Considerations in optimizing CMB polarization experiments to constrain inflationary physics

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 similar to 3 sigma level. A small space-based CMB polarization experiment, with current detector technology and full sky coverage, can detect r similar to 1 x 10(-3) at the similar to 3 sigma 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 sigma) tensor component in a realistic CMB experiment, inflation must either involve large field variations, Delta phi >= 1 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.