Exploration of the cosmic dark ages and lunar based astronomy

The cosmic dark age refers to the era when the Big Bang has just ended and the first generation of stars and galaxies has not yet formed. At this time, the universe is in its primordial state, and much valuable information about its origin is still preserved. The 21 cm signal produced by the neutral hydrogen gas provides an observational probe of this cosmic dark age, but it is now redshifted to the meter wave, decameter wave or even hectometer wave band, where there are also other types of radiation, especially the huge galactic foreground radiation. It is also subject to absorption, refraction and various electromagnetic interferences by the ionosphere when observed from the ground, so its observation is extremely challenging. Observation from the far side of the Moon or along the lunar orbit, however, is ideal, as the ionosphere and low frequency radio interference can be avoided. With renewed interest in the exploration and utilization of our moon, the United States, Europe, India and China are actively planning research on moon-based astronomy, especially in the low-frequency radio band, which will open up a new window for astronomy and can probe the cosmic dark age and origin of the universe. This article will describe the latest research on the cosmic dark age and the cosmic dawn, as well as the current trend to use the moon to carry out low-frequency radio observations, and present a brief overview of the Discovering the Sky at the Longest wavelength (also known as the Hongmeng) lunar satellite array project.