We’ve been trying to detect dark matter directly for a few decades now, but this year, there are a couple of major systems coming online that could finally make it happen, giving us the best shot we’ve ever had. They might not succeed, of course, but that’s the way it goes when you are trying to detect something whose physical nature you know nothing about.
There’s a terrific article in Smithsonian Magazine from February 3 that outlines the large-scale experiments that are going on around the world in this area, including the newest ones, LZ and SuperCDMS, that I’m going to focus on here.
These are expensive, elaborate experiments, but are they worth it? I think so, because dark matter is certainly out there, and it’s probably all around us, too. We’d love to finally know what it is. How do we know it’s out there? A lot of ways….
In 1933, Fritz Zwicky noticed that galaxies in the Coma Cluster appear to move toward each other a lot faster than they would if the stars within them comprised all of their matter. As if there’s much more gravity than there should be. He proposed several explanations for this, one of them being “dunkle Materiel” (dark matter). He also suggested that gravitational lensing could be used to estimate the amount of matter in a galaxy (dark or otherwise). He didn’t live to see that tried, but we do it all the time now. (More on that in a moment).
Vera Rubin, using redshift from the Doppler effect, found that stars in the Andromeda Galaxy (and many others) rotate at the same speed no matter how far from the center you go out. It’s not supposed to work that way. Planets in our Solar System, for example, orbit faster the closer they get to the Sun. But galaxies tend to act more like phonograph records, where the whole thing spins around at the same speed. If you put a bunch of M&Ms on a spinning turntable, the candy will fly off. A galaxy of loose stars should fly apart like that given its speed, but it doesn’t. Some unseen mass holds it all together.
Rubin is often credited these days with being the real discoverer of dark matter because her measurements were a lot more accurate than Zwicky’s, and she proposed specifically that the explanation was dark matter. A lot of people think she should have won the Nobel Prize before she died, but she never did.
Astronomy Magazine laid down this quote about the Nobel committee in 2016 (and you’ve got to tip your hat to them, because there just aren’t that many dark-matter jokes out there):
It’s like the committee cannot see her, although nearly all of astrophysics feels her influence.
So no Nobel Prize, but we are coming around a little bit. Rubin is having a grand observatory named after her, Science News reported on January 10 (even the Senate passed the bill making it so, by unanimous consent), and that observatory will study dark matter and dark energy. It will host the Legacy Survey of Space and Time (LSST), a U.S.-funded project now under construction in Chile that will come online in 2022. Quite fitting and very nice.
Another more-recent clear demo of dark matter’s existence came from colliding galaxies. The Bullet Cluster is actually two clusters of galaxies that have passed through each other “recently” (in galactic terms, anyway). Clusters of galaxies like this often contain a lot more X-ray-emitting hot gas than actual galaxies, so when they collide, most of what does the colliding is the gas. As Neil deGrasse Tyson once graphically explained, it’s like tossing two buckets’ worth of water at each other when each bucket also has a few ping-pong balls in it. The galaxies are the ping-pong balls, while the gas is represented by the water. The balls mostly zip right past each other, but the water lags behind the balls and then gets tied up by colliding in the middle.
Since the gas is the majority of the mass and it gets hung up in the middle like that, we should detect most of the mass in the middle. And if we only look at visible mass, then that’s exactly what we see.
But when we map out ALL the mass by checking out the distortion of galaxies in the background being gravitationally lensed by the whole cluster, we find that the vast majority of the mass is found not around the gas, but around the galaxies. All that extra mass is clearly distorting things that lie behind it, even though we can’t see it. The only way we know it’s there is because it acts, through gravity, like it has mass.
The other really interesting thing is that this invisible mass passes through everything else like a ghost. Because there’s so much of this invisible mass, much more than the mass of the gas, and it appears to be diffuse, not in little packets like the galaxies, it should have behaved like all that gas and gotten hung up in the middle, but it didn’t.
OK, so we have all these indirect ways of knowing that dark matter exists. So where is it? Is there some around us? Can we detect it?
Our Solar System is moving relative to the center of the Milky Way at about 550,000 miles per hour, right through the huge halo of dark matter surrounding the galaxy, so that’s a good guess for how fast dark matter is whizzing through us all every minute, and right through the Earth, too.
The dark matter raining down on us, because of the direction we’re moving, appears as though it’s coming from the constellation Cygnus, which is embedded in the well-known and easy-to-spot Summer Triangle:
So when the weather warms up a little, we can all go out and look up toward the Summer Triangle, knowing we are being blasted by the dark matter wind. If you can feel it, you might want to notify your local university’s astrophysics department, because you are a very special person.
The high velocity of dark matter is good for a couple of reasons. One, it’ll keep zooming through whatever detector we set up, so we get a lot of chances to see it. Its velocity will give it a shot at hitting some unsuspecting atom pretty hard, hard enough to knock some of its electrons off. And we can measure things like that. Two, since the Earth goes around the Sun, the apparent velocity of the dark matter should change with the seasons, and we should see that, offering more convincing proof that we’re observing something real.
And we’ve certainly been trying. But whatever dark matter is, we know from all these unsuccessful experiments that its interaction with regular matter must be super-weak. Yes, there’s the gravity thing, but that’s only effective for gigantic objects like planets. Two cats near each other don’t notice any gravitational force between them, so two atomic-scale particles certainly aren’t going to.
We have a couple of very sophisticated detectors coming online this year, the most sensitive we’ve ever had, hoping to catch a glimpse of the ultra-rare interaction between a dark-matter particle and a “normal” particle. The astro community will be a little anxious. Did they see anything? Have you heard anything?
Rather than try to convey the impact of these new detectors in words, I want to show you a couple of graphs, because they frankly do a better job than the words in any article I’ve read. But first I have to tell you quickly about “cross section”, because it’s the vertical axis on both of them.
When two particles get near each other, there’s a chance they’ll interact in some way. How close they need to get depends on the type of interaction. If you fire a bunch of billiard balls at each other within a known area, you can count the number of collisions you get to figure out how big the balls are. Atoms are fuzzier than billiard balls, but based on how often they interact when we fire them at each other, we can say they act as if they are billiard balls of a certain size. That effective size is pretty much the “cross section”.
Subatomic particles are quite small, so depending on the type of interaction, you’re usually looking at cross sections of between 10-21 and 10-27 square centimeters. The weakest interaction we know of between individual particles is the weak nuclear force, and the typical cross section for that is roughly in the 10-40 square centimeters neighborhood. Dark-matter detection experiments to date have looked in that area and haven’t found anything, so it seems increasingly likely that dark matter interacts with “normal” matter even more weakly/rarely than that. Maybe there’s another type of interaction we don’t know about yet, or maybe dark matter is super-finicky about interacting for some reason. Or it might not interact with “normal” matter at all, other than through gravity.
Here is the history of the sensitivity of dark-matter detectors since around 1990. The vertical axis is the smallest cross section that these detectors would be able to pick up if the incoming dark-matter particle were as heavy as about 50 protons (kind of a big particle). The lower we go, the better.
This is where you can see what difference 2020 will make:
The open blue triangles represent LZ, a detector about to go live in South Dakota this spring. It’ll be at least 10 times as sensitive as anything that has come before it.
A more-expansive history that includes the other new detector, SuperCDMS (in Ontario), is shown below. This is all the dark-matter detection experiments that have happened up to now (and haven’t found anything). Cross section is on the vertical axis again, and mass is on the horizontal axis. “WIMP” means “weakly interacting massive particle”; i.e., dark matter). The yellow stars point you to the lines showing the limits that LZ and SuperCDMS are going to reach. They’re going to cover a lot of area we haven’t covered yet.
You can see that SuperCDMS (going live later in 2020) will cover lower masses, while LZ (this spring) takes on higher ones. The fat dotted orange line shows the limit below which interference from neutrinos will start making detection of dark matter a lot more difficult. If we have to dip into that region, it will probably be time for a new type of detector.
So, how will these detectors actually detect anything? I’ll mostly deal with LZ here; the SuperCDMS principle is similar. LZ will start with a vat of 10 tons of liquid xenon!
That vat will be about a mile underground, in an abandoned mine, to keep out any radiation or particles from the Sun or outer space. Xenon was chosen because it doesn’t react with anything, doesn’t have any unstable isotopes, and can be purified very well, to eliminate all sources of possible background.
The other good thing about xenon is that when it gets impacted by another particle with enough energy, one or more of its electrons can get excited or knocked off completely, and when one of those electrons settles back into its original state, you get emission of a short-wave (high-energy) ultraviolet photon. That’s the little flash of light in the picture. But also, any electrons that get knocked off their atom can be guided to the top of the tank by a magnetic field, and you can detect them when they get there. Measuring the time difference between the flash and the electron arrival tells you where in the tank a xenon atom got bumped.
Xenon in the liquid form is particularly useful, because you can pack a lot of it into the tank, so that the dark matter particles coming through get a lot of chances to hit a xenon atom in just the right way for an interaction to occur. Given the tiny cross-section range we’ve had to stoop to, those interactions only happen once (at most) in a bajillion opportunities, even more rarely than a Boston driver using their turn signal. So we need a lot of xenon atoms and a lot of exposure time.
SuperCDMS is going to use solid crystals of silicon and germanium as the detection material and look for subtle vibrations in the crystal lattice caused by impact with a dark matter particle. You have to keep the crystals at a temperature of near absolute zero so that their normal vibrations are tamped down to near nothing.
If either of these detection systems sees something real, we’ll finally know a lot more about the nature of dark matter. How big the particles are, how they interact with “normal” matter, how many of them are around us.
What are the prospects of really finding something this year … or ever? There is certainly a wide range of views on that.
Leslie Rosenberg, a physicist at the University of Washington who is working on detecting dark matter in still another form called the axion, had this to say about the researchers he might consider his rivals, if he were so inclined:
People are nervous about the WIMP [weakly interacting massive particle], but they’re just being nervous. It would not surprise me if LZ and/or SuperCDMS found the WIMP. I think the community would be surprised because they don’t think big. They get up in the morning, they have their coffee, they read the paper, they go to work, et cetera. … So day-to-day, they don't think about the possibilities. They don't see the light.
Less-sanguine astrophysicists like Ethan Siegel, who wrote a nice article for Forbes in which he discusses the reasons why methods like these are reaching the limits of their utility, still has to concede that the spirit behind all of this is just right:
There is undoubtedly something new out there in the Universe, waiting to be discovered. The only way to know is to look.
Exactly. Don’t you want to know what dark matter is? Don’t you want to understand it better? It’s all around us, and we don’t even know what it is. As Madge the Palmolive lady once said, “You’re soaking in it.”
Will this be the year we finally “see” dark matter for the first time? There’s only one thing we can do:
Try to detect it.
It’s not too late.