Researchers at Brown University in Providence, Rhode Island have narrowed down the size requirements for dark matter — the elusive stuff that pervade the universe, lending mass but emitting no detectable radiation.
Dark matter has so far “existed” only in the calculations of physicists, although observations of actual stars’ and galaxies’ motions do indeed point to some unseen source of mass. In many cases the invisible mass outweighs the visible matter… in fact, it’s believed that visible matter makes up only 4 percent of the universe, while 23 percent is dark matter and the rest is dark energy.
Dark matter is the term for matter that emits no electromagnetic radiation (x-rays, gamma rays, radio waves, visible light, etc.) and dark energy is the force that theoretically is responsible for the ongoing and accelerating expansion of the universe.
Dark matter particles don’t interact with “regular” matter much, which has led physicists to believe they may belong to a class of particles dubiously named “weakly interacting massive particles”… or WIMPs, for short.
Experiments at underground (literally under-the-ground!) particle research facilities in both the U.S. and Europe have been searching for actual physical evidence of dark matter particles. Because they don’t have a strong individual interaction with regular matter particles this has proven to be challenging, to say the least.
In order to detect the particles at all their existence must be inferred through the results of multi-level collision chains between them and their alter-ego antiparticles. (Again, challenging!)
Brown University assistant professor Savvas Koushiappas and graduate student Alex Geringer-Sameth studied seven dwarf galaxies (seen above, circled) which observations have shown must be full of dark matter, since their individual stars’ motions cannot be fully explained by their masses alone. These dwarf galaxies don’t have much hydrogen gas and other common matter, providing the team with a “blank canvas” to better observe dark matter and its effects.
“They’re clean systems,” Koushiappas said.
The Brown University researchers analyzed data collected over three years by NASA’s Fermi gamma-ray space telescope and measured the number of photons emitted from the dwarf galaxies. From the number of photons, they were able to determine the rate of quark production… which in turn allowed them to establish constraints on the mass of dark matter particles.
Based on the known rate of the expansion of the universe and the rate at which particles in the dwarf galaxies canceled each other out, Koushiappas and Geringer-Sameth reported that dark matter must have a mass greater than 40 giga-electron volts (GeV).
“What we find is if a particle’s mass is less than 40 GeV, then it cannot be the dark matter particle,” said Koushiappas.
Previous reports of dark matter WIMPs by researchers at the Gran Sasso facility in Italy showed particles in the 7 to 12 GeV range, much lower than the Brown researchers’ limit. This will help narrow down the range of what defines dark matter particles.
“This is the first time that we can exclude generic WIMP particles that could account for the abundance of dark matter in the universe,” Koushiappas said.
“This is a very exciting time in the dark matter search, because many experimental tools are finally catching up to long-standing theories about what dark matter actually is,” added Geringer-Sameth.
Top image: View of the universe from NASA’s Fermi Gamma-ray Space Telescope. Credit: Alex Geringer-Sameth and Savvas Koushiappas, Brown University