The ΛCDM model is a widely accepted cosmology theory that explains the universe’s structure and evolution. Observational and experimental data strongly support it. However, the two key components—cold dark matter (making up about 25% of the universe) and dark energy (about 70%)—are still mysterious, and scientists are working to understand their true nature.
A Dartmouth professor and a physics-mathematics student propose a new theory on the origin of dark matter—the invisible substance shaping the universe. Their research suggests dark matter may have formed in the early universe from high-energy massless particles colliding and gaining mass instantly after pairing up.
Though still hypothetical, dark matter is believed to exist due to unexplained gravitational effects and is estimated to make up 85% of the universe’s total mass. Unlike previous theories, this new idea can be tested using existing data. If correct, these extremely low-energy particles would leave a unique mark on the Cosmic Microwave Background (CMB), the lingering radiation from the Big Bang.
After the Big Bang, the universe was filled with fast-moving, massless particles, similar to photons—the building blocks of light. Researchers propose that vast numbers of these particles bonded during this chaotic period due to their opposing spins, similar to how magnets attract.
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As the universe cooled, an imbalance in the spins caused a dramatic drop in energy, comparable to steam rapidly turning into water. This process, they suggest, led to the formation of the cold, heavy particles that scientists believe make up dark matter.
Liang, a James O. Freedman Presidential Scholar, found the sudden drop in energy in their model particularly surprising—it connects high-density energy to uneven low-energy regions. As Robert Caldwell, a professor of physics and astronomy and the senior author of the paper, advises, he is further developing the theory through his senior thesis, refining details from their published research and setting the stage for future studies.
Caldwell explains that the particle pairs were preparing to transition into dark matter at a certain stage. This shift helps account for the dark matter observed in the universe today, originating from the early universe’s dense, high-energy particle clusters.
Their study proposes a theoretical particle responsible for this transition. However, they point out that a similar process already occurs with electrons. At low temperatures, electrons can form Cooper pairs, which allow electricity to flow without resistance in superconductors. Caldwell and Liang argue that the existence of Cooper pairs supports their idea that massless particles in their model could have condensed into dark matter.
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They looked to superconductivity for insights into how a sudden energy drop could happen, and Cooper pairs prove that such a mechanism exists.
The team uses a colorful analogy to describe their theory: massless particles start off energetic, like a strong espresso, but eventually settle into a sluggish, dense state, like old oatmeal.
This transformation explains the decline in energy density since the Big Bang while accounting for the increasing density of mass in the universe.
Their theory suggests particle pairs slowed down and grew heavier, entering a cold, nearly pressureless state. This shift would leave a distinct signature on the Cosmic Microwave Background (CMB), the remnant radiation from the Big Bang.
Observatories like the Simons Observatory in Chile and projects like CMB Stage 4 are studying the CMB, and their data could help test Caldwell and Liang’s hypothesis.
Caldwell expresses excitement about their new approach to understanding dark matter. Their theory was inspired by an April 2023 paper on how Cooper pairs might have influenced the early universe.
Liang contacted the study’s authors and found that they hadn’t tested their model at non-zero temperatures.
At the same time, Liang was taking a solid-state physics course with Assistant Professor Rufus Boyack, where he learned a mathematical tool that could extend the existing model. He and Caldwell applied the math and discovered a detailed evolutionary history of particles—from the extreme heat of the early universe to the cooler state we see today.
Journal Reference:
- Guanming Liang and Robert R. Caldwell. Cold Dark Matter Based on an Analogy with Superconductivity. Phys. Rev. Lett. DOI: DOI: 10.1103/PhysRevLett.134.191004
