Measuring the tensor to scalar ratio from CMB B-modes in the presence of foregrounds

摘要

Aims. We investigate the impact of polarised foreground emission on the performances of future CMB experiments aiming to detectprimordial tensor fluctuations in the early universe. In particular, we study the accuracy that can be achieved in measuring the tensor-to-scalar ratio r in the presence of foregrounds.Methods. We designed a component separation pipeline, based on the SMICA method, aimed at estimating r and the foregroundcontamination from the data with no prior assumption on the frequency dependence or spatial distribution of the foregrounds. Wederived error bars accounting for the uncertainty on foreground contribution. We used the current knowledge of galactic and extra-galactic foregrounds as implemented in the Planck sky model (PSM) to build simulations of the sky emission. We applied the methodto simulated observations of this modelled sky emission, for various experimental setups. Instrumental systematics are not consideredin this study.Results. Our method, with Planck data, permits us to detect r = 0.1 from B-modes only at more than 3o-. With a future dedicatedspace experiment, such as EPIC, we can measure r = 0.001 at ～6σ for the most ambitious mission designs. Most of the sensitivityto r comes from scales 20 ≤l≤150 for highrvalues, shifting to lower l's progressively smallerr.This shows that large-scaleforeground emission does not prevent proper measurement of the reionisation bump for full sky experiments. We also investigate theobservation of a small but clean part of the sky. We show that diffuse foregrounds remain a concern for a sensitive ground-basedexperiment with a limited frequency coverage when measuring r < 0.1. Using the Planck data as additional frequency channelsto constrain the foregrounds in such ground–based observations reduces the error by a factor two but does not allow detection ofr = 0.01. An alternate strategy, based on a deep field space mission with a wide frequency coverage, would allow us to deal withdiffuse foregrounds efficiently, but is in return quite sensitive to lensing contamination. In contrast, we show that all-sky missions arenearly insensitive to small-scale contamination (point sources and lensing) if the statistical contribution of such foregrounds can bemodelled accurately. Our results do not significantly depend on the overall level and frequency dependence of the diffused foregroundmodel, when varied within the limits allowed by current observations.