Considerations in optimizing CMB polarization experiments to constrain inflationary physics

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Abstract
We quantify the limiting factors in optimizing current-technology cosmic microwave background (CMB) polarization experiments to learn about inflationary physics. We consider space-based, balloon-borne and ground-based experiments. We find that foreground contamination and residuals from foreground subtraction are ultimately the limiting factors in detecting a primordial gravity wave signal. For full-sky space-based experiments, these factors hinder the detection of tensor-to-scalar ratios of r < 10− 3 on large scales, while for ground-based experiments these factors impede the ability to apply delensing techniques. We consider ground-based/balloon-borne experiments of currently planned or proposed designs and find that it is possible for a value of r = 0.01 to be measured at the ~3 σ level. A small space-based CMB polarization experiment, with current detector technology and full sky coverage, can detect r ~ 1 × 10− 3 at the ~ 3 σ level, but a markedly improved knowledge of polarized foregrounds is needed. We advocate using as wide a frequency coverage as possible in order to carry out foreground subtraction at the per cent level, which is necessary to measure such a small primordial tensor amplitude. To produce a clearly detectable (> 3 σ) tensor component in a realistic CMB experiment, inflation must either involve large field variations, or multi-field/hybrid models. Hybrid models can be easily distinguished from large field models due to their blue scalar spectral index. Therefore, an observation of a tensor/scalar ratio and n < 1 in future experiments with the characteristics considered here may be an indication that inflation is being driven by some physics in which the inflaton cannot be described as a fundamental field.