extra neutrinos we see is going to depend both on the properties of the WIMPs and the mass of the Higgs boson, another scientific puzzle that has yet to be satisfactorily resolved. So it’s not just a matter of building better instruments; we need to nail down a couple of other crucial discoveries before we can properly get to work on finding WIMPs. And that’s if they even exist at all; both the MACHO and WIMP theories might be completely wrong for all we know. Maybe MOND actually has some legs. Maybe it’s one of the more obscure theories, like RAMBOs. Maybe it’s none of these things. Dark matter is one of those intriguing glimpses into the future of physics that’s just out of our grasp because we lack the necessary framework to properly describe it, much like all the not-so-minor discrepancies in our theory of the universe before general relativity and quantum mechanics came along. I suspect unravelling its exact nature is going to be one of the great scientific mysteries of the 21st century, and the one thing you can be sure of is that it’s going to take a lot more than a single experiment on board the ISS to do it. Keep that in mind when the next round of “Dark matter detected” stories hits the news in a couple of years’ time.