Intro to the Dark Sector (Script) (Patreon)

Title Options: The Bizarre Physics of the Dark Sector

Writer Credit: Matt Caplan

By the time I finish this sentence, a billion billion dark matter particles may have streamed through your body like ghosts. The particle or particles of the dark sector make up the vast majority of the mass of the universe - so to them, we're the ghostly ones. Today we're going to try to make contact.

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We see the influence of dark matter in the orbits of stars and galaxies, in way light bends around galaxies and clusters, in the clumpiness of the cosmic background radiation, and more. It’s become disturbingly clear that we can’t see around 80% of the matter in the universe. Even more disturbing is that there doesn’t even seem to be a candidate for dark matter in the known family of particles. We’re faced with the eerie reality of the dark sector - perhaps there’s an entire family of particles that exists in parallel to those we can see - a dark universe that overlaps our own, but so far is hidden from even our most ingenious experiments. Today we’re going to open the gateway to the dark sector and see what we can find.

When we talk about the “dark sector” we typically mean a particle or family of particles that contribute to dark matter. Now it’s possible that dark matter is not particles - it could be black holes or failed stars or even weirder so-called “compact objects”. It might even be that what we perceive as dark matter is really a glitch in the laws we use to describe gravity. But those possibilities are for another time - today we’re focusing on the bizarre physics of the dark sector. So let’s begin with what we do know.

Our best understanding of the subatomic world is given by the Standard Model - which describes the behavior of the known family of particles with incredible success. The visible universe is made of these particles, interacting with each other through the standard model forces - the strong and weak nuclear forces and electromagnetism. Plus gravity.

In general, the behavior of a particle is determined by the forces it interacts with. We can think of forces as the languages that particles use to communicate. Any electrically charged particle experiences the electromagnetic force and can communicate with other charged particles by exchanging photons. But for an electrically neutral particle like a neutrino, electromagnetism is a language it doesn’t speak. Neutrinos are unaffected by that force, and so they are quite literally invisible to photons.  A more technical way to think about this stuff is in terms of quantum fields - where each particle and force is a vibration in its own field. These fields fill the universe, overlapping each other - and if a particle field is connected to - coupled to a force field then it can speak the language of that force.

The force of gravity is a sort of lingua franca, a common language that every particle with mass can speak. But gravity is a little different to the other forces - it’s not part of the Standard Model, and we don’t even know if it has a quantum field.

The main requirement for a dark matter particle is that it doesn’t “speak electromagnetism”. It doesn’t produce light - hence the “dark”. But it also doesn’t absorb light - otherwise we’d be able to detect it when it blocked light from the more distant universe - in the same way we “see” the black lanes of dust that block the light from the center of our galaxy. No, dark matter is both perfectly dark AND perfectly transparent. Good - so it must be electrically neutral like the neutrino.

Dark matter can’t have charge but it must have mass because the only thing we’ve ever actually seen dark matter do is to exert its gravitational influence. Dark matter “speaks gravity”. And we can learn an awful lot from HOW it exerts its gravity. We can map where dark matter is found by how it affects the rotation of galaxies, and how it drives the orbits of galaxies inside galaxy clusters, and by the way it bends light around galaxies and clusters. These tell us something really important: dark matter is far more spread out - more diffuse - than almost all of the visible matter.

And that tells us a lot about any prospective dark matter particle. For one thing, dark matter doesn’t tend to interact with itself - at least not very much. If it did, then giant regions of dark matter would lose energy in those collisions and contract. They might collapse into dark matter galaxies or dark matter stars or dark matter people. But no - dark matter seems to stay puffed up in gigantic halos surrounding the much more concentrated clumps of visible matter. In fact, galaxies are really just shiny dustings of stars, sprinkled deep in the gravitational wells of massive reservoirs of dark matter.

But the fact that dark matter forms those giant halos at all tells us something very important. It gives dark matter a temperature. More accurately, it tells us how far dark matter particles were able to travel in the early universe. This “free-streaming length” of dark matter is how far a dark matter particle could travel before interacting with something - typically another such particle. In the early universe, that distance influenced the size of the seed structures which galaxies would later form from. We’ve talked about that structure formation before. And based on how that structure DID end up forming, it seems likely that dark matter was moving pretty slowly. We refer to such dark matter as “cold”.

So let’s review - if dark matter is a particle, it’s electrically neutral and doesn’t interact much with itself, and it’s relatively slow-moving, and also insanely abundant. For a long time people thought the neutrino might be dark matter - being neutral and the most abundant known particle in the universe. But the neutrinos of the standard model move too fast - they’re “hot” - and there just isn’t enough mass in neutrinos to do the job, due to them being ridiculously light.

There’s really nothing else in the standard model that works - which sounds annoying, but actually gets physicists very excited - because discovering a dark matter particle may be our best for finding a bigger, deeper theory than the standard model. It would also be a no-brainer Nobel prize - and many researchers have devoted their lives to hunting down this particle.

Dark matter hunters come in two breeds. One type searches for new evidence out there in the universe or in our particle experiments here on Earth for evidence of particles that don’t fit the standard model. The other delves deep into theory - in speculative mathematics beyond the standard model for signs of new particles. Today we’re going to focus on the theoretical prospects - because we might as well have some fun before those pesky “observations” ruin everything with their “facts”.

Actually, we don’t have to go too far beyond the standard model to find our first dark matter candidate. Completely independently of our quest for dark matter, physicists have hypothesized a new type of neutrino - the so-called sterile neutrino. I won’t describe these in detail because we’ve been over them before - but in short: as ghostly as neutrinos are, sterile neutrinos are far ghostlier. They don’t even interact by the weak force, which means they’re almost impossible to detect. There are some exceedingly clever experiments to do so - like we saw that time we visited FermiLab. If sterile neutrinos exist AND are massive and slow-moving enough, they’re a great dark matter candidate.

Another candidate we’ve discussed is the axion. This is a weird little particle that popped up in the math when physicists were trying to solve another mystery of physics - the so-called CP problem. Axions, if they exist, would be incredibly light - maybe 1% or less the mass of the already-puny neutrino.  So to account for dark matter they’d need to exist in prodigious numbers … but according to pro-axion physicists, that may well be the case.

OK, enough with the things we’ve already talked about. Explorations of the theoretical landscape have led physicists to multiple possibilities for dark matter particles. One of the most promising ideas comes from supersymmetry. We’ve also talked about supersymmetry, but not about how it could give us dark matter.

Supersymmetry is an extension of the standard model which proposes that all the regular particles - both matter and force-carrying - have twins - counterparts on the opposite side of the table. Every matter particle or fermion has a supersymmetric force-carrier, or boson. And every boson has its fermion twin.

It’s expected that these supersymmetric particles are much heavier than their standard model counterparts - and that may explain why we haven’t seen them in our particle accelerators - perhaps we just haven’t produced enough energy to make one yet. But they may have been produced in the insanely energetic early universe, and the leftovers from that time could still be throwing their weight around, so to speak.

The simplest kind of dark matter we get from supersymmetry is called a ‘neutralino.’ It’s a sort of ‘three in one particle’ where the electrically neutral superpartners of the Z boson, photon, and Higgs particle, all mix together. In some models these are the lightest supersymmetric particles possible - ”LSPs” - but they’re still incredibly heavy. And while normally heavy things tend to decay to lighter things, if they can’t decay to Standard Model particles then they’d be stable and long lived- an almost perfect dark matter particle.

There are other dark matter candidates in different flavours of supersymmetry - all of them “LSPs” - for example the counterparts of the neutrino or the graviton. The expected mass of these particles is eerily close to the mass expected for a certain type of dark matter - which some would say is a point in favor of supersymmetry. This seeming coincidence is sometimes called “the WIMP miracle”. But for that to make sense I should probably explain what a WIMP is.

Supersymmetric dark matter particles like the neutralino are examples of a general dark matter particle type called the WIMP, or “weakly interacting massive particle”. The idea of the WIMP was proposed independently of any actual WIMP candidates. It’s a description of what some physicists thought dark matter particles had to be like- which is to say, weakly interacting and massive. The massive part is obvious enough - it helps if you want to make up 80% of the mass in the universe, and also slows the particle down - helps make it “cold”. We also covered weakly interacting - it helps dark matter halos stay puffed up. But it turns out that the interaction strength of dark matter is extremely important - it may have governed how every interesting thing in our universe first formed.

The idea is this: In the first fractions of a second after the Big Bang, particles and their antimatter counterparts would have been popping into existence constantly, borrowing energy from the crazy radiation of the time. And then when the particle bumps into its antiparticle they both annihilate, releasing that energy again. As the universe cooled and energy dropped, that process ceased. We were left with a universe full of particle-antiparticle pairs that would then just annihilate over time. But its possible some particles may not have been able to find an antiparticle counterpart before the expanding universe pulled them too far apart. Things like electrons and antielectrons, or positrons, interact very strongly via the electromagnetic force - which means they find each other too easily. The universe didn’t expand fast enough to throw these particles apart, and so almost all annihilated.  But a WIMP, with its extremely weak interaction, would more easily dodge its antimatter buddy - and so countless may have survived to this day.

So it turns out you can do a calculation of what interaction strength such a relic particle would need to have to survive in sufficient numbers to give us dark matter. And that interaction strength is about the same as the weak interaction. Ergo, WIMPs interact by the weak force only, or something weaker.

OK, so we have multiple possible members of the dark sector - and we didn’t even cover all of them. Perhaps none exist, but perhaps several do. It’s possible that an entire ecosystem of particles are going about their dark business across the universe -  interacting by dark forces, all of them oscillations in their own dark quantum fields - perhaps with its own complexity and diversity. Because dark matter is weakly-interacting, our light sector is probably more complex - probably. But to know one way or another one of the many brilliant experiments currently in progress or planned will need to bear fruit. We’ll talk more about those experiments another time. For now, we’ll just have to enjoy knowing that our light-weight light sector exists in parallel to this completely invisible and vastly more massive sector of dark spacetime.