ask a physicist

Janx said:

What's the deal with Dark Matter? Is it just a placeholder for the math (until they find the real thing and name it Umbranium, or is there really funky stuff out there?

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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 version
First, why we know that there's something new there (ie, why we have a known unknown):

1. 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.
2. 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).
3. 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.
4. 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).

It's worth mentioning that all these observations require about the same amount of extra matter.

What are the possibilities for explaining this?

1. 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.
2. 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.
3. 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.

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:

1. 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.
2. 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!
3. 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.]

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:

1. 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.
2. 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.
3. When you use good resolution and include normal matter, at least some current simulations resolve a number of the "problems" people have talked about.
4. Very small interactions of dark matter particles with each other also sometimes fix some of the problems.

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.

After that wall of text yesterday, I think I'll go with a couple short ones today.

Janx said:

What's the coolest physics question you've answered?

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Given your other question, I'm going to assume you mean "question" as in from the public or a student. This is a pretty hard question in that I'm not quite sure how to quantify the coolness of a question. For me, it's pretty cool when a student asks a question that shows he/she is really getting something or thinking about what they're learning. I'm also very impressed with a lot of the questions at EN World --- in this thread and elsewhere --- because they are well-informed and curious. And generally on cool topics like wormholes, transporters, etc.

One that stands out (for all the above reasons) was from what I think was a high-school or early college student at a public panel I did on dark matter last fall. In the question period, he asked about dark energy and multiverses, which was certainly off-topic for the panel. But it was on a cool topic, showed enthusiasm (ie, this was clearly someone who'd been reading about science), and took advantage of a pretty rare opportunity in Winnipeg.

Janx said:

What's the coolest Physics problem you've solved or contributed to?

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Geez, this is like asking me to choose my favorite pet or something. More seriously, if you're in this line of work and don't find yourself thinking you're working on something cool all the time, you're either getting burnt out or aren't in enough control over your work.

I think in terms of "wow" factor, the work I did on "tachyon condensation" in string theory sounds the coolest. In a less exotic context, tachyon condensation is really similar to a ball rolling down a hill into a valley, and the Higgs field of the Standard Model of particle physics does this in the early universe. Yes, it would be pretty nasty to live through, but it's a comprehensible enough process. About 10 years ago, some of my colleagues showed that certain string theory tachyons actually destroy entire dimensions of space when they "roll down the hill." My work was about applying this in the context of the Big Bang and understanding some details --- one of the results was that the story is actually a bit more complicated, and parts of the original calculations may not be 100% reliable (though that isn't necessarily a surprise when something that dramatic happens).

But I'm also very excited about some work I'm doing right now, which definitely also is cool. This is about black hole formation in "anti-de Sitter" spacetime, or AdS, which is important in string theory for reasons I'll mention in another answer. What's interesting is that AdS has a gravitational potential even though it's "empty." In normal flat spacetime, if you throw a ball away, it will just keep going. In AdS, if you're at the center, a ball you throw away will come back. So, in flat spacetime, matter that tries to collapse into a black hole but isn't concentrated enough will just fly away, but in AdS, it will fly off and then fall back and get another chance to form a black hole. And over and over. (As usual, I'm oversimplifying a bit, but hey.) To track how this works and when a black hole forms takes fairly heavy duty computation, and this is my first time doing real computational physics, so that's personally exciting. And it's a relatively hot topic. As for the coolness, besides the work itself, the calculations lend themselves to making animations. Simple ones, but still --- MOVIES! So that's cool.

A nice little video on Dark Matter and Dark Energy. Doesn't tell you much that wasn't already said, but these folks do a nice video series more people should know about.

[video]https://youtu.be/QAa2O_8wBUQ[/video]

Umbran said:

A nice little video on Dark Matter and Dark Energy. Doesn't tell you much that wasn't already said, but these folks do a nice video series more people should know about.

[video]https://youtu.be/QAa2O_8wBUQ[/video]

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Thanks for the link! It's worth saying that there are lots of great video and blog resources out there.

It's also worth saying after the big dark matter post I did that most of the energy in the universe is really just energy, not particulate matter, which people have named "dark energy." We know very little concrete about it, but the simplest idea for it is that it's the cosmological constant, which is another way of just saying the constant energy of the vacuum (completely empty space). I'm of course willing to entertain questions about that too.

Thanks for answering my generic questions. I figured I could at least find some questions you liked answering by asking for what you liked

There are several questions related to the general theory of relativity/Einsteinian gravity, so I'll do a series of answers over the next couple of days.

garnuk said:

How does Einstein's theory of bent space for gravity work, when there is no "gravity" acting on the object to keep it in the well created by the bent space-fabric?

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I'm going to assume that you're asking about, say, how the earth moves in orbit around the sun, why a dropped tennis ball accelerates downward here on earth, etc, in the context of general relativity (rather than how spacetime gets bent in the first place). One way to think about it is to go back to one of Newton's laws that "an object at rest stays at rest and an object in motion stays in motion unless acted upon by an outside force" like you might have learned in high school or introductory university physics. I want to phrase it a little differently: objects stay in the same state of motion unless acted on by an outside force.

In normal Newtonian mechanics (which is flat spacetime in relativistic terminology), staying in the same state of motion means motion at constant speed in the same direction --- no acceleration. In general relativity, though, as you mention, spacetime is curved. If you go through the math, you find that "no acceleration" means something different in curved spacetime. What it means is "freefall." In other words, astronauts in the International Space Station float because they (and the ISS) are in a constant state of motion --- freefall around the earth. (In Newtonian gravity, you'd say instead that the gravitational force accounts for the centripetal acceleration, so the space station doesn't have to push on the astronauts). So that's it, really: curved paths like orbits are really the "no acceleration" paths in curved spacetime.

This really does change how we have to think about gravity. In Newtonian gravity, we say that we stand on the earth with no acceleration because the downward force of gravity is balanced by an upward push from the ground. In general relativity, we say that we are actually accelerating upward compared to our natural state of motion (falling toward the center of the earth) due to an unbalanced upward push from the ground. It's remarkably different if you stop and think about it. But it's also a pretty simple statement that has many consequences that have been verified experimentally many many times.

Janx said:

Thanks for answering my generic questions. I figured I could at least find some questions you liked answering by asking for what you liked

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Not a bad idea. The truth is, I like pretty much all my research. As I said, in this line of work, you should find anything you do either important, exciting, or cool (even if it's just because it's simple enough to have undergraduates work on it). But the coolness of some projects is much more obvious to a non-physics audience than other projects.

freyar said:

There are several questions related to the general theory of relativity/Einsteinian gravity, so I'll do a series of answers over the next couple of days.



I'm going to assume that you're asking about, say, how the earth moves in orbit around the sun, why a dropped tennis ball accelerates downward here on earth, etc, in the context of general relativity (rather than how spacetime gets bent in the first place). One way to think about it is to go back to one of Newton's laws that "an object at rest stays at rest and an object in motion stays in motion unless acted upon by an outside force" like you might have learned in high school or introductory university physics. I want to phrase it a little differently: objects stay in the same state of motion unless acted on by an outside force.

In normal Newtonian mechanics (which is flat spacetime in relativistic terminology), staying in the same state of motion means motion at constant speed in the same direction --- no acceleration. In general relativity, though, as you mention, spacetime is curved. If you go through the math, you find that "no acceleration" means something different in curved spacetime. What it means is "freefall." In other words, astronauts in the International Space Station float because they (and the ISS) are in a constant state of motion --- freefall around the earth. (In Newtonian gravity, you'd say instead that the gravitational force accounts for the centripetal acceleration, so the space station doesn't have to push on the astronauts). So that's it, really: curved paths like orbits are really the "no acceleration" paths in curved spacetime.

This really does change how we have to think about gravity. In Newtonian gravity, we say that we stand on the earth with no acceleration because the downward force of gravity is balanced by an upward push from the ground. In general relativity, we say that we are actually accelerating upward compared to our natural state of motion (falling toward the center of the earth) due to an unbalanced upward push from the ground. It's remarkably different if you stop and think about it. But it's also a pretty simple statement that has many consequences that have been verified experimentally many many times.

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Thanks! Learn something new everyday...
However, my question just now shifts to what causes the "freefall". I imagine that if you take a ball and funnel into space, and push the ball around the edge, that without some other force, (like blowing on it), the ball would just spin out of the funnel all together.

garnuk said:

Thanks! Learn something new everyday...
However, my question just now shifts to what causes the "freefall". I imagine that if you take a ball and funnel into space, and push the ball around the edge, that without some other force, (like blowing on it), the ball would just spin out of the funnel all together.

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If you recall the idea of inertial motion from introductory physics like I mentioned in my answer --- the idea that objects move in a straight line at constant speed unless acted on by an outside force --- that's all freefall is. In space, very far from a large mass, there's "no gravity" (in relativity, spacetime is flat), and a free object will just move along at constant speed in a straight line. It's just that near a large mass (like a star or planet), the curviness of spacetime means that "inertial motion" includes behavior like falling toward a planet or orbiting it. I don't think you can really say there's a cause of freefall or inertial motion. It's just how things work, and I think it's really important to remember that "freefall" in general relativity is the same idea as Newton's laws, just in a broader framework.

In your example, the ball and the funnel are pushing (exerting force) on each other, so that's how the ball can roll around the edge of the funnel. But, yes, eventually it would run out of funnel and move off inertially --- in empty space, that would be a straight line, but, near a planet, that might be an orbit.

Good questions. Maybe if there are more follow-up questions, it would be good to post a new thread in the Misc. Geek Talk forum. Thanks!

Always good to see another Physicist here. My study was astrophysics, specifically exoplanet detection with the Kepler Satellite data as well as modeling the planet and it's system.