In the 18th century, physicists were trying to understand what happened when things burned. It was theorized that combustible substances such as wood and coal contained something called phlogiston, which was released during burning. This theory seemed to explain some of the results of experiments, but of course it was completely wrong.
In what may turn out to be a similar flight of fancy, physicists today are enamored of the theory of dark matter. Like phlogiston, dark matter itself has never been observed; it’s proposed as a way to explain certain things that have been observed.
The problem that the dark matter theory attempts to address is quite real. The problem is that the outer parts of galaxies are spinning too rapidly. It’s possible to measure the spin of some galaxies — those that are tilted so that we observe them somewhat edge-on. This is possible because the light on one edge of the galaxy (the edge that’s spinning toward us) will be blue-shifted, while the opposite edge is red-shifted because it’s receding from us. This is basic physics. I couldn’t do the math, but I understand the concept.
We can also estimate the mass of a galaxy. This is done by estimating the number of stars in it (based on its brightness) and multiplying that estimate by the average mass of a star. Mass causes gravitational attraction, and gravity causes stuff to orbit the center of mass, in exactly the way that the Earth orbits the sun. The speed of the orbiting body depends on both the diameter of the orbit and the amount of mass around which the object is orbiting. Again, I couldn’t do the math, but this is basic stuff.
When the rate of spin of the outer parts of nearby spiral galaxies is calculated, it quickly becomes apparent that there’s not nearly enough mass to explain the speed of rotation. This means one of two things: Either there’s a bunch of mass that we don’t see, or we don’t understand how gravity works at galactic distances. The idea that the law of gravity needs to be revised is not popular, though some theorists are working on it. The general consensus is that these galaxies are embedded in a halo of dark matter — stuff we can’t see, but that adds significantly to the mass of the galaxy.
One idea, which seems not to be panning out, is that galaxies are studded with “brown dwarfs.” A brown dwarf is a a lot bigger than Jupiter, but a lot smaller than the sun. It’s small enough that nuclear fusion has failed to ignite; thus it doesn’t put out much in the way of visible light. It’s brown, and it’s a dwarf star. But while there are certainly brown dwarfs floating around, a survey of our own galactic neighborhood suggests that there aren’t nearly enough of them to account for the rapid spin of spiral galaxies like our own.
A more popular notion is that the dark matter is a cloud of non-baryonic particles. Protons, neutrons, and electronics are baryonic; they’re the stuff we’re made of. We can’t see this non-baryonic matter, so the theory goes, because it neither absorbs nor emits light. However, it has mass, so it generates a gravitational field. (Don’t ask me whether “generates a gravitational field” is how physicists would talk about it. I don’t know.)
I have no problem with the idea that the universe is filled with particles that we know nothing about. But I have yet to read an explanation of how this massive dark matter is supposed to be behaving.
I also have a problem with how confident some authorities are that such a mysterious thing exists. In poking around on the Web, I quickly found a site (detailing the findings of the Wilkinson Microwave Anisotropy Probe) that asserts, baldly, this: “The WMAP science team has … completed a census of the universe and finds that dark matter (matter not made up of atoms) is 24.0%.” Well, imagine that. They can’t see it; they don’t know what its properties might be; but they’ve done a census. Shee-it.
The idea that they’re treating as gospel is this: The clouds of dark matter are supposed to produce gravitational fields within which the baryonic matter (clouds of hydrogen, to start with) congregates, condenses into stars, and so forth. But if this cloud is imagined as consisting of zillions of tiny particles (perhaps not much larger than a proton), it doesn’t seem, to my muddled way of thinking, to be behaving in a sensible way. Some of these particles will be moving rather rapidly; some will be moving more slowly. That seems indisputable. Those that are moving too rapidly will have enough velocity to escape from the cloud. They’ll be gone. So there’s a maximum velocity that the dark matter particles (they’re called WIMPS — weakly interacting massive particles) can have, and some will be dawdling along more slowly than that.
Baryonic matter forms clumps under the influence of gravity. We call these clumps stars. So why hasn’t the dark matter formed clumps? Any variation in density of a dark matter cloud, no matter how slight, will gradually attract more and more of the slower-moving WIMPS. After a few billion years you won’t have a diffuse cloud anymore; you’ll have clots of the stuff. These clots will be drifting around within our galaxy. They will be invisible, but they will cause gravitational perturbations, because some of them will be rather massive.
No such perturbations are observed.
Not only that, but a massive object like a star will naturally acquire its own halo of dark matter. Slower-moving WIMPS that drift in close to our own sun won’t have enough velocity to escape. And we know for certain that this hasn’t happened. If there was any such halo around the sun, Newton’s law of gravitation would never have been discovered, because the planets in our own solar system would be orbiting more quickly than they are.
Thus the theory requires that dark matter (a) remain in a stable galaxy-sized cloud rather than drifting off into the cosmos but also (b) not form clumps. We haven’t the least idea what the characteristics of WIMPS might be, so we can’t actually rule that out, but it does seem rather implausible, doesn’t it?
When rain falls on a large flat paved area, you’ll soon see shallow pools of water. The pavement is never perfectly flat. I find myself wondering why physicists think the universe itself (spacetime) is perfectly flat except where there’s mass. One way of looking at gravitation (this is Einstein stuff) is that a massive object distorts spacetime. The sun, for instance, creates the three-dimensional equivalent of a large and very deep dimple in the fabric of spacetime. That’s what gravity is.
But why should we assume that mass is the only thing that can warp spacetime in this way? The supposed galactic halo might not be a cloud of massive particles at all; it might simply be a slightly lower place in spacetime, a sort of shallow 3D puddle of slightly enhanced gravity. The cloud of primordial hydrogen would naturally coagulate in such places, and that would create galaxies. It’s known that there are slight anisotropies (uneven places) in the hot, dense plasma that erupted in the Big Bang. Why shouldn’t some gravitational anisotropies still be hanging around?
Of course there’s no physics theory that would explain such a gravitational puddle — but there’s no theory that explains what dark matter is, either. We can be fairly sure it’s not phlogiston, but beyond that, who knows?