On the night of May 11th, 2018, the 22nd session of the “New Worlds, New Horizons” Public Astronomy Lecture series-The Dark Ages and the Dawn of the Universe was held in the Back Hall Auditorium in the Main Building of Tsinghua University. The lecturer, Dr. Xuelei Chen is a leading researcher from the National Astronomical Observatory of China. The lecture attracted an audience of over 500, including students, teachers and the general public.

Dr. Chen recalled the history of modern cosmology: the Universe started from a hot Big Bang and then gradually cooled. As it cools, it entered a dark age after 380 thousand years; only the emergence of the first-generation stars and black holes ended this dark age approximate 250 million years later. The universe began its reionization when the ultraviolet radiation emitted by the stars ionized the intergalactic gases. However, due to the high density of gases within the galaxies themselves, there should be some neutral hydrogen atoms remaining, which formed in the Big Bang. These atoms would send out special signals with 21cm-long wavelength, and therefore researching on such signals would help us reveal the mysterious history of the universe.

Dr. Chen introduced many cutting-edge experiments, such as the EDGES, LOFAR, WMA, and 21CMA in China. However, thus far no 21cm emission has been detected. Dr. Chen discussed the corresponding technology of data processing, the collection and extraction of signal information, their difficulties and possible solutions. After the lecture, Dr. Xuelei Chen interacted with the audience by answering many questions from the audience.