Strong stuff
WIMP stands for weakly interacting massive particle, but what if the particles of dark matter aren’t so wimpy? It is possible that dark matter particles interact via the strong force. If so, when a gas of these particles is pulled together by gravity, their strong interactions will resist the squeeze – they will bounce off each other and create shock waves of dark matter. Instead of a spike in the density at the centre of galaxies, you would get a smoother bump, which fits the observations.
This strongly interacting dark matter has its drawbacks, however. Those interactions, and the resulting shock waves, tend to make the clumps of dark matter spherical, whereas galaxies are ellipsoids in general – and that goes for the dark matter haloes surrounding them too.
This rules out the simplest kind of interactions within dark matter, and forces theorists to postulate a more complex kind of interaction whose strength depends on how fast the particles approach each other. Another possibility is that dark matter interacts weakly with normal matter, and interacts strongly only with itself. The best test will be to look at dark matter distributions in more detail. If future observations reveal that the dark matter cores of galaxies are ellipsoids, then strongly interacting dark matter will be ruled out, once and for all.
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Warm stuff
When the specialists talk about cold or hot dark matter, what they really mean is matter whose particles are either slow or fast-moving. The WIMPs of cold dark matter are relatively heavy particles, each with a mass many times that of a proton. The trouble with slow-moving dark matter is that it ought to gather together very densely in the centre of galaxies – which doesn’t fit observations. But if the dark particles were hotter and faster, many of them would escape from the galactic core as the clump forms. This would smooth out the cusp in the dark matter density curve, making it too small for astronomers to detect.
Warm dark matter is not to everyone’s taste, however. Dark matter is supposed to help galaxies grow by clumping together soon after the big bang and dragging in gas. But if it is too warm, the clumps would grow too slowly.
Warm dark matter is still a possibility, but its breathing space is getting very narrow as observations improve. If NASA’s James Webb Space Telescope, the successor to the Hubble Space Telescope, reveals clumping in the very early universe, warm dark matter will bite the dust.
Crumbly stuff
Renyue Cen at Princeton University has come up with the idea of decaying dark matter: particles that fall apart of their own accord without hitting anything, like radioactive atoms. If dark matter behaves in this way, then over billions of years it will gradually turn into photons and neutrinos and other particles that will zip straight out of the galaxy, and any central density spike will dissipate.
This is quite easy to test. If dark matter decays, then there would have been a lot more of it in the early universe than there is now. And because dark matter’s gravity creates structure in the universe, there would have been a lot more structure on smaller scales – more miniature galaxies, for example. So astronomers should be able to test the theory if they can look at distant galaxies in great enough detail.
If Cen is right, the future is a strange one: as dark matter disappears, its grip on galaxies will weaken. The Milky Way will gradually spread out and disperse, and eventually every star in it will be alone in the universe. Fortunately, like most astrophysical apocalypses, this one would not happen for several billion years.
Fuzzy stuff
Perhaps the strangest kind of dark matter is that devised by Wayne Hu of the Massachusetts Insitute of Technology. His idea is that dark matter particles are exceedingly light. They would have a mass of 10-22 electronvolts, which would mean that a proton would weigh as much as a thousand billion billion billion of them. Common sense would say that these particles must be very small if they are so light, but they’re not. Their quantum nature defies common sense: Heisenberg’s uncertainty principle dictates that the lighter a particle is, the less well defined is its position. So Hu’s ultra-light dark matter particles would be diffuse clouds of existence, with each vanishingly light particle spread throughout a vast volume several thousand light years across. That rules out any possibility of having a dense concentration of dark matter.
Exploding stuff
What if dark matter particles exploded on contact? That would certainly stop the stuff from getting too dense. If enough of it were to collect in a small area, collisions would happen all the time, and most of the dark matter would be destroyed.
The neat thing about this process is that it only takes place in the centre of galaxies, where the density problem arises. Another plus is that it’s a “self-quenching” process: when the density falls, the particles stop annihilating. That means you get roughly the same final density in every galaxy – a prediction borne out by observations.
It seems like a tidy solution, but its inventor, Manoj Kaplinghat at the University of California, Davis, is quick to point out its shortcomings. While warm or strongly interacting particles require only relatively modest tweaks to the theory of particle physics, it is more difficult to find a theoretical basis for annihilating dark matter. “It has the most unpalatable particle physics of all dark matter models,” Kaplinghat says.