About 85% of the matter in the universe is invisible and labeled Dark Matter. We do not know the particles that make up this substance. We know that dark matter exists based on its gravitational influence on ordinary matter that we detect in the form of stars and gas. This is as reliable as the information reporters get about my family members by speaking only with me.
One of the proposed models for dark matter which became popular recently involves particles with a small electric charge, so-called millicharged dark matter. If the charge to mass ratio is much smaller than its value for electrons, the interaction of dark matter with light would be negligible, explaining its darkness.
When my brilliant collaborator, Misha Medvedev, entered my office a few months ago on the first day of his sabbatical stay at Harvard’s Institute for Theory and Computation (ITC), he suggested that we discuss millicharged dark matter. Within ten minutes we came across unprecedented constraints that were not derived before, which tightly limit the electric charge of dark matter to insignificant levels. The reason we were fast in deriving these limits is because both Misha and I were trained as plasma physicists during our graduate studies.
What is a plasma? It is a mix of charged particles, like the soup of electrons and protons resulting from breaking hydrogen atoms into their building blocks. In the first 400,000 years after the Big Bang, the cosmic gas and radiation were hot enough — above 3,000 degrees Kelvin, for collisions to break the hydrogen atoms. Afterwards, for the next billion years, cosmic hydrogen remained in the form of atoms until the first generation of stars produced enough ultraviolet radiation to break again these atoms into a soup of free electrons and protons. As a result, most of the ordinary matter in the universe is in a plasma state.
Misha and I reasoned that if the dark matter were millicharged, then it would also behave as a plasma.
Electromagnetism is much stronger than gravity. But given its equal number of positive and negative charges, a plasma behaves as a nearly neutral fluid under the influence of gravity. Even if the plasma particles do not collide with each other, tangled magnetic fields would keep charged particles in rotation, confined to within the so-called Larmor distance.
Two decades ago, observers mapped the distribution of matter in a cluster of galaxies, called the Bullet Cluster, composed of two subcomponents of a giant merger at a relative speed of 4,500 kilometers per second, about 1.5% of the speed of light. The hot electron-proton plasma was mapped through its X-ray emission at a temperature of 200 million degrees, a dozen times hotter than the center of the Sun. The dark matter was mapped through its gravitational lensing of the images of background galaxies far behind the cluster.
Amazingly, the dark matter centroid was shifted relative to the collision interface of the electron-proton plasmas, as expected if ordinary matter was stopped at a collision shock front, whereas the dark matter, just like the collision-free stars, passed through the collision interface between the colliding subcomponents. This suggests that the dark matter does not interact with itself or the magnetic fields embedded in the cluster.
Clusters of galaxies show strong magnetic fields with an amplitude of order a millionth of a gauss. Adopting this value, Misha and I used the constraint that the dark matter was not stopped at the collision interface to argue that its Larmor distance was larger than the separation between that interface and the dark matter centroid. This implies a charge to mass ratio for dark matter particles which is a hundred trillion times smaller than the value for protons. Another constraint, associated with the scale of plasma instabilities, suggested a limit that is a hundred times tighter than that.
Based on these considerations, we constrained the dark matter particles to have a completely negligible electric charge, consistent with our inability to detect these particles through their interaction with light.
Although we cannot see dark matter, the hope is that we will detect its signature in another way, either in laboratories through its interaction with ordinary matter or in space through its secondary signatures. So far, physicists have invested decades of research and billions of research funds in searching for dark matter without success. When I asked one of the experimentalists the question: “how long will you continue to search?”, he answered: “as long as the National Science Foundation awards me grants to do it.” As experimentalists continue to search for dark matter, the limits on existing models will improve and so will the need for new ideas to explain it.
Since dark matter has little influence on our daily life, my advice to federal funding agencies is to allocate similar funding to the search for extraterrestrial civilizations. If we find them, not only will our perspective on our cosmic existence change, but we might also be able to ask them whether they know the nature of dark matter. This could save us time and money. If they know how to engineer dark matter, their most advanced stealth spacecraft might be invisible to us, giving a new answer to Fermi’s paradox: “Where is everybody?”
ABOUT THE AUTHOR
Avi Loeb is the head of the Galileo Project, founding director of Harvard University’s — Black Hole Initiative, director of the Institute for Theory and Computation at the Harvard-Smithsonian Center for Astrophysics, and the former chair of the astronomy department at Harvard University (2011–2020). He is a former member of the President’s Council of Advisors on Science and Technology and a former chair of the Board on Physics and Astronomy of the National Academies. He is the bestselling author of “Extraterrestrial: The First Sign of Intelligent Life Beyond Earth” and a co-author of the textbook “Life in the Cosmos”, both published in 2021. His new book, titled “Interstellar”, was published in August 2023.
