There aren’t many mysteries in the entire universe that loom as huge as dark matter. We know that the regular matter in our Universe and the known laws of gravity cannot explain what exists based on the gravitational effects we experience, which occur constantly and on scales ranging from the individual galaxy and above. However, all of the indirect evidence for dark matter derives from astrophysical measurements that are insufficient without that one essential component. Despite the fact that that single addition of dark matter resolves a vast range of issues and conundrums, none of our direct detection efforts have been successful.
There’s a good reason for this: each direct detection technique we’ve explored is predicated on the idea that dark matter particles couple to and interact with some sort of ordinary matter. This isn’t a poor assumption because it’s the kind of interaction we can now constrain and evaluate. However, there are many physical conditions that arise in the Universe that we are simply unable to replicate in the lab at this time. If dark matter interacts with normal matter under those circumstances, it will be the Universe’s laboratory rather than an experiment on Earth that reveals the particle nature of dark matter. Here are some reasons the next supernova in the Milky Way may be the ideal candidate for this.
Although there are many different kinds of supernovae that can happen in the universe, the vast majority of those that we observe are core-collapse (or type II) supernovae. Every time stars are created in huge quantities, they have a particular mass distribution, with less massive stars forming frequently while more massive stars, albeit small in number, account for a sizable fraction of the overall mass of the young stars. In a matter of a few million years, the most massive stars ever to form—those with masses greater than 8–10 times that of the Sun—will explode in a core-collapse supernova.
The vast majority of a core-collapse supernova’s energy gets carried away in the form of neutrinos, a class of particles that only interacts very weakly with all other forms of matter but which plays a significant role in nuclear processes. Although the supernova signals we’re used to seeing occur across the electromagnetic spectrum — in various wavelengths of light — neutrinos are a class of particles that play a major role in nuclear processes. 99% of the energy produced in a core-collapse supernova is released as neutrinos, which quickly leave the star’s interior and transport energy very effectively. In most core-collapse supernovae, this process results in the core’s implosion and the creation of either a neutron star or a black hole.
Neutrinos are very, very seldom found in the particle physics experiments we run in the lab. This is due to three characteristics of neutrinos.
- Only the weak nuclear force, which is substantially suppressed in comparison to either the strong nuclear force, which holds atomic nuclei together, or the forces that control charged particles, electric currents, and light, interacts with neutrinos (the electromagnetic force).
- The cross-section of neutrinos with ordinary stuff, such as atoms, protons, etc., is incredibly small. It would require roughly a light-worth year’s of lead as a detector to have a 50/50 chance of having your neutrino interact with them for a typical neutrino created in a Sun-like star, for instance.
- Furthermore, the neutrino cross-section increases with neutrino energy; the greater your neutrino’s energy, the more probable it is to interact with matter. The likelihood of a neutrino produced by a supernova, a solar neutrino, or (most challenging of all) a neutrino left over from the Big Bang interacting with matter is significantly higher than that of a neutrino produced by ultra-high energy cosmic rays.
You have to get extremely close and wait a long time before you can be certain you have firmly detected the neutrino signal you’re looking for if something only produces a modest amount of neutrinos. However, the detectors that are active all over the world will not be able to avoid the neutrino signal that is permeating the entire planet if something produces a huge quantity of high-energy neutrinos, either all at once or over a very short period of time. Supernovae are known to be produced by galaxies like the Milky Way about once every century, with certain actively star-forming galaxies creating more than one each decade and other, less active galaxies making them only a few times per millennium. We are not the fastest, but we are on the slower side as a big, peaceful galaxy.