The low frequency instrument in the ESA Planck mission

Measurements of the cosmic microwave background (CMB) allow high precision observation of the cosmic plasma at redshift z similar to1100. After the success of the NASA satellite COBE, that in 1992 provided the first detection of the CMB anisotropy, results from many ground-based and balloon-borne experiments have showed a remarkable consistency between different results and provided quantitative estimates of fundamental cosmological properties [1]. During the current year the team of the NASA WMAP satellite has released the first improved full-sky maps of the CMB since COBE, leading to a deeper insight in the origin and evolution of the Universe. The ESA satellite PLANCK, scheduled for launch in 2007, is designed to provide the ultimate measurement of the CMB temperature anisotropy over the full sky, with an accuracy that will be limited only by astrophysical foregrounds, and robust detection of polarisation anisotropy. PLANCK will observe the sky with two instruments over a wide spectral band (the Low Frequency Instrument, based on coherent radiometers, from 30 to 70 GHz and the High Frequency Instrument, based on bolometric detectors, from 100 to 857 GHz). The mission performances will improve dramatically the scientific return compared to WMAP. Furthermore the LFI radiometers (as well as some of the HFI bolometers) are intrinsically sensitive to polarisation so that by combining the data from different receivers it will be possible to measure accurately the E mode and to detect the B mode of the polarisation power spectrum. PLANCK sensitivity will offer also the possibility to detect the non-Gaussianities imprinted in the CMB.