A team of researchers from Dartmouth College has proposed a new hypothesis that could verifiably explain the central enigma of cosmology: What is the dark matter that makes up 85 percent of our universe, and how is it formed? We have never been able to see it, but we know it exists because we can observe its gravitational effects on galaxies. Although some claim otherwise.

According to the study, published in the journal Physical Review Letters, dark matter could have formed in the earliest moments of the universe after the Big Bang from the interaction between massless particles, which, when combined, suddenly lost energy and acquired considerable mass. This transformation would resemble the physical process in which steam condenses into water, according to the authors.

What we are proposing is a radical transformation: that dark matter began as nearly massless particles, similar to light, and ended up as cold, heavy clumps responsible for the structure of galaxies, says Robert Caldwell, professor of physics and astronomy at Dartmouth and lead author of the paper. This contrasts with the usual view that identifies dark matter as a cold and passive substance, unconnected to energetic relativistic states.

After the great explosion that marked the beginning of the universe about 13.7 billion years ago, the cosmos was dominated by extremely hot and fast particles similar to photons, the elementary particles of light. According to the researchers, it would be in that primordial soup where large quantities of these massless particles began to pair up, attracted by the opposite orientation of their spins, similar to the attraction between opposite poles of a magnet.

dark matter water vapour
Distribution of dark matter detected in the Coma galaxy cluster region (green). The background is an image taken with the HSC. The team investigated the dark matter distribution by precisely measuring the effect of the very slight distortion of the shape of galaxies taken by the HSC due to the presence of dark matter (weak gravitational lensing effect). The image shows dark matter radiating from the center of the galaxy cluster (center of the image). Credit: HyeongHan et al.

As the universe expanded and cooled, the researchers propose that an imbalance in the orientation of these spins occurred, causing a sudden drop in the system’s energy—a collapse that would trigger a phase transition. The most surprising part of our model is precisely that sudden drop in energy, which acts as a bridge between a densely energetic universe and one characterized by cold, clustered particles, explains Liang.

According to the authors, this transition phenomenon is reminiscent of what happens with Cooper pairs in materials physics. At very low temperatures, two electrons can form a pair that conducts electricity without resistance, a key property in superconductivity. Caldwell and Liang believe this analogy strengthens the plausibility of their proposal: Cooper pairs demonstrate that there is a physical mechanism capable of generating this kind of transition, says Caldwell.

According to the authors, the extremely cold and almost pressureless particles they believe make up dark matter would leave a distinctive signal in the Cosmic Microwave Background (CMB), the thermal echo of the Big Bang that still permeates the entire universe. This signature could be identified using data already collected by projects like the Simons Observatory in Chile or the future CMB Stage 4 initiative.

The beauty of the model lies in its simplicity: it doesn’t require complex theoretical frameworks to work, it’s based on physical concepts and timelines we already know, notes Liang. Moreover, the theory offers a coherent explanation for the mystery of the universe’s current energy density, which is markedly lower than in its early stages. Today’s structures acquire their mass thanks to the density of cold dark matter, but there must also be a mechanism that accounts for the drop in energy density to the levels we observe today, he concludes.

Although this is a theoretical hypothesis, the fact that it is based on mathematical models compatible with already available observational data allows for future testing. According to Caldwell, we are proposing a new way to think about—and possibly identify—dark matter. And the most exciting part is that we can put it to the test.

SOURCES

Darthmout College

Guanming Liang(梁冠铭), Robert R. Caldwell, Cold Dark Matter Based on an Analogy with Superconductivity. Physical Review Letters, 134, 191004. doi.org/10.1103/PhysRevLett.134.19

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