Where did dark matter come from? The answer may be surprisingly simple

Scientists have proposed a new explanation for dark matter and how the elusive substance that anchors galaxies might have taken shape in the newborn universe.

The scenario begins with weightless particles zipping through the cosmic furnace and ends with a chill population of massive pairs that could still be drifting through space today.

The work – carried out by physicists at Dartmouth College – offers predictions that astronomers can check against existing observations of the Cosmic Microwave Background, the after-glow of the Big Bang.

Particles after the Big Bang

“Dark matter started its life as near-massless relativistic particles, almost like light,” said Robert Caldwell, senior author of the study. “That’s totally antithetical to what dark matter is thought to be – it is cold lumps that give galaxies their mass.”

In the model designed by Caldwell and undergraduate researcher Guanming Liang, countless high-energy quanta filled the universe just after the Big Bang. These particles, similar to the photons that carry light, hurtled about at near-light speed.

Under certain conditions, two such quanta could lock together because their spins pointed in opposite directions, rather like the attraction between the north and south poles of tiny magnets.

The moment they bonded, the new entity’s energy plummeted and its inertia skyrocketed. Within a flash it went from a racing ray to a hefty relic.

A sudden drop in energy

“The most unexpected part of our mathematical model was the energy plummet that bridges the high-density energy and the lumpy low energy,” Liang explained.

The researchers liken the transformation to steam condensing into water – a hot, diffuse gas suddenly collapses into a cooler, denser phase.

“At that stage, it’s like these pairs were getting ready to become dark matter,” Caldwell said. “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.”

Cooper pairs as a clue

The paper introduces a brand-new particle to trigger the shift, yet the authors argue that nature already provides a parallel. In certain superconducting metals, two electrons with opposite spins form so-called Cooper pairs that glide without electrical resistance.

“We looked toward superconductivity for clues as to whether a certain interaction could cause energy to drop so suddenly,” Caldwell noted. “Cooper pairs prove that the mechanism exists.”

If the analogy holds, the relic pairs born in the cosmic dawn would have cooled into a nearly pressure-free state – a key hallmark of the unseen matter whose gravity sculpts galaxies.

A simple model for dark matter

Dark matter accounts for roughly eighty-five percent of all mass, yet the universe’s overall energy density has fallen dramatically since its first moments.

Liang points out that the new theory addresses both facts at once: “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.”

The abrupt condensation of massless quanta into massive pairs drains energy from the radiation budget while boosting the matter budget, squaring theory with observation.

“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,” said Liang. “It builds on concepts and timelines we know exist.”

A signature written in ancient light

Because the transition would have left the paired particles almost motionless, the team predicts a distinctive imprint on the cosmic microwave background.

The CMB has already been mapped by satellites and ground-based projects, and more precise surveys are under way at the Simons Observatory in Chile and in planned CMB Stage-4 experiments.

“It’s exciting,” Caldwell said. “We’re presenting a new approach to thinking about and possibly identifying dark matter.”

The researchers emphasize that their proposal is testable: if future CMB data reveal the subtle pattern predicted by their equations, the case for heavy pairs born from light-like quanta will strengthen. If not, the idea will need revision or replacement – exactly how science should proceed.

Future of dark matter research

For now the paper stakes out a middle ground between theoretical elegance and observational reach.

Upcoming missions will either expose the predicted CMB trace or tighten the limits on how dark matter could have formed. Either outcome will refine our view of the invisible framework that holds the cosmos together.

Should the ancient sky bear the scar of those primordial pairings, physicists might finally glimpse how ghostly matter traded light speed for lasting weight – and reshaped the universe in the process.

The study is published in the journal Physical Review Letters.

Image Credit: NASA, ESA and M. Montes (University of New South Wales)

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