C-BASS is a new project to image the whole sky at a wavelength of six centimetres, measuring both the brightness and the polarization of the sky. The main uses of this survey will be to help us make better images of the cosmic microwave background (CMB) and to study diffuse radiation from our Galaxy. The CMB is the very faint afterglow of the Big Bang. It has a temperature of less than three degrees Celsius above absolute zero. By making images of this radiation, scientists are able to see the universe as it was just after the Big Bang. This radiation is also polarized - in a similar way to sunlight reflected off water - and this can be measured in a way similar to wearing polarized sunglasses.  The polarization is predicted to have two distinct patterns; one due to variations in density, the other due to the presence of gravitational waves. The polarization patterns from gravitational waves are expected to be extremely faint (less than 1 millionth of a degree Celsius) but if they can be measured, they will provide information about the state of the universe when it was less than a billion billion billionth of a second old.

Many telescopes are now being designed and built to detect these very faint polarization patterns in the CMB but, in order to clearly see the signals, we need to accurately remove all the other contaminating signals from the sky which obscure our view of the early universe. Most of this contamination comes from our own Galaxy, the Milky Way.  This is where C-BASS will play an essential role in this quest for understanding the origin and evolution of our universe. Operating at a wavelength of 6cm, C-BASS will make an extremely accurate map of the contaminating signal from our Galaxy. This will allow the contamination to be subtracted with great accuracy from high-frequency measurements such as those to be made by Planck.

The C-BASS instrument will consist of highly sensitive microwave amplifiers, cooled to within a few degrees of absolute zero, and configured to measure tiny differences in temperature and polarization. It will be mounted on two separate telescopes - one at the Owens Valley Observatory in California, the other in South Africa.  This will allow C-BASS to observe both in the northern and southern hemispheres and hence map the whole sky.

The all-sky maps produced by this survey will be key to enabling accurate subtraction of contaminating signals from the data collected by specialized CMB telescopes in order that they can reveal the true fluctuations in the microwave background. In addition to playing an essential part in the quest for revealing the tiny fluctuations in the CMB and understanding the origin and evolution of our universe, C-BASS will vastly increase our understanding of the physics of the gas between the stars in our own Galaxy, for example by mapping out the magnetic field in the Galaxy.

The C-BASS project is funded by the National Science Foundation (NSF) in the U.S. and the Universities of Oxford and Manchester in the U.K.