How to measure CMB spectral distortions with an imaging telescope

International audienceWe propose the implementation of an imaging telescope in combination with an interfrequency calibrator to measure the spectral shape of the microwave sky by exploiting the differences in the sky intensity between multiple pairs of frequency channels. By jointly sampling the cosmological and foreground parameters in a Bayesian framework for 600 instrument configurations, we determine the minimum calibration accuracy required in order to obtain measurements of spectral distortions and simultaneously measure spectral and spatial fluctuations of the cosmic microwave background (CMB). We demonstrate the feasibility of this technique for a Probe of Inflation and Cosmic Origins (PICO)-like CMB mission and show that a 10-σ measurement of the y distortion along with a 2 orders of magnitude improvement on the Far-Infrared Absolute Spectrometer (FIRAS) μ-distortion limit is feasible from this technique. We argue that longer-term applications may be envisaged at even higher sensitivity, capable of attaining the μ-type spectral distortions that provide a robust prediction of the Lambda Cold Dark Matter (LCDM) model and even primordial recombination lines of hydrogen and helium, in the context of the European Space Agency Voyage 2035–2050 program