Three New Experiments May Discover the Nature of Dark Matter
Dark matter is theorized to comprise 85% of the total matter in the universe, but we have no idea what it is. It is a hypothetical type of matter, predicted from its observed effects, that does not emit or absorb any kind of electromagnetic radiation, including light. Scientists have been attempting to discover the nature of this invisible matter for decades, and now three new approved experiments might yield an answer.
Historically, the most popular candidate for a dark matter particle is WIMPs, or weakly interacting massive particles. WIMPs are theoretical particles that do not interact through electromagnetism, and in fact do not interact with any particles with the exception of other WIMPs. Two of the experiments will explore the WIMP theory of dark matter, which remains the prevailing theory, while ADMX-Gen2 will explore the alternative theory that dark matter is composed of axions. Axions are, hypothetically, extremely small particles with no electric charge that interact only minimally with ordinary matter. Although both WIMPs and axions account for observed effects in the universe, no definitive evidence for their existence has been found.
That will almost certainly change, as the experiment to discover axions is expected to provide some kind of definitive outcome, whether it's the discovery of axions or reasonable proof that axions do not exist. "At very high confidence, this generation-2 experiment can either detect the axion or reject the hypothesis," said spokesperson for the experiment Leslie Rosenberg of the University of Washington. The experiment will use a giant magnet in order to detect the small amount of electromagnetic radiation that is released when axions (if they exist) convert into microwave photons.
Advocates for the WIMP theory of dark matter were concerned that budgetary problems would lead to only one WIMP experiment advancing to the next round of dark matter experiments. They were relieved when two were chosen for the next generation, since according to Stanford University physicist Blas Cabrera, continuing only one WIMP experiment "doesn't make a lot of sense." He went on to explain that the two experiments are attempting to interact dark matter with different types of materials, and many theories of WIMP dark matter predict that it will interact differently with different types of particles. He also asserted that the WIMP and axion theories of dark matter are not mutually exclusive, as a combination or an interaction between the two particles could be the source of dark matter. "Nature may be more complicated than we initially thought, and we should be thinking more broadly," Cabrera said.
If the nature of dark matter is revealed through these experiments, the discovery would have a profound impact on cosmology. Dark matter explains many astrophysical mysteries, from certain properties of galaxy formation to the large scale structure of our entire universe. The universe seems to be hierarchically organized, with stars organized into galaxies, galaxies organized into galaxy groups, which are organized into galaxy clusters and so on and so forth. The largest structures seem to be superclusters and filaments, at which point there is no more organization. They are randomly distributed, making the universe seem smoothly distributed on the large scale, a phenomenon that is poetically called the End of Greatness. A significant amount of dark matter is necessary to explain this type of organization of the universe.
Although we're all hoping that these experiments yield a long-awaited answer to the mystery of dark matter, in the worst case scenario several predominant theories would be eliminated, which is nearly as groundbreaking. Cabrera said, "In science the ruling out is as important as discovery, but of course discovery is always more fun."