Dark matter neither reflects nor emits light, making it nearly impossible to observe directly.
For decades, scientists have been unraveling the mysteries of dark matter—a substance that makes up about 27% of the universe’s mass-energy. Unlike the matter we know, which forms everything from oceans to exoplanets, dark matter neither reflects nor emits light, making it nearly impossible to observe directly. But a groundbreaking study has found new ways to track its elusive presence during the universe’s earliest days.
Dark matter plays a pivotal role in shaping the universe’s vast structures, but its behavior on smaller scales has long puzzled researchers. While it’s known that dark matter interacts weakly with other particles, scientists suspect it might influence the formation of smaller cosmic structures—especially during the “cosmic dawn,” the period when the first galaxies began to emerge from primordial gas.
A study led by Jo Verwohlt from the University of Copenhagen takes a step closer to uncovering these mysteries. Published in the journal Physical Review D, her team explored how dark matter might have left subtle imprints in hydrogen signals from this ancient era, now stretched and redshifted due to the expansion of the universe. Their work could help distinguish between competing theories about the nature of dark matter.
The Role of Dark Radiation
One fascinating theory proposes that dark matter interacted with a hypothetical force known as “dark radiation,” akin to how electromagnetic forces are mediated by photons. If true, this interaction could have caused heating in the dense, early universe, leading to the formation of “dark matter halos.” These halos are thought to be gravitationally bound regions where dark matter gathers, much like galaxies today.
These halos may have also undergone cycles of density fluctuations, known as “dark acoustic oscillations.” Similar to sound waves, these oscillations could have impacted the formation of the first galaxies. While these ripples would have faded quickly, their influence might still be detectable in signals from the cosmic dawn.
A Cosmic Signature in Hydrogen
Verwohlt’s team focused on a specific signature: the 21-centimeter hydrogen line. This signal occurs when neutral hydrogen atoms undergo a hyperfine spin-flip transition. During the universe’s early stages, these atoms either absorbed or emitted 21-centimeter photons from the cosmic microwave background, leaving traces of the interaction.
By studying these traces, researchers aim to uncover evidence of dark matter’s impact on small-scale cosmic structures. Using advanced models of cosmic structure formation, the team linked the 21-centimeter signal to star formation rates, finding that this data could shed light on whether dark acoustic oscillations occurred.
The study suggests that South Africa’s HERA radio telescope could be instrumental in this search. According to the researchers, around 18 months of observations would be required to determine whether these oscillations—and by extension, dark radiation—ever existed.
Such findings could offer unprecedented insights into dark matter’s properties and its role in shaping the early universe, potentially answering one of cosmology’s biggest unanswered questions.
