CMB $\mu$ distortion from primordial gravitational waves

TL;DR

This paper introduces a novel mechanism for generating $$ distortion in the CMB caused by primordial gravitational waves, which can provide new constraints on their amplitude and spectral index on small scales.

Contribution

It proposes a new $$ distortion generation mechanism from primordial gravitational waves, distinct from scalar perturbations, enabling independent small-scale constraints.

Findings

Estimated $$ distortion size from gravitational waves.

Provides a method to constrain gravitational wave amplitude independently.

Suggests wider-range spectral index constraints when combined with CMB anisotropy data.

Abstract

We propose a new mechanism of generating the $\mu$ distortion in cosmic microwave background (CMB) originated from primordial gravitational waves. Such $\mu$ distortion is generated by the damping of the temperature anisotropies through the Thomson scattering, even on scales larger than that of Silk damping. This mechanism is in sharp contrast with that from the primordial curvature (scalar) perturbations, in which the temperature anisotropies mainly decay by Silk damping effects. We estimate the size of the $\mu$ distortion from the new mechanism, which can be used to constrain the amplitude of primordial gravitational waves on smaller scales independently from the CMB anisotropies, giving more wide-range constraint on their spectral index by combining the amplitude from the CMB anisotropies.