Gravity waves goodbye

TL;DR

Detecting primordial gravitational waves via CMB anisotropies is highly challenging due to the dominance of scalar perturbations and theoretical constraints on tensor-to-scalar ratios, making such detection unlikely with current methods.

Contribution

This paper analyzes the limitations of current CMB observations and models in detecting the stochastic gravitational wave background, highlighting the challenges and theoretical constraints involved.

Findings

CMB temperature anisotropies can only probe T/S down to about 10%.

Current models suggest T/S is less than 0.5, making detection difficult.

Optimistic polarization measurements only slightly improve detection prospects.

Abstract

The detection of a stochastic background of long-wavelength gravitational waves (tensors) in the cosmic microwave background (CMB) anisotropy would be an invaluable probe of the high energy physics of the early universe. Unfortunately a combination of factors now makes such a detection seem unlikely: the vast majority of the CMB signal appears to come from density perturbations (scalars) - detailed fits to current observations indicate a tensor-to-scalar quadrupole ratio of T/S < 0.5 for the simplest models; and on the theoretical side the best-motivated inflationary models seem to require very small T/S. Unfortunately CMB temperature anisotropies can only probe a gravity wave signal down to T/S \sim 10% and optimistic assumptions about polarization of the CMB only lower this another order of magnitude.