Dark matter formed when fast particles slowed down and got heavy: Study

WASHINGTON: 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 per cent 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 theorise 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.