A study by Dartmouth researchers proposes a new theory about the origin of dark matter, the mysterious and invisible substance thought to give the universe its shape and structure.
The researchers report in Physical Review Letters that dark matter could have formed in the early life of the universe from the collision of high-energy massless particles that lost their zip and took on an incredible amount of mass immediately after pairing up, according to their mathematical models.
While hypothetical, dark matter is believed to exist based on observed gravitational effects that cannot be explained by visible matter. Scientists estimate that 85% of the universe's total mass is dark matter.
But the study authors write that their theory is distinct because it can be tested using existing observational data. The extremely low-energy particles they suggest make up dark matter would have a unique signature on the Cosmic Microwave Background, or CMB, the leftover radiation from the Big Bang that fills all of the universe.
"Dark matter started its life as near-massless relativistic particles, almost like light," says Robert Caldwell , a professor of physics and astronomy and the paper's senior author.
"That's totally antithetical to what dark matter is thought to be—it is cold lumps that give galaxies their mass," Caldwell says. "Our theory tries to explain how it went from being light to being lumps."
Hot, fast-moving particles dominated the cosmos after the burst of energy known as the Big Bang that scientists believe triggered the universe's expansion 13.7 billion years ago. These particles were similar to photons, the massless particles that are the basic energy, or quanta, of light.
It was in this chaos that extremely large numbers of these particles bonded to each other, according to Caldwell and Guanming Liang, the study's first author and a Dartmouth senior.
They theorize that these massless particles were pulled together by the opposing directions of their spin, like the attraction between the north and south poles of magnets.
As the particles cooled, Caldwell and Liang say, an imbalance in the particles' spins caused their energy to plummet, like steam rapidly cooling into water. The outcome was the cold, heavy particles that scientists think constitute dark matter.
"The most unexpected part of our mathematical model was the energy plummet that bridges the high-density energy and the lumpy low energy," Liang says.
"At that stage, it's like these pairs were getting ready to become dark matter," Caldwell says. "This phase transition helps explain the abundance of dark matter we can detect today. It sprang from the high-density cluster of extremely energetic particles that was the early universe."
The study introduces a theoretical particle that would have initiated the transition to dark matter. But scientists already know that the subatomic particles known as electrons can undergo a similar transition, Caldwell and Liang say.
At low temperatures, two electrons can form what are known as Cooper pairs that can conduct electricity without resistance and are the active mechanism in certain superconductors. Caldwell and Liang cite the existence of Cooper pairs as evidence that the massless particles in their theory would have been capable of condensing into dark matter.
"We looked toward superconductivity for clues as to whether a certain interaction could cause energy to drop so suddenly," Caldwell says. "Cooper pairs prove that the mechanism exists."
The metamorphosis of these particles from the cosmic equivalent of a double espresso into day-old oatmeal explains the vast deficit in the energy density of the current universe compared to its early days, Liang says. Scientists know that density has declined since the Big Bang as the universe's energy expands outward. But Liang and Caldwell's theory also accounts for the increase in the density of mass.
"Structures get their mass due to the density of cold dark matter, but there also has to be a mechanism wherein energy density drops to close to what we see today," Liang says.
"The mathematical model of our theory is really beautiful because it's rather simplistic—you don't need to build a lot of things into the system for it to work," he says. "It builds on concepts and timelines we know exist."
Their theory suggests that the particle pairs entered a cold, nearly pressureless state as they got slower and heavier. This characteristic would make them stand out on the CMB. The CMB has been studied by several large-scale observational projects and is the current focus of the Simons Observatory in Chile and other experiments such as CMB Stage 4.
Existing and future data from these projects could be used to test Caldwell and Liang's theory, the researchers say.
"It's exciting," Caldwell says. "We're presenting a new approach to thinking about and possibly identifying dark matter."
