Oscillating modulation to B-mode polarization from varying propagating speed of primordial gravitational waves

TL;DR

This paper explores how variations in the speed of primordial gravitational waves during inflation can cause oscillating features in the B-mode polarization spectrum, offering a new way to test theories beyond standard cosmology.

Contribution

It demonstrates that step-like changes in the gravitational wave speed induce unique oscillations in the polarization spectrum, which are difficult to replicate with other parameters.

Findings

Oscillating modulation in B-mode polarization spectrum due to $c_T$ variation.

Modulation intensity linked to $c_T$ dynamics.

Potential to constrain theories with high-precision CMB data.

Abstract

In low-energy effective string theory and modified gravity theories, the propagating speed $c_T$ of primordial gravitational waves may deviate from unity. We find that the step-like variation of $c_T$ during slow-roll inflation may result in an oscillating modulation to the B-mode polarization spectrum, which can hardly be imitated by adjusting other cosmological parameters, and the intensity of the modulation is determined by the dynamics of $c_T$. Thus provided that the foreground contribution is under control, high-precision CMB polarization observations will be able to put tight constraint on the variation of $c_T$, and so the corresponding theories.