Prospects for measuring cosmic microwave background spectral distortions in the presence of foregrounds

Measurements of cosmic microwave background (CMB) spectral distortions have profound implications for our understanding of physical processes taking place over a vast window in cosmological history. Foreground contamination is unavoidable in such measurements and detailed signal-foreground separation will be necessary to extract cosmological science. In this paper, we present Markov chain Monte Carlo based spectral distortion detection forecasts in the presence of Galactic and extragalactic foregrounds for a range of possible experimental configurations, focusing on the Primordial Inflation Explorer (PIXIE) as a fiducial concept. We consider modifications to the baseline PIXIE mission (operating similar or equal to 12 months in distortion mode), searching for optimal configurations using a Fisher approach. Using only spectral information, we forecast an extended PIXIE mission to detect the expected average non-relativistic and relativistic thermal Sunyaev-Zeldovich distortions at high significance (194s and 11 sigma, respectively), even in the presence of foregrounds. The Lambda CDM Silk damping mu-type distortion is not detected without additional modifications of the instrument or external data. Galactic synchrotron radiation is the most problematic source of contamination in this respect, an issue that could be mitigated by combining PIXIE data with future ground-based observations at low frequencies (nu similar or equal to 15-30 GHz). Assuming moderate external information on the synchrotron spectrum, we project an upper limit of vertical bar mu vertical bar < 3.6 x 10(-7) (95 per cent c. l.), slightly more than one order of magnitude above the fiducial Lambda CDM signal from the damping of small-scale primordial fluctuations, but a factor of similar or equal to 250 improvement over the current upper limit from COBE/ Far Infrared Absolute Spectrophotometer. This limit could be further reduced to vertical bar mu vertical bar < 9.4 x 10(-8) ((9)5 per cent c. l.) with more optimistic assumptions about extra low-frequency information and would rule out many alternative inflation models and provide new constraints on decaying particle scenarios.