“Right now, as we speak, there are 10,000 dark matter particles going through your body per second,” said Savvas Koushiappas, assistant professor of physics, referring to the substance that comprises around 80 percent of our universe, yet one we know very little about.
Koushiappas, in collaboration with a team of physicists, recently published findings that are poised to open the doorway to a greater understanding of this mysterious yet ubiquitous substance. The team was led by Koushiappas’ former student, Alex Geringer-Sameth PhD ’13, now a postdoctoral research associate at Carnegie Mellon University, and their associate Matthew Walker, assistant professor of physics at Carnegie Mellon.
The team found evidence of gamma radiation emanating from the nearby dwarf galaxy Reticulum II. A dwarf galaxy is a small collection of stars that orbits a larger galaxy, according to a press release from Fermi National Accelerator Laboratory, the research institution that collected the telescopic data. Dwarf galaxies may comprise fewer than 100 billion stars but are typically composed of a few billion stars. In comparison, average-sized galaxies such as the Milky Way are made up of hundreds of billions of stars. In searching for evidence of dark matter, dwarf galaxies are an ideal target because they consist primarily of dark matter.
The structure and properties of dark matter cannot be explained by the Standard Model of particle physics. Dark matter cannot be observed with standard tools such as a telescope. A summary of Walker’s research explains that the existence of dark matter typically has been measured by the discrepancy between two standard indexes of mass. The first method involves multiplying the mass of a typical star by the number of stars in the system. In the second method, one must measure “the orbital positions and speeds of those same stars, plug the numbers into the law of gravity (which relates mass to position and speed) and solve for mass,” the report states. The trouble arises because these two calculations yield vastly different results.
“For nearly every galaxy that has been observed, the ‘luminous mass’ obtained by counting visible objects is smaller than the ‘dynamical mass’ inferred by applying (Newtonian) gravity to the motions of visible objects,” according to Walker’s website. This discrepancy in mass is what is understood to be dark matter, and the problem that results has been around for more than 80 years. Today, there are many other different experimental observations that all point to the presence of dark matter.
The mystery of dark matter is analogous to the previous mystery of atomic particles, said John Beacom, professor of physics at Ohio State University, who was not involved in the study. “We are familiar with water as a substance, but it’s really composed of microscopic particles. And people have only known that since I think the 19th century.”
Similarly, the current understanding of dark matter is in the same position as matter made up of atomic particles — scientists have observed its existence, but “right now it’s just a thing, like water used to be just a thing,” Beacom added.
Until recently, only around 25 dwarf galaxies had been identified. About a month ago, nine of these new systems were discovered, Geringer-Sameth said.
“A lot of them have been found in the Northern sky. A new telescope has found a whole bunch more in the Southern sky, so the whole story of Snow White just got a lot longer and the names of the Dwarves got harder to pronounce,” Beacom said.
These newly discovered galaxies provided the Brown-Carnegie Mellon team with the opportunity to run a new model — one constructed by Geringer-Sameth while he was completing his PhD work with Koushiappas. Geringer-Sameth developed a novel statistical way to combine large data sets from many different sources, allowing for more sensitive detection of dark matter signals.
The difficulty we have in observing dwarf galaxies stems from their size and distance. “Finding these dwarf galaxies is really hard. It’s sort of like, imagine you’re in a lamp store at night, and you are looking out the window past all the lamps and you’re trying to see a cloud of fireflies. We’re in the disc of the galaxy, and any direction we look we see Milky Way stars,” Beacom said.
Tools sensitive enough for this research have only recently been developed, Beacom said. “Because we’re right on the threshold where the most sensitive existing measurements ever made are barely detecting hints, nature could be tricking us,” he added.
Gamma rays are emitted when dark matter undergoes a process called annihilation. “The idea is that dark matter is distributed in huge amounts over large scales but is actually composed of individual particles … when they get close together, their mass is converted into normal particles, elementary particles that we know about,” explained Geringer-Sameth, adding that these particles then lead to the emission of gamma rays, which have the shortest wavelengths of all forms of light.
This means that they are the highest energy form of light. Gamma rays can be emitted from not only dark matter interactions, but other kinds of particle interactions as well. The type of interaction that generates a gamma ray can be determined by measuring the number of photons per energy interval, a measurement that is referred to as a spectrum.
Different processes generate different spectral shapes. The results of the Brown-Carnegie Mellon study align closely with expectations for what dark matter gamma radiation spectrum might look like.
But these findings are by no means conclusive. Rather, they set the stage for further investigation.
The study provides the first evidence of gamma rays coming from dwarf galaxies, but it is “not a bulletproof case,” said Dan Hooper, an associate scientist at Fermilab and assistant professor of astronomy and astrophysics at the University of Chicago. Though the results are “very suggestive,” there is a chance they that other explanations could exist.
Though the Brown-Carnegie Mellon group is the first to “talk about it in detail,” two other groups have also followed up on the results and found the same results.
Hooper’s group has examined similar phenomena in its own research, which he has conducted with his collaborator Tim Linden, a postdoctoral fellow at the Kavli Institute for Cosmological Physics at the University of Chicago.
The findings are “very similar,” Hooper said, adding that though addressing different questions, both groups “detect a fairly significant excess of gamma rays with similar characteristics and … discuss different possible interpretations, including the possibility that dark matter might be responsible,” Hooper said.
Koushiappas said he expects his research is only at the beginning of what will likely be 30 additional years of experimentation in the field. For example, the Large Hadron Collider at the European Organization for Nuclear Research — CERN — will restart soon, with an aim to look for signatures of dark matter, Beacom said.
A previous version of this article misattributed an analogy between the mysteries of dark matter and water to Dan Hooper. In fact, it was John Beacom. The article also misidentified the individual who asserted that a study led by Savvas Koushiappas was not conclusive and who conducted research with Tim Linden. In both cases, it was Hooper, not Beacom. The Herald regrets the errors.
