  3D mapping of matter distribution in the universe through the 21 cm radio
emission of atomic hydrogen is a complementary approach to optical surveys for
the study of the Large Scale Structures, in particular for measuring the BAO
(Baryon Acoustic Oscillation) scale up to redshifts z <~ 3 and constrain dark
energy. We propose to carry such a survey through a novel method, called
intensity mapping, without detecting individual galaxies radio emission. This
method requires a wide band instrument, 100 MHz or larger, and multiple beams,
while a rather modest angular resolution of 10 arcmin would be sufficient. The
instrument would have a few thousand square meters of collecting area and few
hundreds of simultaneous beams. These constraints could be fulfilled with a
dense array of receivers in interferometric mode, or a phased array at the
focal plane of a large antenna.
