Sensitivity of the Hongmeng 21cm experiment to scattering dark matter

Deep in the history of our universe, long before the first stars ignited to light up the cosmos, there existed a vast, dark era known as the cosmic dark ages. During this time, the universe was filled with a thin, cooling gas of hydrogen, invisible to our eyes but detectable through a faint radio whisper. This whisper, a signal at a specific wavelength called the 21-centimeter line, carries the thermal history of that ancient gas. For decades, astronomers have hoped to listen to this signal to map the early universe, but the signal is incredibly faint, buried beneath a cacophony of radio noise from our own planet and the Milky Way. Recently, a puzzling signal was reported that seemed far deeper and colder than standard physics predicted, leading some to wonder if invisible particles of dark matter were interacting with ordinary gas to cool it down faster than expected.

A new study by Junsong Cang and colleagues explores whether a future mission called Hongmeng, a satellite array designed to orbit the Moon, can solve this mystery. The researchers did not build the satellite or collect new data; instead, they created a sophisticated computer simulation to predict how well Hongmeng would perform if it were launched. They modeled the entire experiment, including the expected cosmic signal, the overwhelming radio noise from Earth, and the complex physics of how dark matter might scatter off gas atoms. By running these simulations, they calculated the sensitivity of the instrument and determined how much it could improve our current understanding of dark matter interactions.

The team's work centers on a specific type of interaction where dark matter particles bounce off protons and electrons in the early universe, transferring heat away from the gas. If this happens, the gas cools more rapidly, making the 21-centimeter signal appear deeper and more pronounced. The researchers simulated the Hongmeng mission operating for five years, but with a very cautious approach: the satellite would only take measurements when the Moon was positioned between the Earth and the Sun, using the lunar body as a natural shield to block radio interference from our planet. Even with this limited observation time, the simulation showed that Hongmeng would be a powerful tool. After just one month of operation, the mission could improve upon current limits on dark matter scattering by a factor of four. By the end of the full five-year mission, it could tighten these constraints by a factor of thirty-nine, pushing the limits of what we know about dark matter to a level far beyond what is currently possible with other methods.

The study also delved into the messy reality of separating the cosmic signal from the foreground noise. The researchers found that the ability to detect dark matter depends heavily on how well we understand the formation of the very first stars, known as Population III stars, which appeared during the era Hongmeng aims to observe. If the properties of these ancient stars are unknown, it becomes harder to distinguish their effects from the subtle cooling caused by dark matter. The simulations showed that while fixing the properties of these first stars would make the dark matter limits appear even tighter, doing so might give a false sense of security. The most robust results come from allowing the model to vary the properties of these stars alongside the dark matter parameters, ensuring that the final limits are reliable even if our understanding of early star formation is imperfect.

Furthermore, the team discovered that the relationship between dark matter and the early universe is not always a simple story of cooling. Depending on the mass of the dark matter particles and how fast they are moving, the interaction could either cool the gas or, in some cases, heat it up due to friction. This dual nature creates a complex landscape for data analysis. The researchers noted that if the dark matter particles are in a specific mass range where the cooling and heating effects nearly cancel each other out, the signal might vanish, making it difficult to detect. This complexity means that simply looking for a single type of signal is not enough; the analysis must account for the possibility that the signal could behave in unexpected ways, potentially leading to false alarms or missed detections if the data is not interpreted with extreme care.

Ultimately, this paper serves as a roadmap for what is possible with the Hongmeng mission. It confirms that by placing a radio telescope on the Moon, we can bypass the noisy atmosphere of Earth and listen to the universe with unprecedented clarity. The simulations suggest that even a conservative observation strategy could revolutionize our search for dark matter, potentially ruling out entire classes of theories or providing the first direct evidence of dark matter interacting with ordinary matter in the early universe. While the mission has not yet flown, the mathematical groundwork laid by this study provides a clear expectation: if the instrument performs as designed, it will offer the most stringent test yet of whether dark matter is truly invisible or if it leaves a subtle, cooling fingerprint on the dawn of time.