The imprint of primordial gravitational waves on the CMB intensity profile

TL;DR

This paper investigates how primordial gravitational waves influence the CMB intensity profile by analyzing the induced geometry on photon beams, revealing potential observational signatures of tensor modes in the early universe.

Contribution

It introduces a novel method to characterize gravitational shear effects on CMB photons using spin-weight-two variables and derives the power spectrum of tensor mode contributions.

Findings

Tensor modes induce a pseudo-scalar in the CMB intensity profile.

Analytical power spectrum of the pseudo-scalar at superhorizon scales is obtained.

Potential observational signatures of primordial gravitational waves are discussed.

Abstract

We use the induced geometry on the two dimensional transverse cross section of a photon beam propagating on a perturbed Friedmann-Robertson-Walker (FRW) spacetime to find the Cosmic Microwave Background (CMB) photon distribution over a telescope's collecting area today. It turns out that at each line of sight the photons are diluted along a transverse direction due to gravitational shearing. The effect can be characterized by two spin-weight-two variables, which are reminiscent of the Stokes polarization parameters. Similar to that case, one can construct a scalar and a pseudo-scalar function where the latter only gets contributions from the tensor modes. We analytically determine the power spectrum of the pseudo-scalar at superhorizon scales in a simple inflationary model and briefly discuss possible observational consequences.