Pedagoguery

We know that there is a lot more to the universe than we observe directly. A big portion of that is dark matter, which is six times more abundant than normal baryonic matter. The conventional view of dark matter is that it is boring. The most commonly conceived type of dark matter are called WIMPs, – Weakly Interacting Massive Particles. They are believed to act only through the two weakest forces: the weak nuclear force, which is both very weak and very short-range, and gravity. But what if this view is not correct. Scientists are even now investigating alternatives.

In the conventional view of the early universe, WIMPs were created in the first 10 nanoseconds of the universe. Since they are very massive, they require a lot of energy to create, thus there must be sufficient density and energy of particles to create them. However, since WIMPs are their own antiparticles, too high a density of them would result in too many WIMP collisions, thus lowering the density. As the universe expanded, both sorts of collisions would become less frequent, until at the 10 nanosecond mark, there was no longer enough energy in the particles for the creation of new WIMPs, and the overall density of WIMPs was low enough that their collisions with each other were rare enough to ignore. At that point, the total number of WIMPs in the universe became fixed. Observations of the total amount of dark matter in the universe today, as well as models of the production of WIMPs in the early universe are in remarkable agreement – they both come up with about the same number.

However, there are other possibilities. One possibility is that WIMPs are unstable. If that were the case, then, why is there dark matter in today's universe? The answer is that WIMPs would decay into normal matter as well as something else that interacts only through gravity. This particle has been amusingly termed the Super-WIMP. How could we tell if dark matter is composed of WIMPs or Super-WIMPs? According to theory, when a WIMP decays into a Super-WIMP, the resulting Super-WIMP would be traveling at nearly the speed of light. It would take time for them to slow down, and this would have an affect on galaxy formation, since it would take longer for the dark matter halos that seed the formation of galaxies to emerge. Astronomers are now checking the density of galactic halos to determine which scenario is more likely. Super-WIMPs also mean that WIMPs may be more interactive than we at first supposed. If there are none around in today's universe, it is possible that they could have had electric charge, for instance.

Another possibility is that dark matter interacts with other dark matter through new, unrecognized forces. While that would not mean that there are parallel worlds out there with dark matter people on dark matter worlds, it could mean that dark matter is far more active than it has been given credit for. Suppose, for instance, that there is some kind of dark matter electromagnetism. It would have to be weak, or else it would have affected the evolution of the distribution of dark matter in ways we could detect, but it would mean that there is a whole other world out there invisible to our eyes. One way to determine whether something like that exists is to look at places in the universe where galaxy clusters pass through each other. One such cluster is called the Bullet Cluster. Using gravitational lensing, it is possible to determine the distribution of dark matter in the two clusters. To a large degree, it remains aligned with the galaxies in each cluster, which means that it not react strongly to the dark matter in the other cluster.

While most current evidence supports a more traditional view of dark matter, that does not preclude the fact that it might lead a hidden life that we have yet to detect.

Next time, where did Earth's water come from?