Dark Matter, Part II

Between an incessant news cycle and uninformed journalism, reports of scientific discovery have become numbingly commonplace. The effects of misreported discoveries do not appear transformative, if only because the world prior to discovery was never fully characterized.

Previously, I wrote on the history of the discovery dark matter to clarify that dark matter had been discovered many decades ago, but remains unidentified. To truly appreciate the impact of what identifying dark matter would mean, we must dispense with the paraphrase and summary science journalism relies on, and instead look at the essence of the theories.

So what is dark matter? Why does it matter?

As mentioned in my initial post, “dark matter” is a term for something that has been identified in its interactions with other matter, but not identified as to its true nature. Its existence was identified by the rotation curves of galaxies; wherein stars far away from the center of spiral galaxies moved faster than one would estimate based on the gravitation attractive of all the stellar matter in a galaxy. This speedy rotation implied the existence of a halo of non-luminous particles surrounding a galaxy which itself was heavier than its host galaxy. The fact that it does not appear to be luminous, or interact with other matter or decay producing light, be it radio waves, visible light, x-rays, or gamma rays, resulted in its rather unimaginative euphemism: “dark matter.” (n.b. black holes are not the same colloquial dark matter in that they are regions of space-time which are sufficiently warped by a sufficiently dense mass that light does not escape.) Of course, euphemisms aren’t formal names but just colloquial obfuscations of sinister truths we care not to acknowledge. The sinister truth is that there are several theories for what dark matter really is, of which two have been the focus of attention in recent research. These are the more imaginatively named WIMPs, which itself is a parent class of theories, and Axions, itself a specific theory.

Be they WIMPs or Axions, these theories of dark matter come from a fundamental problem of physics that dates to the first few moments of universe. Just after the Big Bang, about ~10-32 seconds after “second” had any meaning, the universe was very energetic and very hot. This ratio of heat to volume, or energy density, allowed for a class of very fundamental, tremendously heavy particles to exist. As the universe rapidly expanded, the energy density dropped and these particles decayed into the particles we know and love in our every day lives today, such as the electron and photon. Taking the electron and photon as an example, we can observe a few fundamental distinctions. The electron has electrical charge; the photon does not. The electron has mass; the photon does not. But most relevantly, the electron has spin, an intrinsic angular momentum like a spinning top, but the photon does not. In fact one of the most fundamental distinctions of particles is not electrical charge, but that of spin.

One theory to try to explain this distinction of particles with and without spin is the theory of Supersymmetry—where each particle with spin has a twin particle without spin. They are twins because they were born from that same decaying parent particle, too heavy to be sustained as the energy density of the universe rapidly decreased. When the decay occurs, supersymmetry is broken and the child particles possess different spin. An extension of the theory of supersymmetry posits that there must be a Lighest Supersymmetric Particle (LSP) that will not decay.

(Physicists like to think in terms of what is and is not conserved so they can track what happens during a specific interaction. Baryon and lepton number, effectively quark-based and non-quark based matter with non-integer spin, is not conserved under all interactions of supersymmetry. A theory posits there is some relationship between number of leptons, baryons, and spin that is conserved. This conserved quantity, or symmetry before and after an interaction is R-Parity. If R-Parity is conserved, we would predict the LSP; otherwise all supersymmetrical particles would decay without violating important symmetries and LSP, as a WIMP, would not be an accurate theory. You can read more about R-Parity and supersymmetry here.)

Such a parent particle would not have charge, be it electric or color (color being yet another imaginative term to describe an additional type of charge possessed by quarks beyond the well-known electric charge that is conserved by the strong force, one of the four fundamental forces that keeps the otherwise repulsive nuclei of atoms together. Having neither of these charges, LSP would only interact by the nuclear weak force and gravity. There are a few types of LSP, based on the classes of child particles.

Well we do know is that this elusive dark matter interacts with gravity, but not electromagnetism. So if you believe this LSP would be a good candidate for dark matter you’d share the company of many in the physics community. These “Light” supersymmetric particles are still quite heavy, as they would still be the parent particles of particles we know today, but not interactive. Thus, Weakly Interacting Massive Particles. LSP has been the dominant WIMP theory over the past several decades (n.b. there are several WIMP theories that do not rely on supersymmetry! For instance, one general theory is the Majorana fermion, a fundamental particle with spin that is its own antiparticle. This concept may buttress non-supersymmetric theories, but some WIMP theories do depend on it.) Supersymmetry has a problem though—we have been able to establish energy densities mirroring that of the very early universe in particle colliders—but have not found any of these parent, supersymmetric particles.

Axions, like the Majorana fermion, do not rely on supersymmetry. After the success of Quantum Electrodynamics (QED) in describing how light and matter interact as a quantum relativistic field theory, the same field theory approach was then applied to the strong force. After many decades, a theory of Quantum Chromodynamics (QCD) emerged. (The “Chromo-“ prefix is due to the color charge mentioned earlier.) An issue with QCD was that, in the theory, there was no prediction that the symmetries of charge and parity (a fundamental “even” and “oddness” in nature) had to be conserved. Oddly though, within experimental error charge and parity have always been preserved. Whether in accord, or in spite of, Occam’s razor, a new theory for a new field was developed within the confines of QCD to unravel this paradox. The Peccei–Quinn theory developed by Roberto Peccei and Helen Quinn changed one of the terms in the equation that defines the strong force to add a factor defining a new field for a decaying, no-spin axion particle. This updated factor subtracts out the symmetry, violating terms of the equation, and producing a theory that corresponds to observed results.

The funny thing about the axion is that it shouldn’t have a mass like many other no-spin particles. They are pseudoparticles, or ephemeral particles that exist only by virtue of fluctuations of the quantum vacuum. It is this fluctuation that imparts a very tiny, variable mass. So what we see cosmically as dark matter may be the exchange of many, many short-lived axions that help to stabilize the strong force to keep it from violating fundamental symmetries.

So LSP, representing WIMPs, and axioms are two promising candidates for dark matter. They are not the only candidates. Nor are they inherently mutually exclusive. These and the other theories for dark matter are contentious and have not been validated in a meaningful way, despite plentiful new reports. If any of these theories are validated by clear experimental evidence it would have at least some impact on the big questions of physics, but maybe remarkably to outsiders, the work of theory has continued without validation and even validation may not lead to a dramatically different worldview but a shift within the community to continue on a single track. For now, these theories provide educated guesses consistent with the framework of the proven physics before them about what underlies our reality. Either way, the reality of dark matter isn’t friendly to discovery. It may just want to hide in the dark.

Next post, I’ll discuss some notable experiments that have tried to determine dark matter’s true identity.

Note: The author would like to acknowledge the contributions of Dr. Michael C. Ogilvie, in the Department of Physics in Arts & Sciences at Washington University in St. Louis, to this blog entry.