Tianlai: A 21cm intensity mapping experiment proceedings of the XXXIst URSI general assembly and scientific symposium to be held in Beijing, China (CIE), August 17–23, 2014

The Tianlai project is a radio astronomical experiment dedicated to the study of the nature of dark energy. By observing the baryon acoustic oscillation (BAO) features in the large scale structure power spectrum through 21cm intensity mapping, i.e. observe the intensity variation of the 21cm line emission of the neutral hydrogen at redshifts 0<;z<;3 without resolving the individual galaxies which host these neutral hydrogen atoms. The BAO features in the power spectrum can be used as standard ruler for distance measurement, by measuring it at different redshifts, the dark energy equation of state can be determined. In order to achieve high precision, it is necessary to observe large sky area with high precision, and the design of the Tianlai experiment is optimized for this. The full scale experiment requires a compact array with a geometric dimension of (100m)², and filled with receiver elements. The Tianlai pathfinder, which is being built in a radio quiet site in Xinjiang, China, consists of 3 cylinders of 15m wide and 40 m long. The project is an international effort with participants from China, France, the USA and Canada.