A groundbreaking study by Dartmouth researchers proposes a new theory regarding the origin of dark matter, the elusive and invisible substance believed to shape the universe. This innovative theory suggests that dark matter emerged from particles that rapidly condensed, akin to steam transforming into water. The researchers shared their findings in the esteemed journal Physical Review Letters, indicating that dark matter could have formed during the universe's early life through the collision of high-energy massless particles that, upon pairing, quickly gained an immense amount of mass.
According to mathematical models presented by the researchers, hypothetical dark matter is inferred to exist based on observed gravitational effects that visible matter alone cannot explain. Scientists estimate that an astonishing 85% of the universe's total mass is comprised of dark matter. What sets this study apart is its potential for testing; the theory can be evaluated using existing observational data.
The researchers propose that the extremely low-energy particles constituting dark matter would leave a distinct signature on the cosmic microwave background (CMB), the remnant radiation from the Big Bang that permeates the universe. Robert Caldwell, a professor of physics and astronomy and the senior author of the paper, states, "Dark matter began as near-massless relativistic particles, almost like light." This concept starkly contrasts with the traditional view of dark matter as cold lumps that contribute mass to galaxies. Caldwell adds, "Our theory attempts to explain the transition from being light to becoming lumps."
In the chaotic aftermath of the Big Bang, which is believed to have initiated the universe's expansion 13.7 billion years ago, hot, fast-moving particles dominated the cosmos. These particles resembled photons, which are the massless quanta of light. During this tumultuous period, a significant number of these particles bonded together, as explained by Caldwell and the study's first author, Guanming Liang, a senior at Dartmouth.
The authors theorize that these massless particles were drawn together by the opposing directions of their spin, similar to how the north and south poles of magnets attract. As the particles cooled, an imbalance in their spins caused a dramatic decline in energy, akin to steam condensing into water. "The most unexpected part of our mathematical model was the energy plummet that links high-density energy to the lumpy low energy," Liang notes.
This phase transition elucidates why we can detect such a significant amount of dark matter today, stemming from the high-density cluster of energetic particles that characterized the early universe. The study introduces a theoretical particle responsible for initiating the transition to dark matter. Interestingly, scientists are already aware that subatomic particles like electrons can undergo a similar transition. Caldwell and Liang highlight how, at low temperatures, two electrons can form Cooper pairs, which can conduct electricity without resistance and are crucial in certain superconductors.
The existence of Cooper pairs serves as compelling evidence that the massless particles in their theory could condense into dark matter. Caldwell remarks, "We looked towards superconductivity for insights into whether a particular interaction could cause such a sudden drop in energy." This transformation of particles from a high-energy state to a low-energy state explains the stark deficit in the energy density of the current universe as compared to its formative years.
Liang emphasizes that while the density of the universe has decreased since the Big Bang due to its expansion, their theory also accounts for the increase in mass density. "Structures derive their mass from the density of cold dark matter, but a mechanism must exist where energy density drops to approximate current observations," Liang states. Their mathematical model is not just intricate; it is also elegantly simple, relying on established concepts and timelines.
The researchers propose that as these particle pairs slow down and gain mass, they enter a cold, nearly pressureless state, making them distinguishable in the CMB. Several large-scale observational projects have examined the CMB, with current focus on initiatives like the Simons Observatory in Chile and the upcoming CMB Stage 4 experiment. Existing and future data from these significant projects could be instrumental in testing Caldwell and Liang's theory.
In conclusion, Caldwell expresses his excitement about presenting a new perspective on identifying and understanding dark matter. "We're introducing an innovative approach that could potentially lead us to uncover the mysteries of dark matter," he says. The implications of this research could significantly advance our understanding of the universe and the fundamental forces at play.
