Friday, June 8, 2018

The Dark Side

In the 1920's a number of experiments observed that certain nuclei would decay by emitting a "beta" particle in a process called beta decay. Although the beta particle itself was shown to simply be an electron, the process of beta decay exhibited some strange and unexplained properties. In particular, the decay products did not conserve energy, momentum, or angular momentum. Up until that time, all known physical processes had conserved these three quantities. That is, the amount of energy, momentum, and angular momentum at the end of any process was exactly the same as the amount at the beginning of the process. However, beta decay seemed to violate these well known conservation laws. A number of explanations were proposed including the possibility that these conservation laws were not absolute. However, one explanation seemed much more simple and elegant. The physicist Wolfgang Pauli proposed that another particle was also emitted during beta decay along with the electron, but this other particle was nearly impossible to detect. For all practical purposes it was invisible to any experimental detectors of the time. Now it may seem crazy to postulate an unknown, nearly undetectable particle simply to preserve well-established laws of physics, but consider the genius of this idea. By simply proposing the addition of one unknown entity all of the problems with beta decay could be solved. Enrico Fermi named the unknown particle a "neutrino," or "little neutron" in Italian. It took about 25 years to experimentally confirm the existence of this particle, but eventually the neutrino was discovered in 1956. The simple but elegant introduction of a nearly undetectable neutral matter particle was the solution to multiple problems.

Today we have a similar situation to that of the 1920's. When we observe the cosmos we find that there are a number of problems that seem to violate well established laws of physics. Einstein's theory of general relativity, which describes how gravity works, is a remarkably successful theory with tremendous predictive power, but when we try to use that theory to explain certain observed effects, the theory doesn't quite work. For instance, when we watch how fast the stars in galaxies rotate about the galactic center we find that the outer part of the galaxy doesn't obey Einstein's theory if we assume that we can actually see all of the matter in the galaxy. (See the graph at the end of this blog post.) We also find that the large scale distribution of galaxies throughout the universe has some problems if we assume that we can observe all of the matter that is there. (See the opening figure of this blog post.) In addition, we know that gravity can actually bend the path of light, but the amount of bending we observe is much greater than what we would expect using the theory of general relativity and the amount of visible matter.

So how should we solve these problems (and a half dozen more that I haven't mentioned)? One possibility would be to discard Einstein's theory of general relativity and assume it doesn't work in the regions where our observations seem to contradict it, even though it works so perfectly everywhere else. That is the solution favored by some scientists. However, most scientist propose a different solution. If we postulate the existence of some unknown kind of matter that is nearly impossible to detect but can interact with other matter by the force of gravity, then amazingly we can solve all of the above problems simultaneously (and the half dozen other problems not mentioned). This solution sounds quite similar to the solution proposed for the beta decay problem. By simply proposing the existence of some kind of matter that has not yet been discovered and is nearly invisible to our current particle detectors we can retain the known physical laws that seem to work so well and we solve a whole bunch of seemingly unrelated problems with one simple yet elegant solution. We call this proposed new type of matter "dark matter."

Quite remarkably, all of these unrelated problems can basically be solved by introducing the same amount of dark matter into our equations that describe different aspects of the universe. That is, we don't just postulate dark matter to solve all of these various problems, but we postulate the same amount of dark matter to solve these various unrelated problems. That makes the proposed solution even more simple and elegant. It turns out the amount of dark matter required to solve these problems would account for about 80% of all the matter in the universe. In other words, all of the elements of the periodic table, all of the known particles like quarks and leptons in the universe, only make up about 20% of the matter in the universe, and the rest is this mysterious dark matter.

There are many current experimental searches for dark matter conducted in outer space, in particle colliders and in other earth-based experiments. So far all of these searches have not yet directly discovered any dark matter. We certainly know that there are effects we clearly see in nature that are not explained by known physics. Either we will eventually discover new physics or we will eventually discover this new kind of matter.

My Christian friends and colleagues who believe the universe is young, only 6,000 years old or so, point to dark matter as an indication that scientists don't really understand the origin of the universe. They claim that scientists must invent this strange kind of matter to try to salvage the Big Bang as a reasonable explanation for the origin of the universe and that such a desperate move shows how flimsy the current scientific explanations are. Most people I read with this viewpoint then point to some other unknown laws of physics as the explanation for phenomena like galaxy rotation and gravitational lensing that don't really have anything to do with the origin of the universe. As I read these criticisms of dark matter they seem very arbitrary. The general strategy is to simply cast doubt about anything that scientists have discovered about the origin of the universe, but to do so, the critics must then invoke other varying explanations for lots of different phenomena. Most of these other explanations are actually possible explanations held by other scientists (like the scientists that dismissed conservation of energy and momentum to solve the beta decay problem). But in reality, although the existence of dark matter is the simplest and most elegant solution for the problems mentioned above, almost all scientist would agree that there are other less probable solutions.

The proposal that dark matter exists is not an indication that scientists must invoke strange new ideas to salvage Big Bang cosmology, nor is it the only possibility for explaining what we observe. It is simply the most simple and elegant because with one idea many unrelated problems can be solved. This is how science is done. Over and over again, new ideas are suggested to solve problems and to explain phenomena we don't currently understand. As we have seen with the case of beta decay, those solutions may involve strange undetectable particles that take decades to eventually discover. These ideas are not invented as a frantic strategy to save some anti-biblical idea about the origin of the universe, but as a natural consequence of our quest to understand a vast and amazing creation. If we take beta decay as a test case, then we will eventually discover the dark matter. If that happens, what will be the next momentarily unsolved problem in physics that my young earth creationists friends will point to as evidence that us scientists have it all wrong? To me, the unsolved problems are simply evidence that the creator of the universe is far more clever, powerful, and inventive than I can imagine. Just when we discover one amazing thing about his creation, there is another unsolved problem waiting on the horizon. That is what makes physics fun!

Of course there is another known mystery of the universe that is even bigger than dark matter. It is what we call "dark energy" and we know less about it than we know about dark matter. We'll explore that subject in the next blog post.

This is a plot of the rotational velocity (v) of the stars in a galaxy designated M33 as a function of their distance (R) from the center of the galaxy. The dotted line is what is expected from our theory of gravity with no dark matter. The data points are what we actually see and the solid line is what would be expected if dark matter exists in the galaxy.

The opening figure shows a computer simulation of the large scale structure of the universe. Each little dot is a galaxy. From observations, we know that the galaxies in our universe form this filament-like structure. Our computer simulations must include the existence of dark matter in order to reproduce the structure that we actually observe.


  1. Thank you Dr. Strauss. Your explanation is simple and elegant as well. Looking forward to the next post!

  2. Michael Does the dark matter and dark energy interchange according to Einsteins equation and if so what is the driving force. I would suppose in decay its related to instability to a simpler in some sense state. Could there be a cosmological reason for such dark interchange that varies the ratio of these dark entities over eons of time in accord with the big bang theory, expansion..etc. Where does the second law and entropy enter into the issue...if at all.

    1. I'm not totally sure of the answer to either of your questions. However, I don't think there is any mechanism for exchange of one to the other. That is done in the normal matter sector through matter and antimatter annihilation, so there would have to be an abundance of "anti-dark matter" and that would have annihilated early in the universe. So I think the relative amounts are pretty much invariant after the very early universe.