Enhancing the tensor-to-scalar ratio in simple inflation
V. Mukhanov, A. Vikman
TL;DR
The paper demonstrates that allowing a nontrivial kinetic term in the inflaton Lagrangian can significantly boost the tensor perturbations relative to scalars, leading to a larger tensor-to-scalar ratio and brighter B-mode polarization signals in the CMB. By adopting a generalized slow-roll framework with $p(\phi,X)$ and exploring slow-roll driven by the potential but with a kinetic-term modification, the authors show that the sound speed $c_S$ can exceed 1 and be tuned to amplify gravitational waves. They present a concrete model with $p(\phi,X)=\alpha^{2}[\sqrt{1+2X/\alpha^{2}}-1]-\frac{1}{2}m^{2}\phi^{2}$ in which $c_S>1$ and derive that the tensor-to-scalar ratio scales like $9c_*/N$, enabling potentially observable B-modes for suitable parameter choices. The results imply a higher inflation scale and a different phenomenology than canonical slow-roll, though they rely on some degree of fine-tuning in the kinetic sector; future CMB observations will be crucial to test and constrain such scenarios.
Abstract
We show that in theories with a nontrivial kinetic term the contribution of the gravitational waves to the CMB fluctuations can be substantially larger than that is naively expected in simple inflationary models. This increase of the tensor-to-scalar perturbation ratio leads to a larger B-component of the CMB polarization, thus making the prospects for future detection much more promising. The other important consequence of the considered model is a higher energy scale of inflation and hence higher reheating temperature compared to a simple inflation.