Correction appended.
Sometimes even theoretical physicists — scientists pushing the boundaries of modern science to new heights — make the fatal mistake of using the wet erase marker on the dry erase board. Though Assistant Professor of Physics Savvas Koushiappas was using the wrong marker, he was able to make a discovery that may change science completely.
Koushiappas, along with Alex Geringer-Sameth GS, published a paper in Physical Review Letters yesterday that offers a new lower limit for the mass of dark matter particles. Their conclusion that a dark matter particle must have a mass of at least 40 giga-electron volts rules out other existing theories that predict a mass less than that number or are based on the assumption of a lower mass.
Dark matter theories have existed now for almost 30 years, Geringer-Sameth said. These findings offer strong new evidence in support of some of these theories, while challenging others.
But what is dark matter, anyway? It seems a concept reserved for abstract physicists, something the average person cannot understand. But according to Geringer-Sameth, it is much more entwined in our lives than people think.
"If you open your hand, you've got about one dark matter particle in it at any given time — that's how much dark matter there is around us here on earth," he said. Dark matter is matter that we cannot see or feel but that accounts for about 23 percent of the universe. Matter we can see only accounts for about 4 percent.
"We believe that we are living in a sea of dark matter particles, which passes right through us and through the earth all the time," Geringer-Sameth said.
But how do we know dark matter exists if we cannot see or feel it? According to Geringer-Sameth, astronomers seeking to measure the amount of matter in the universe tried two methods of doing so — summing the masses of all visible matter like stars and gas and using gravitational equations to estimate the amount of mass in the universe, based on the movements of universal bodies. But when scientists discovered there was a large discrepancy between the two calculations, some theorized that there was other matter exerting gravitational force that accounts for the difference.
Koushiappas and Geringer-Sameth used a novel approach in tackling the question of the mass of dark matter. "Basically, we worked backward," Geringer-Sameth said. They looked to seven dwarf galaxies — which are known to contain large amounts of dark matter because of the unusual motion of their stars — and measured gamma-ray emissions resulting from the collision of dark matter particles and their anti-particles, a process known as annihilation.
Using this gamma-ray data, the duo was able to determine the approximate rate of annihilation occurring in the dwarf galaxy and therefore determine bounds for the mass of the dark matter particles involved in these collisions.
A previous version of this article's headline incorrectly stated that dark matter may be lighter than previously thought. In fact, the results of the study suggest it may be heavier. The article also misspelled Alex Geringer-Sameth's GS name. The Herald regrets the errors.
