“You’ve heard of electric cars and e-books, but now we are talking about electric dark matter,” said Julian Munoz of Harvard University. “However, this electric charge is on the very smallest of scales.”
“We are constraining the possibility that dark matter particles carry a tiny electrical charge – equal to one millionth that of an electron – through measurable signals from the cosmic dawn,” says Avi Loeb of the Harvard-Smithsonian Center for Astrophysics (CfA) about the nature of ne of the enduring mysteries of the Universe. “Such tiny charges are impossible to observe even with the largest particle accelerators.”
Astronomers have proposed a new model for the invisible material that makes up most of the matter in the Universe. They have studied whether a fraction of dark matter particles may have a tiny electrical charge.
Dark Matter Interacts with Normal Matter by the Electromagnetic Force
Julian Munoz, who led the study, and his collaborator, Avi Loeb of the Harvard-Smithsonian Center for Astrophysics (CfA), explored the possibility that these charged dark matter particles interact with normal matter by the electromagnetic force.
Their new work dovetails with a result from the Experiment to Detect the Global EoR (Epoch of Reionization) Signature (EDGES) collaboration. In February, 2018 scientists from this project said they had detected the radio signature from the first generation of stars, and possible evidence for interaction between dark matter and normal matter. Some astronomers quickly challenged the EDGES claim. Meanwhile, Munoz and Loeb were already looking at the theoretical basis underlying it.
“Nature of Dark Matter is One of the Biggest Mysteries”
“We’re able to tell a fundamental physics story with our research no matter how you interpret the EDGES result,” said Loeb, who was the chair of the Harvard astronomy department. “The nature of dark matter is one of the biggest mysteries in science and we need to use any related new data to tackle it.”
The story begins with the first stars, which emitted ultraviolet (UV) light. According to the commonly accepted scenario, this UV light interacted with cold hydrogen atoms in gas lying between the stars and enabled them to absorb the cosmic microwave background (CMB) radiation, the leftover radiation from the Big Bang.
This absorption should have led to a drop in intensity of the CMB during this period, which occurs less than 200 million years after the Big Bang. The EDGES team claimed to detect evidence for this absorption of CMB light, though this has yet to be independently verified by other scientists. However, the temperature of the hydrogen gas in the EDGES data is about half of the expected value.
Was Hydrogen Cooled By Cold Dark Matter?
“If EDGES has detected cooler than expected hydrogen gas during this period, what could explain it?” said Munoz. “One possibility is that hydrogen was cooled by the dark matter.”
At the time when CMB radiation is being absorbed, any free electrons or protons associated with ordinary matter would have been moving at their slowest possible speeds (since later on they were heated by X-rays from the first black holes). Scattering of charged particles is most effective at low speeds. Therefore, any interactions between normal matter and dark matter during this time would have been the strongest if some of the dark matter particles are charged. This interaction would cause the hydrogen gas to cool because the dark matter is cold, potentially leaving an observational signature like that claimed by the EDGES project.
Only small amounts of dark matter with weak electrical charge can both explain the EDGES data and avoid disagreement with other observations. If most of the dark matter is charged, then these particles would have been deflected away from regions close to the disk of our own Galaxy, and prevented from reentering. This conflicts with observations showing that large amounts of dark matter are located close to the disk of the Milky Way.
Dark Matter Lives in Our Solar Vicinity
“We know that there ought to be some dark matter in our vicinity,” wrote Julian Munoz in reply to an email from The Daily Galaxy. “Astronomical measurements of its density tell us that there are roughly 0.3 protons per centimeter cube worth of DM near the Sun. If all DM was charged, it may be ejected from the Milky Way disk, which does not agree with data! In our paper we suggested that only a fraction of DM is electrically charged, while the rest is neutral (for example forming dark atoms). Thi charged fraction can be roughly 0.1% to 1%, so most (>99%) of the DM would not get ejected. These small fractions actually mirror what happens in the visible sector, where after the CMB is emitted only a fraction of ~0.05% of electrons are free charges, while the rest form neutral hydrogen atoms.
“So in summary,” wrote Munoz, “ we proposed that only a tiny fraction of the dark matter is electrically charged today, so the rest can behave as cold and neutral dark matter, and thus it lives in the Solar vicinity.
“In terms of further developments, so far this model is alive and well!’, Munoz writes. “Other researchers have proposed extensions to this model with further interactions that open even more parameter space to explain EDGES (for example allowing heavier DM particles). Upcoming particle-physics experiments like LDMX will be able to test a lot of the possible DM models that can generate the EDGES signal, which is exciting. It is not every day that particle-physics and astronomical observations can complement each other this way. Finally, data from 21-cm interferometers like HERA, MWA, LOFAR, and the SKA will be able to provide a more robust measurement of hydrogen during cosmic dawn, independent from EDGES, which would really shed light onto this important issue.”
Lincoln Greenhill also from the CfA is testing the observational claim by the EDGES team. He leads the Large Aperture Experiment to Detect the Dark Ages (LEDA) project, which uses the Long Wavelength Array in Owen’s Valley California and Socorro, New Mexico.
Image credit: at top of page was produced by a simulation showing the evolution of dark matter in the universe. (Milennium-ditII Simulation)