What if there's no dark matter?

An article in today’s Toronto Star talked at length of a lab deep in a Sudbury mine trying to find dark matter, the stuff that is supposed to make up most of the matter in the universe which for some reason you cannot detect. Dark matter is hypothesized by math and observations of gravity; nobody has yet found any.

I’m not a genius but dark matter is one of those things I’ve always sort of half expected might, in the 22nd century, be amusingly regarded as the “luminifous ether” of our name, the silly nonsense we made up to explain something we REALLY didn’t understand. But I am probably wrong. Probably. (If there is dark matter everywhere wouldn’t the Earth be gathering it and getting noticeably more massive that could be explained by space debris?)

Still… Let’s suppose they spend ten years trying to find dark matter and say “Holy shit, everything that should have detected failed!” What the hell do we think then? What is Explanation B?

Alternate theories include things like “the force attributed to dark matter is actually space itself doing something weird” and “gravity equations have extra terms in them” and “our measurements are bad” and “alien spaceship engines are distorting the movement of far off mass by abusing something about the universe to get thrust without expending rocket fuel”.

Frankly, you can pretty much make alternate theories all day or just say “for reasons unknown, the stars move with <special equation>”. At a certain level of physics, everything comes down to “something does something according to <complex bit of math> because it does”.

One thing to appreciate with dark matter is that it’s not just a fancy word for “placeholder”.
The hypothesis that there are vast amounts of matter interacting only via the gravitational force has been used to make predictions and so far has only been validated while competing hypotheses have been refuted or have yet to make a prediction we can test. That’s why it’s gained the support it has.

Like much of modern science, it’s unlikely to be outright wrong but may eventually look like a crude model in retrospect, and specific particle models for dark matter are (as everyone would admit) speculative.

I remember in college, my teacher was telling me this story about how he and his coworker were working on a piece of machinery that wasn’t working properly. The thing is, every time they hooked the voltmeter up to it, the machine would work right. As soon as they took it off, the machine would stop working. Out of frustration, they eventually said “fuck it” they left the voltmeter connected to it, jammed it in the guts of the machine and put the casing back on. Engineering at it’s finest.

Anyway, I suspect dark matter is pretty much like that voltmeter. They don’t know what the fuck it is, but the math sure does make everything look pretty.

No. Why would it? Dark matter falling toward the earth would not be stopped by anything, because it doesn’t interact with anything. Nothing would slow it down enough to be “gathered”. It would just zip by/through us. All gravity would do is help it get moving a little faster as it gets closer, and a little slower as it gets farther away.

This is not really accurate. For a theory to be useful, it has to be able to make predictions. “Aliens are messing with the universe” and “space itself is doing something weird” aren’t very useful for predicting stuff.

Just to clarify a bit - I found a pretty good paragraph on the dark matter page for wikipedia to show an example of something dark matter explains.

Most of the mass of this group of two galaxies that just crashed into each other is located away from the center. Most of the regular matter is concentrated in the center. A theory of dark matter predicts this neatly, where no other explanation currently can. The regular matter in the galaxies just smashed into each other and collected somewhere in the middle, while the dark matter continued sailing past, completely unaffected.

Various cosmological experiments indicate that certain gravitational effects we’ve noticed don’t match the mass we observe; their effects indicate more mass than we’ve actually found. To resolve that disparity, there are three options:

  1. There’s extra mass there, but we can’t see it. Our interactions with it are pretty much limited to the electromagnetic force, so that leaves gravity, the weak interaction, and the strong interaction. The latter isn’t a good candidate because of color confinement. If dark matter doesn’t participate in any of the forces besides gravity, it’s unclear how it would originate. That leaves the weak force, and there are some indications that the math could work out right if some weak-style interactions are added into the mix.

  2. There’s no extra mass there, and our understanding of gravity is flawed. It’s possible, but general relativity has proved amazingly, insanely accurate so far and is on an entirely reasonable mathematical and physical foundation.

  3. Something else is seriously wrong: dark matter interacts via an as-yet unknown fifth force; some complicated gravitational interaction in string theory or brane theory is resposnible, etc.

The first is the most likely. As Mijin said, ‘dark matter’ is not just a placeholder for some bit of science we’re clueless about. We’re seeing gravitational effects that we don’t have enough mass to explain. The most likely explanation is that we can’t detect the missing mass, which would indicate that it doesn’t participate in the electromagnetic interaction.


Actually, those crackpot reactionless thrusters allegedly work exactly like this. It’s precisely the opposite problem - if we ever get unequivocal proof that these impossible “thrusters” really work, one way to explain them is that they are interacting with the rest of the mass in the universe and in fact are speeding up expansion.

I guess it depends on the speed the dark matter is travelling at, but if it interacts with gravity, which is what it’s supposed to do, one thing that could stop it is, well, gravity. Some of the dark matter would pass by, but some, one would think, might not achieve escape velocity.

Is all dark matter stationary relative to Earth’s movement and we’re hammering through it at incredible speed?

You don’t seem to have a good grasp of what dark matter is. It’s matter that interacts normally through gravitation but not electromagnetism. Electromagnetism is responsible for things like friction in the air and your floor being solid enough to stand on, so we’re never going to hold a baseball-sized chunk of dark matter or be able to look at dark matter, because both of those would require it to interact electromagnetically, which I think is what you’re thinking of as ‘observing’ it. Because it doesn’t interact electromagnetically, it doesn’t clump like regular matter, it ends up forming an orbiting ‘halo’ around (and through) a regular matter galaxy.

We’ve already spent more than ten years trying to directly observe a dark matter particle in a lab setting, but it’s not easy to catch something that passes through a block of solid steel as easily as it does hard vacuum. Current theory leans towards dark matter being some kind of Weakly Interacting Massive Particle, and there are a number of candidate particles that fit with the Standard Model of physics. If those end up not panning out, there are alternate theories, and doing enough accelerator experiments to rule out current ideas will probably lead to new models. The observed behavior of galaxies and of light being redshifted by passing through mass fit the theory of gravity if there is dark matter, and don’t if there isn’t, so any explanation that amounts to ‘there isn’t any’ would have to include some major changes in physics.

Also, back when I was in school (majoring in Physics) I never encountered professors talking about Luminiferous Ether as something that was silly nonsense or just some dumb thing people did. It was always taught as an example of a good theory, because it had explanatory value, made useful predictions, and was falsifiable. The fact that it turned out not to be correct doesn’t mean that people who came up with it are regarded as laughable.

Gravity doesn’t work like that. It can capture things in an orbit, but aside from black holes it doesn’t normally bring anything to a halt, just makes it’s path curve or makes it loop a lot. It’s the electromagnetic interactions that make the molecules in a cloud of matter lose velocity and gain heat, and eventually form a solid lump like a planet or star. Most dark matter would have significant velocity relative to the Earth to start, and gain velocity by falling in due to Earth’s gravity. Without electromagnetism, it doesn’t slam into the ground or slow from friction with the atmosphere, it just wooshes right through and out the other side.

There is some thought that the Alpha Magnetic Spectrometer, currently mounted on the ISS has detected evidence of dark matter. At least evidence consistent with one possible theory of it.

I wonder - it seems that neutralinos of mass of about 800Gev are consistent with the AMS observations. How much support to super-symmetry do these observations provide? So far the LHC has not provided much support.

To be fair, you can capture things using gravity, just like you can with electromagnetism. The thing is, with either force capture is impossible using just one capturing object, and it gets easier the more objects you have. With electromagnetism, there are gazillions of capturing objects, because you’ve got every single proton and electron to work with, but gravitationally-interacting objects tend to be much fewer in number.

It’s worth mentioning, by the way, that just because dark matter is more abundant than “ordinary” matter (in scare quotes because it’s apparently the dark matter that’s “ordinary”), doesn’t mean that it’s more abundant everywhere. Dark matter is much more uniform in distribution than “ordinary” matter. Our sort of matter, meanwhile, is very clumpy, and we happen to live in one of the clumps (hardly a surprise, since we ourselves are clumps of matter). Within our Solar System, say, our sort of matter is far more abundant than dark matter. But solar systems are so far apart that, in the vast spaces in between where there’s almost none of our kind of matter, the dark matter is dominant.

Dark matter also isn’t particularly mysterious. We know that there are many different kinds of particles, and that not all particles interact via all forces. Heck, we even know of a few specific particles that don’t interact electromagnetically: The neutrinos. Why is it so surprising that there might be a few more like that that we haven’t discovered yet?

Dark matter isn’t that fast (cold dark matter must be the dominant form of dark matter), but the point is that it doesn’t have to be fast to not be captured in significant amounts by an object as small as the Earth or the Sun. Dark matter hardly interacts with anything, including itself, so it has no way of transferring momentum (and angular momentum) that is needed for it to show the strong clumping behaviour of baryonic matter. It has to move near enough to the Sun, and at a speed that is small relative to the Sun, to be captured by the solar system, which ultimately means not a lot of dark matter gets captured by the solar system.

My belief this week is that the effects ascribed to dark matter are regions of warping in space(-time?) left over from the Big Bang. These warped areas led to the creation of galaxies, superclusters, etc. and contribute to the rotational speed of galaxies appearing off. But the delta warpage isn’t large enough at the “local” level to notice it at the solar system scale.

Sort of based on stuff like this recent announcement.

So, no actual dark matter, per se. No WIMPs are anything.

Next week, I’ll be believing something else. (When are they going to solve this?)

One theory I’ve heard is that some things we assume are universal constants like gravity or the speed of light might not be. The discrepancies we’re observing are caused by small variations in these constants across really large amounts of time or space.

People have come up with a number of theories along those lines, but none of them really work. Either they don’t fit data or require too much fine tuning to match what we see. Back when dark matter was first theorized there wasn’t as much data, so that kind of theory looked more promising.

Just a random semi-related question.

Does dark matter (as theorized/ evidenced by the data) clump/interact with itself in any fashion?

It doesn’t clump with itself, if it did then the dark matter around the galaxy would start interacting and eventually form discrete lumps instead of a halo around the galaxy, and it wouldn’t fit what we’ve observed. Some theories of WIMPs (Weakly Interacting Massive Particles) can interact with each other through nuclear forces but not electromagnetism, but without EM they don’t cluster enough for this to be significant. Some highly speculative theories have dark matter interacting through a force or forces that we haven’t seen yet, and theorize that the matter we see is a sort of fringe thing to the real action in the universe, though they’re really more in the ‘wild guess’ category.

Thanks for that.

But to refine the question a bit. Are the models/data saying there is NO interaction between dark matter/dark matter or dark matter/matter or just slight?

Of course if there is absolutely NO interaction between dark matter and matter then any experiments to detect it are a bit hopeful :slight_smile: