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

Abstract

Aims. We investigate the impact of polarised foreground emission on the performances of future CMB experiments aiming to detect primordial tensor fluctuations in the early universe. In particular, we study the accuracy that can be achieved in measuring the tensorto-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 foreground contamination from the data with no prior assumption on the frequency dependence or spatial distribution of the foregrounds. We derived error bars accounting for the uncertainty on foreground contribution. We used the current knowledge of galactic and extragalactic foregrounds as implemented in the Planck sky model (PSM) to build simulations of the sky emission. We applied the method to simulated observations of this modelled sky emission, for various experimental setups. Instrumental systematics are not considered in this study. Results. Our method, with Planck data, permits us to detect r = 0.1 from B-modes only at more than 3σ. With a future dedicated space experiment, such as EPIC, we can measure r = 0.001 at ∼6σ for the most ambitious mission designs. Most of the sensitivity to r comes from scales 20 ≤ ≤ 150 for high r values, shifting to lower 's for progressively smaller r. This shows that large-scale foreground emission does not prevent proper measurement of the reionisation bump for full sky experiments. We also investigate the observation of a small but clean part of the sky. We show that diffuse foregrounds remain a concern for a sensitive ground-based experiment with a limited frequency coverage when measuring r < 0.1. Using the Planck data as additional frequency channels to constrain the foregrounds in such ground-based observations reduces the error by a factor two but does not allow detection of r = 0.01. An alternate strategy, based on a deep field space mission with a wide frequency coverage, would allow us to deal with diffuse foregrounds efficiently, but is in return quite sensitive to lensing contamination. In contrast, we show that all-sky missions are nearly insensitive to small-scale contamination (point sources and lensing) if the statistical contribution of such foregrounds can be modelled accurately. Our results do not significantly depend on the overall level and frequency dependence of the diffused foreground model, when varied within the limits allowed by current observations.

Modelling and simulation of large-scale polarized dust emission over the southern Galactic cap using the GASS Hi data

“…The BICEP2 experiment 1 puts an upper bound on the amplitude of the CMB B-mode signal, parameterized by a tensor-to-scalar ratio (r) at a level of r < 0.09 (95% confidence level BICEP2 and Keck Array Collaboration et al 2016). Therefore, the major challenge for a BICEP2-like experiment with limited frequency coverage is to detect such an incredibly faint CMB B-mode signal in the presence of foreground Galactic contamination (Betoule et al 2009). The dominant polarized foreground above 100 GHz come from thermal emission by aligned aspherical dust grains (Planck Collaboration Int.…”

Joint 3D modelling of the polarized Galactic synchrotron and thermal dust foreground diffuse emission

“…Of particular cosmological interest is the possibility of measuring the so-called polarization B modes, the existence of which implies that tensor fluctuations from primordial gravitational waves are generated during inflation. Planck should be able to measure the tensor-to-scalar ratio, r, down to 0.1 (Betoule et al 2009; in the case of a nominal mission (2 full-sky surveys) and to 0.05 with an extended mission of 4 full-sky surveys ). The value of r sets the energy scale of inflation (Peiris et al 2003) and then provides constraints on inflationary models (Baumann 2009).…”

“…As these component separation techniques will be mainly based on Planck data alone, one of the main issues will be to estimate the residual foreground contamination on the final CMB temperature and polarization maps. These residuals will translate into systematic biases and larger error bars on the estimation of the temperature and polarization power spectra of the CMB fluctuations (see Betoule et al 2009, for a recent study). Thus, they will affect the precision to which cosmological information can be retrieved from the Planck data.…”

Iterative destriping and photometric calibration forPlanck-HFI, polarized, multi-detector map-making

“…Polarized signals for thermal dust and synchrotron are also added. More details about the sky model used for this work can be found in Leach et al (2008) and Betoule et al (2009).…”