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Today's topic is on Dark Matter, a strange, fascinating and mysterious substance, though in some ways we think it is considered fascinating because we call it Dark. Dark Matter, Dark Energy, and Dark Flow, each topics we’d like to cover in the future, have no special relationship with each other. We call them Dark because we can’t see them. It is a bit like how every new jet plane or weapon gets called X something-or-other, the X means it’s a prototype and that gets dropped later on. Dark Matter, Dark Energy, and Dark Flow have as much relation to each other as a prototype assault rifle, fighter jet, and naval cruiser. What Dark Matter and Energy both have in common is that they are thought to make up a huge chunk of the Universe.
What they also have in common is that they have both captured the attention of the public, which is mostly good, but also means that an awful lot of scientifically dubious stuff that has popped up around them. We will talk about some of the weirder things we might be able to use the stuff for near the end, it does have potential uses, but it’s also collected a lot of rubbish over the years as people tapped it for its mystery value. And again the stuff is quite the mystery, but at the same time it also isn’t. We’ve suspected this stuff existed for almost a century now back to when we first realized we lived in a galaxy. Back in the days of yore, when particle physics had just established that atoms included protons and electrons and not much else, we thought the universe was probably an object of infinite size and age, the Steady State Model, on the grounds that if it wasn’t infinite in size then if it was infinite in age everything should have smashed together into one big ball under the influence of gravity. And of course the Universe had to be infinite Lloyd because if it was not you would have to wonder about what was around before it and where it came from. But we were getting good enough with our telescopes to be able to measure the relative speed of a lot of stars and it established that they were by and large moving in an orbit of what we eventually realized was the galaxy. This is where the problem started with the missing mass.
We were getting pretty good at doing orbital calculations at this point and realized that based on the number of stars we could see and their mass, everything should be rotating around the galaxy at a certain speed depending on how far from the center it was. Much like how the planets rotate around the sun, only a good deal more complicated since while the sun is one big enormous mass surrounded by a handful of smaller planets who perturb each other’s orbits just a little bit, the distribution of the galaxy makes the influence of them on each other a much bigger factor in things. We won’t go through that math, but there was agreement there was a lot of matter we couldn’t see, an awful lot too, way more than planets around stars would justify. This wasn’t too big a deal though, we just assumed it was a lot of gas and rock and such and the orbital models were hardly precise things at the time.
Now we had realized galaxies existed quite some time before that, a generation before the United States came into existence Immanuel Kant had nicknamed them Island Universes and it had been speculated a lot of them were flattened disks, like our own is. But we had no idea how far away they were until about the End of World War One when comparisons of the brightness of novae in them to ones here was noticed to be a lot dimmer, and dimmer to a degree that would indicate they were millions of light years away. This caused what was called the Great Debate, about the size and scale of the Universe. They still weren’t getting called galaxies at that time, spiral nebulae was the normal term, and I’ve never been able to find out when that fell out of use and galaxies came in.
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| Spiral Galaxy Source: ESA/Hubble / CC BY (https://creativecommons.org/licenses/by/4.0) |
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| Supernova Source: ESO/M. Kornmesser / CC BY (https://creativecommons.org/licenses/by/4.0) |
You will see in a lot of very old sci-fi stories or articles references to visiting the Andromeda Nebula for instance. Now in 1933 Fritz Zwicky, the fellow who coined the term supernova, was at Caltech studying clusters of the nebulae or galaxies when he noticed that they had to have way more mass than we could see to be behaving as they did. He estimated the one he was looking at must have some 400 times more mass than he could see, and he called this matter dark matter, though being Swiss he used his native tongue.
Now let we give you a quick example of how you could estimate a Galaxies mass, simplified to avoid all the heavy math you need to do this when you’re looking at effective snapshots of billion year processes.
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| Andromeda Galaxy Source: Michael S Adler / CC BY-SA (https://creativecommons.org/licenses/by-sa/4.0) |
Our neighbor, the Andromeda Galaxy, is actually moving toward us. That’s not the norm, virtually every galaxy is moving away from us, red shifting, as you know, and that’s what killed the SteadyState model in favor of the Big Bang. We know what frequencies of light certain types of stars and phenomena give off, so we can look at distant things and say that’s the spectrum they should be emitting, but everything on it is moved a bit over. It’s red-shifted, meaning it’s traveling away from us quite quickly. Blue-shifting, where the spectrum of a star moves the other way, indicates the opposite. So if we had millions of years to watch the Andromeda galaxy approaching us we could say it was going at such and such a speed when we started looking and is now going faster, being speed up by gravity. If we calculated how much faster we’d know how much force the gravity had exerted and be able to say how massive our galaxy was. Same as we could estimate the gravity and mass of Earth by dropping a rock and seeing how fast it got as it fell. In a nutshell if you do this for our two galaxies you’d find out they both massed a lot more than the stars in them would suggest they did.
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| Kuiper Belt and Oort Cloud Source: NASA This SVG image was created by Medium69.Cette image SVG a été créée par Medium69.Please credit this : William Crochot / Public domain |
Now that’s not a problem, we already know a lot more hydrogen and gas is floating around than the stars contain. Add to that, the year before Jan Oort, for whom the Oort Cloud was named, had tweaked to a similar idea, that the mass of our own galactic plane was much larger than we thought. Problem was, his calculations were erroneous and Zwicky himself was off by an order of magnitude. For a long while, because our data and modeling just wasn’t sufficient to really nail it down probably, the idea kind of hung out in limbo. Everyone agreed there was a lot more matter than what star contained but this was somewhat shrug worthy. Seeing gas clouds of hydrogen is a bit tricky but conceptually straight forward. Everything tends to absorb or reflect light, and any given atom or molecule has certain characteristics of which frequencies it does this for. So if we are looking at a star we know there’s some gas and dust between us and it, and if we look at one twice as far away we’d expect, on average, there’s be about twice as much. Start looking at thousands of them and you can start calculating pretty exactly how much gas and dust is between you and each of them and get a good idea how much junk is lying around in between and even its distribution. We’ve gotten way, way better at this as the years rolled by, and way better at measuring the mass of galaxies, and it started getting very obvious that there just wasn’t enough mass out there, in terms of stars and gas and dust, to equal the gravitational force we saw. The discrepancy was just way too big and way too concrete to ignore anymore.
This gives us our first two properties of dark matter, because we know it has to have mass, or at least exert gravity, generally considered the same thing, and we know it has to be transparent to light. Because it isn’t sucking up visual light or any of those other frequencies like radio we look at. Or at least, if it does interact with light, it’s got to do it very minimally, either by being compacted into huge dense chunks or just interacting so little we don’t notice, being very weakly interacting. That’s how things stood in 1980, when Vera Rubin submitted her results on the galactic rotation problem showing rather firmly that most galaxies were carrying about 6 times more mass than we could account for from stars and gas and dust to be spinning at the rates they were. This wasn’t a ground breaking and surprising paper exactly, it had been suspected for a while, but she had compiled such a compelling hard argument and collection of data that it wasn’t so much a nail in the coffin as a stake through the heart of the idea that it might just be a mundane source of mass we were estimating badly. This was hard, she’d used the best modern equipment of the time to measure this stuff and spent years compiling it, and every model agreed that for stars to orbit their galaxies like they were doing there needed to be a lot more mass than we could otherwise detect, either because it was emitting light or blocking it.
Now a lot of options were put forth, we had discovered neutrinos for instance, they’d been theorized since the 1930’s and first detected in 1946, and had some of those expected properties. Neutrinos don’t interact with virtually anything, that’s why they can glide untouched through entire planets.
The problem with using them for dark matter is two fold.
First neutrinos come about essentially as the byproduct of nuclear reactions. When you get a nuclear decay or a fusion or fission reaction some of that excess energy flies off as a neutrino. Several trillion of them pass through you from the sun every second as a result of the fusion of hydrogen into helium. That’s the problem though, only a small fraction of the mass of a star is converted into neutrinos, and hydrogen is hands down the most abundant material in the Universe, meaning most mass hasn’t gone and been fused and produced neutrinos yet. So neutrinos can’t make up the majority of the mass and energy of the Universe because they simply haven’t been produced in large enough amounts yet. And since they fly off virtually unimpeded you can expect those from far and ancient corners of the Universe to arrive intact to our detectors.
Second there’s no reason to think they got produced in huge quantities at some point early in the Universe and we can’t see them now. Neutrinos have very little mass compared to their total energy, they move within spitting distance of the speed of light. And virtually nothing stops them. So they ought not to be hanging around galaxies. They ought to be fairly evenly distributed throughout the Universe, more even then photons since light does get absorbed by a lot of things.
That’s our third property of dark matter. It can’t be moving too quickly or it wouldn’t be clumped up around galaxies. It’s not evenly distributed throughout the Universe, it clumps around galaxies a lot. That means it can’t be moving too fast on average. The escape velocity of galaxies is higher than Earth’s or the solar system but it’s nowhere near the speed of light. So we can assume this missing mass isn’t moving near the speed of light like neutrinos do, or even at decently relativistic speeds, or it would be spread out a lot more evenly, whereas the models that show it exists by making galaxies spin at the wrong speed indicate its spread out in a spherical halo around the galaxy. Since it clearly doesn’t interact with light or normal mass there are no collisions making particles of dark matter slow down and tend toward a disc-around-a-sphere distribution like many galaxies and solar systems exhibit. Gravity would slow them down as they try to pass us and speed them up as they neared, so you’d get nice orbits for those moving slow enough to stay captured, but they will not generally clump into balls like normal matter does to form planets and stars because they’re not colliding with anything to average out their momentum. So that is 3 properties.
so far, they interact with gravity, because they exert it on things and we can see it clumping around galaxies. They don’t interact with light or normal matter, or at least do so even less than neutrinos do. And they do not have much speed. We might as well add a fourth too, that they demonstrably make up the majority of the mass in the Universe. So that is the state of play for the last couple generations, we have a learned a bit more but mostly why various alternative explanations don’t work.
Here are some of the more common ones and their strengths and flaws:
Wimps – Short for Weakly Interacting Massive Particle, this is the current favored candidate for dark matter. You could think of this like a neutrino, though in the way a glacier creeping across the landscape is like a snowball. Which is to say they are both made of ice and moving, but the one is much faster and smaller. If a Neutrino is a weakly interacting low mass particle, a WIMP is any even less interacting heavy mass particle. Neutrino mass is about a millionth of an electron’s mass and billionth of a proton or neutron’s mass. WIMPS are thought to be as massive as a proton to maybe a thousand times more massive, currently. Nor would they have to be a single type of particle, there could be a whole particle zoo of these which would make sense. since they’re the majority of matter. After all there are 6 flavors of quarks and6 types of leptons of which the electron is one and the neutrino is too, well actually three. Of the four fundamental forces, gravity, electromagnetism and the weak and strong nuclear force, leptons can interact with all but the strong force, whereas quarks interact with all four.
But the gauge bosons, the particles that transmit those forces, usually are said to only interact with the force type they transmit and gravity, though gravity is always kind a tricky to talk about at the subatomic scale. Very loosely though we’d say each particle has traits that let it interact with a given type of force and some particles are just missing those. We sometimes think of it like a module or receiver that lets it pickup that forces signal, as it were, and apparently most of the matter in the universe didn’t come with that installed for anything other than gravity. Gravity is a weird force, it is way weaker than the other three and has defied a lot of out attempts to unite the forces in Grand Unified Theories. It’s fairly popular nowadays to say it’s not entirely real or maybe not a force, so if we stop calling it a force to me that would like having a definition of automobile that excluded the old Model-T. Keeping it simple though, WIMPs would be particle that just don’t have the device installed to listen to those other three forces, or have only a very cheap system that barely hears them. Most other particles have the receivers installed for one, two, or all three of those forces. We find this analogy makes it easier to understand why apparently most of the particles in the Universe are lacking this trait. Now again the WIMP is the lead candidate, so it’s got more in its favor then against it. Starting with quite a few models predicting something like them ought to exist. The big thing against them is trying to detect the wretched things. We’ve mostly managed to figure out what they aren’t. Like neutrinos, WIMPS ought to be flying around and through us all the time, but it is kind of hard to detect something whose only known property is it does not interact with anything for you to detect. We mean if you had a box full of the stuff you could shine a flashlight right through it and see nothing, wave your hand through it and feel nothing, and while you could detect it by feeling its mass when you picked up the box, they would just slide right through the box, which couldn’t have kept them inside in the first place anyway.
The big long standing competitor to WIMPs was MACHOs(Massive Compact Halo Objects). MACHOs and WIMPs are fairly inspired acronyms. since they are quite descriptive. Where the Wimp is a tiny particle that doesn’t interact with virtually anything, a MACHO is a huge macroscopic object that just doesn’t emit much light. This is everyone’s favorite first suggestion too. Brown Dwarf stars, big gas giants, neutron stars that have cooled down, and black holes.
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| Brown Dwarf
Source: NASA/JPL-Caltech / Public domain
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| neutron star Source: Casey Reed - Penn State University / Public domain |
Now Halo is important in there because it means the objects are out in the halo of the galaxy. Remember we need a distribution of matter that implies the missing mass enfolds the galaxy as a loose sphere. A MACHO definitely absorbs light, it just doesn’t particularly emit it. They’re very dense compared to dust clouds so we’d expect they don’t block much light. This has generally not been a popular solution with the people studying the problem, but more the solution popular with folks not studying it, since it basically represents folks saying “Do we actually need to make up some freakish new material composing the supermajority of the Universe?” In a nutshell, especially these days as we’ve compiled more data and more accurate data, the answer is yes. Now there is a lot of reasons this doesn’t work, many of them highly technical, but let us offer a more intuitively simple one that’s a variation of the one we use with the Fermi Paradox we call the Time Elapse Argument, or TEA, because I like acronyms too. If I see a weird looking galaxy a billion light years away, and know that it looked like that a billion years ago, then spot a similar one a similar distance away in a different direction and another in third direction we can say that those are unrelated, if artificial in origin, and further more that in a volume with a radius of a billion light years we’d expect 3 of them. So if we look out to a radius of 2 billion light years, which would have 2-cubed or eight times the volume, we ought to expect about 8 times as many of them or about 24 of them. Going out to 3 times the distance we’d expect 27 times as many or 81 of them, give or take. Going out to, say, five billion we’d expect to find about 400 of these things. But if they are artificial in origin, you’d think they’d be more common as the universe got older and more folks arose to build one. As time elapses you can argue they should be more common, so the further away you look, and the further back in time you look, the less common they should be. If you are seeing a phenomena which appears evenly distributed throughout the universe in space, and therefore time, it is not too likely to be artificial in origin. This works quite well for a lot phenomena folks have jumped the gun on and attributed to aliens, which has often included dark matter. But it also includes a lot of MACHOs. Planets take time to form, there are more of them now than they used to be… which is a good caveat to the time elapse argument since we can’t assume just because something is now more common that it is artificial, just that if it is not more common nowadays it probably is not artificial. Planets take time to form, Neutrons stars take time to form as a star must form, live, and die, then cool a lot to being invisible to us. So we should expect that a long time ago in a galaxy far, far away it ought to have a not formed so much of a Massive Compact Object Halo. Weirdly what we instead see at huge distance away and back in time are things like dwarf galaxies composed of mostly dark matter instead. But we certainly are detecting dark matter around ancient galaxies absurdly far away in time and space. So that along with it not being neutrinos tells us that Dark Matter is also old. It is old, it is cold, and it is massive. As best as we can tell it has stayed constant in quantity this whole time too so it presumably does not have a half-life or get created by interactions between normal matter. There are tons of other weaknesses for it being MACHOS but I think that is the one that is easiest to understand without needing lots more science and technology background on it.
The next candidate is the Axion. This particle was first theorized to deal with some problems in strong nuclear force interactions. It is very like the WIMP, except it isn’t heavy. Whereas WIMPS can be thought of as heavy version of neutrinos, weighing as much or more as a proton whereas neutrinos weigh in at about a billionth of that, the axion weighs even less than the neutrino, anywhere from about the same mass to a millionth of neutrino mass. Now the axion is thought to be a particle with very tiny mass and no electric charge, but is also thought to switch into photons in magnetic fields. Axions are nice candidates since they are hard to detect but not nearly as hard as WIMPs, both from their properties and that they ought to be somewhere between a billion to a billion, billion times more common. But while it would make a huge difference to cosmology if it were WIMPS or Axions making up dark matter they are pretty similar objects. Theories for Axions do place their creation all the way back to the early time of the Universe too. In some ways I would say the Axion beats the WIMP out as a candidate for dark matter in the sense that it will be a lot easier to detect, so focusing on proving it or disproving either gets us the answer or gets it out of the way. And we have a number of experiments just getting under way or coming on line soon that ought to settle the matter.
We’re going to skip the Kaluza-Klein Particle, an option for dark matter under string-theory. It has not been ruled out yet but it is supposed to be easier to detect than most of the others, relatively speaking of course, and we haven’t got a whiff of it in the experiments we expected to get a whiff from. We will also skip the gravitino, a particle predicted under Super-symmetry, they were a great candidate but the blows to super-symmetry in recent years demote them we think.
Similarly MOND, Modified Newtonian Dynamics, loosely the idea that gravity doesn’t fall off as the inverse square at very long distances, has fallen strongly out of favor so we will skip it too.
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| flaring black hole Source: NASA / Public domain |
That leaves our last big candidate, tiny primordial black holes. Normal black holes just don’t form enough to account for all that mass, but a lot of our notions about the moments right after the big bang gives us some cause to think lots of small black holes might have formed then. A small black hole with no Hawking Radiation is a WIMP for all practical purposes, one would fly right through you without you even noticing. But if they do evaporate, we can say they have to mass at least a hundred megatons or they wouldn’t be around anymore. One with a mass less than that wouldn’t have lived this long and would have been quite bright. We can push that up a bit too, because there is dark matter in our own solar system, some models say now it might be denser than expected, possible a whole large asteroids worth scattered about, and if that’s the case we can put an upper limit on how much radiation they could be spitting out without us noticing them. We’d say realistically we would see them if they massed less than about 100 billion tons, because that at that point you’d expect them to give off a bit less than a megawatt each of radiation and number about a million. If they massed less they would each be way brighter and more numerous. Chop a black hole into ten small ones each of equal mass and they would each be a hundred times brighter than the original and again ten times more numerous. We’d have difficulty imaging us missing things brighter than a megawatt and more numerous than asteroids. 100 billion ton black holes would be about as bright as small asteroids and about as numerous. Not that they would be giving off visible light, it would be gamma, but We’d have a hard time imagining us missing even the 100billion tons ones in gamma. Smaller than that and we’d say no way. Bigger than that is an option but not a lot bigger or we would detect them messing with planetary orbits and picking up accretion disks.
Now what’s interesting is that while there is cosmic microwave background radiation, there’s also infrared and x-ray background radiation, and recently NASA’s Alexander Kashlinsky and others noticed that there was a lot of matchup in those two, with the most obvious candidate for such a match being primordial black holes. At the same time we have just recently gotten our first detections of gravitational waves from the LIGO detector, our first sight of black holes and not just things glowing while they fell into them, and it has been suggested some of those detections were of the merger of primordial black holes. but while primordial black holes have been a solid if minor contender for dark matter for a long while, their stock seems to be on the rise the last couple years. while quite a few of the others have lost steam during that same time. So WIMPs remain the strong first place, and we’d say axions after that, but we would promote primordial black holes to third place and we can pursue both them and axions aggressively in the next decade to either prove them or disprove them. There are tons of other dark matter candidates, from the plausible and credible if minor candidates to the downright zany. The first and most obvious use is just as filler material. If you can find some substance or field that lets you interact with dark matter, then there’s lots of uses for something that basically is just mass with no interactions. We’ve discussed quite a few megastructures that use most of their mass just for gravity, like shell and disc worlds, and we would usually assume you would use hydrogen and helium for this job, since they are so abundant compared to other elements like carbon and iron. Dark matter would for instance be even better. You don’t have to worry about undergoing any weird transitions into things like metallic hydrogen or start up fusion if you stick too much in one place. Dark stars, hypothetical stars composed mostly of dark matter that might have been around when the Universe was young, are thought to have been able to get a big as solar system. Since some hypothetical forms of dark matter are their own anti-particle, you’d expect the rare collisions of two of them to produce a matter-anti-matter reaction generating light. This light is why we call them stars, or dark matter stars or dark stars. You probably wouldn’t want to use dark matter as a dense filler in such a case, though it would depend on how often those collisions were happening, but that would make that an excellent power supply too. Obviously we have no concept for a material or field that could contain dark matter but if we did have one, and if dark matter was its own anti-particle, you could imagine sticking it in a big balloon or bladder you could squeeze tighter to make more collisions occur or expand out to decrease those. Giving you essentially a massively plentiful source of energy equal to antimatter but way safer, easy to throttle, and naturally abundant, super abundant even.
BUT if you could do that with dark matter and it was its own anti-particle you’ve an amazingly mass efficient fuel supply evenly distributed throughout the galaxy that practically has a sign on it saying perfect ‘rocket fuel’. Ships able to gather that up as they plowed by ought to have no problem going quite close to the speed of light and not needing to worry about running out fuel. If it isn’t its own anti-particle then we can still use it, and not just for filler mass, it could be dumped into black holes as power supplies for instance. Keeping something like that from collapsing on itself is a lot easier with something like dark matter, you could send a beam between relay points, or even two overlapping beams going opposite directions, and unlike normal matter you can send your laser message right down that beam without needing to worry it would scatter. It’s not actually faster than light, but from a practical standpoint it would be since along that path space would be contracted so the message would get there faster than normal. If we cannot find an easy way to manipulate the stuff though, there is again the black hole route. Dark matter would be eaten by a black hole whose event horizon they crossed.
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| Event horizon Source; Deutsch: Black_Hole_Milkyway.jpg: Ute Kraus, Physikdidaktik Ute Kraus, Universität Hildesheim, Tempolimit Lichtgeschwindigkeit, (Milchstraßenpanorama im Hintergrund: Axel Mellinger)abgeleitetes Werk: Sponk (talk)English: Black_Hole_Milkyway.jpg: Ute Kraus, Physics education group Kraus, Universität Hildesheim, Space Time Travel, (background image of the milky way: Axel Mellinger)derivative work: Sponk / CC BY-SA 2.0 DE |
Event horizons are quite tiny compared to galaxies but in the long term you’d expect black holes to eat a lot of them, gaining their mass energy and angular momentum. So there are actually some uses for the stuff inside the realm of plausible science, albeit we have to stretch a bit. As we learn more about the stuff we may learn other uses for it or way to manipulate it, and we’d say we can be optimistic on that score for now, though we also need to be patient, it isn’t likely we will crack the mysteries of dark matter for some years. And it is mysterious stuff, though hopefully a bit less so now we’re done for the day.
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