By Harry Keller
Editor, Science Education
You may have seen the headlines already. “What is dark matter made of? Galaxy cluster collisions offer clues,” shouts the Christian Science Monitor.1
Here you have a great moment to engage students in something exciting and to use critical thinking. Dark matter is a hypothesis to explain why stars in galaxies circle around their centers faster than the observable matter says that they should, as well as other more sophisticated reasons. It’s called “dark” because it does not emit light and because you cannot see it. You might also have called it “invisible” matter, but much matter seems invisible. Besides, “dark” implies spooky, and this stuff is definitely spooky because ordinary matter moves right through it as though it isn’t there, except for gravitational effects.
The new finding just reported tell us that dark matter behaves just the same way with itself. One patch of dark matter moves right through another as though it’s not there at all (again, except for gravitational effects). This result pushes back against the most popular idea about the identity of dark matter, that it’s WIMPs — weakly interacting massive particles — because particles don’t just pass right through each other.
Two explanations come to mind. Some physicists doing the work still are seeking particles, but these other explanations could help get around that barrier. There are some truly exotic explanations going around as well, such as the existence of a “mirror universe.” It’s best to stick with the simplest ones, though. Occam’s Razor tells us so.
One explanation is that gravity just doesn’t work exactly as we expect it to, especially when dealing with very large masses spanning very large volumes of space. This is not a very popular explanation. The other is that dark matter is really energy. Because of the equivalence of energy and matter demonstrated by Einstein and captured in his famous equation, E=mc2, energy is affected by and causes gravity.
Imagine ripples in a pond. They are energy, and they pass right through one another, coming out the other side unaffected by the interaction. Might dark matter be like those ripples? Could dark matter really be knots in spacetime having energy, and so having mass, but not being particles at all? The Big Bang could have distorted spacetime so much that it left drifting spacetime knots (possibly some sort of standing wave).
This is where the idea becomes really interesting. So far none of the measurements have set a limit on the size of dark matter elements, except that they are likely considerably smaller than galaxies. They do not have to be microscopic. They could be as big as a house or a planet or even a star. If they really are some sort of spacetime knot, then their size could even vary with their effective mass, which is proportional to the amount of energy stored in them.
There’s still much to learn about this hidden part of our universe. We are entering a period in cosmology that reminds me of particle physics in the first half of the twentieth century. It began with three particles: the electron, the neutron, and the proton. Measurements showed that there were other things going on. In particular, loss of energy and other non-conservation effects in certain nuclear reactions suggested a particle that was very light, had no charge, and interacted so weakly with other matter that it could pass right through the Earth readily. This was the neutrino, which was finally discovered in 1956, more than 25 years after its existence had first been postulated.
We seem to be in a similar period now. Although the concept was first proposed in 1932, real evidence was only found about 50 years ago. Finding dark matter has proven more difficult that finding neutrinos. We may be nearing the end of the search. Certainly, this new data will reinvigorate the search for explanations.
1 Calla Cofield, “What is dark matter made of? Galaxy cluster collisions offer clues,” CSM, 26 Mar. 2015.