Reply to thread

Message: <blockquote data-quote="freyar" data-source="post: 6675845" data-attributes="member: 40227">This is a popular topic on EN World, so I'll try to give a thorough answer for it.  Here's the tl;dr version: there's very good evidence that normal matter isn't enough to account for the gravity needed to hold things like galaxies together (or get them to form in the first place), so there must be something there with certain characteristics.  We call this dark matter.  At a basic level, it's already incorporated in the math of our understanding of cosmology (history of the universe), but we don't know its detailed properties.Long versionFirst, why we know that there's something new there (ie, why we have a known unknown):<ol> <li data-xf-list-type="ol">The first accepted evidence came from looking at the orbits of stars in other galaxies.  Basically, if what we see is all the matter there is, the orbital speeds should decrease far from the center of a galaxy, but instead they stay the same. That means there has to be a lot of mass distributed even at large distances from the centers of galaxies, a total of 5 to 6 times as much as we can see.</li> <li data-xf-list-type="ol">If you look at clusters of galaxies, you also find that the galaxies in the clusters are moving far too fast to stay together unless there's more mass than we can see.  This was actually the first evidence discovered for dark matter, but it wasn't widely accepted, perhaps because people suspected the observations just didn't see a lot of the gas between galaxies (which I think ended up to be the case).</li> <li data-xf-list-type="ol">We can also look at light from galaxies farther away passing those clusters.  That light is gravitationally lensed by the mass in the cluster, and we can use the lensing as a measure of the mass, which is again more than the normal visible matter.</li> <li data-xf-list-type="ol">The cosmic microwave background (aka CMB) shows us the clumpiness of matter in the very early universe.  Up till that time, normal matter was a plasma, and the pressure of the plasma strongly resists clumping.  It turns out that the amount clumping we see is consistent with having a large amount of pressureless matter (not normal stuff).</li> </ol>It's worth mentioning that all these observations require about the same amount of extra matter.What are the possibilities for explaining this?<ol> <li data-xf-list-type="ol">Gravity/mechanics don't work how we think.  The most popular option is called "modified Newtonian dynamics" (MOND), which takes the idea that Newton's second law (F=ma) should be changed for very small accelerations.  This works pretty well for stars in galaxies, less well for clusters (and see below) and not really at all for the CMB.  The relativistic version of MOND is very messy, and it just doesn't seem like you can explain the CMB data without extra matter unless you say the gravitational force didn't point toward mass but in an apparently random direction in the early universe.  It's very hard to make sense of that, and all but the most hard core MOND supporters (like the people who invented it) say as much.</li> <li data-xf-list-type="ol">There could just be lots of "dark" normal matter, like gas clouds, super-Jupiter planets, etc.  (The planet-like option used to go by the name of massive compact halo objects, or MACHOs.)  There was a big search for MACHOs in the 1990s, which didn't find enough, and CMB data now tells us the excess mass can't be made of normal matter.</li> <li data-xf-list-type="ol">There is some new kind of elementary particle, generically called "dark matter."  There are several possible types of particle that could be dark matter, but the simplest to describe is a weakly interacting massive particle or WIMP (MACHOs vs WIMPs, get it?  Weren't those physicists so clever?).  This is basically a very heavy kind of new particle (usually considered to be at least as heavy as a Higgs boson, though not necessarily) that doesn't interact with normal stuff very much.  There are actually a number of reasons to have this type of particle from a theoretical point of view --- they arise in solutions to other problems in particle physics, for example. With the right amount, this fixes all the observational problems above and helps to explain the formation of structure (like galaxies) in the universe.</li> </ol>So most physicists prefer particle dark matter, and WIMPs are the most popular, though that's partly because they're easiest to look for compared to other particle alternatives (like looking for your keys under the lamppost rather than in the middle of the dark street).How are we looking for dark matter?  3 ways, primarily:<ol> <li data-xf-list-type="ol">Colliders: dark matter or perhaps other new particles related to dark matter could be produced at a particle collider experiment, like the Large Hadron Collider that discovered the Higgs boson and more recently pentaquarks.</li> <li data-xf-list-type="ol">Direct detection: the earth is passing through a cloud of dark matter (if the dark matter particle mass is the same as for a Higgs boson, there would be about one dark matter particle per liter here).  Every once in a while, a dark matter particle could hit an atomic nucleus and cause a recoil.  So there are lots of experiments set up to look for those extremely rare recoils. The problem is, until we do the measurement, we don't know how rare the recoils will be!</li> <li data-xf-list-type="ol">Indirect detection: dark matter may have rare interactions out in space, which we might see just because there's enough dark matter that rare events still happen a lot!  The most common thing to look for is dark matter annihilating with anti-dark matter (every particle has an antiparticle, after all).  Presumably, this would eventually produce "normal" Standard Model particles that eventually produce photons we can see.  [Most of my work has been on this area.]</li> </ol>These are the main experimental means of searching for WIMPs, and there are variations for other types of particle dark matter.  These are very difficult experiments, so there have been a number of controversies and false alarms.  Still, the experiments are reaching into very interesting parts of "theory space," so there is hope that the next decade will tells a lot more about what dark matter is.One other point to make is that dark matter, being most of the matter in the universe, has a lot to do with how the structures of our universe (galaxies, clusters of galaxies, etc) formed.  By and large, simulations of dark matter particles moving under the influence of each others' gravity give results similar on a large scale with observations of our universe.  On a smaller scale --- like the center of a galaxy like ours or like the dwarf galaxies that orbit our Milky Way galaxy --- some people have noted discrepancies (and some EN World posters have asked about these previously).  This is a point that MOND adherents like to bring up, since MOND does an ok job explaining some of them (at least according to abstracts of some papers I've seen).  There are, though, some reasons not to get worked up about this:<ol> <li data-xf-list-type="ol">Doing gravitational calculations of many dark matter particles is ridiculously hard.  Even solving for the motion of the planets in the solar system is very difficult if you include the gravitational effects of the planets on each other (this problem is generally called the "N-body problem").  Even low resolution simulations require the gravitational interactions of billions of "dark matter particles" each with mass many times that of the sun and take years and years of CPU time to run.  These have improved a lot, so the point is that there is still work to be done in understanding the theoretical prediction.</li> <li data-xf-list-type="ol">A lot of the "problems" noticed were in comparing the real world to simulations that involve only dark matter.  People are only in the last few years incorporating normal matter in with the dark matter in their simulations --- things like star formation, supernovas, etc.  These can have some big and complex effects.</li> <li data-xf-list-type="ol">When you use good resolution and include normal matter, at least some current simulations resolve a number of the "problems" people have talked about.</li> <li data-xf-list-type="ol">Very small interactions of dark matter particles with each other also sometimes fix some of the problems.</li> </ol>So the jury is out, but it's relatively encouraging.  At the very least, its' way too early to say there is a crisis for the dark matter paradigm.I think that's basically it.  Hope that gives you something to read for a while. <img src="https://cdn.jsdelivr.net/joypixels/assets/8.0/png/unicode/64/1f609.png" class="smilie smilie--emoji" loading="lazy" width="64" height="64" alt=";)" title="Wink    ;)"  data-smilie="2"data-shortname=";)" /></blockquote>

Verification