Measuring the Largest Angular Scale CMB B-mode Polarization with Galactic Foregrounds on a Cut Sky

TL;DR

This study evaluates foreground cleaning methods for detecting primordial gravitational wave signals in CMB B-mode polarization at large angular scales, demonstrating improved upper limits on the tensor-to-scalar ratio r using simulated data from the CLASS experiment.

Contribution

It introduces a simulation-based analysis of foreground cleaning effectiveness for measuring the tensor-to-scalar ratio r at large scales with the CLASS survey, achieving tighter constraints.

Findings

95% C.L. upper limit of r<0.017 without primordial gravitational waves

Recovery of r=0.012 with uncertainties around ±0.006 to ±0.011

Improved upper limits to r<0.008 when including multipoles 30≤ℓ≤100

Abstract

We consider the effectiveness of foreground cleaning in the recovery of Cosmic Microwave Background (CMB) polarization sourced by gravitational waves for tensor-to-scalar ratios in the range $0<r<0.1$. Using the planned survey area, frequency bands, and sensitivity of the Cosmology Large Angular Scale Surveyor (CLASS), we simulate maps of Stokes $Q$ and $U$ parameters at 40, 90, 150, and 220 GHz, including realistic models of the CMB, diffuse Galactic thermal dust and synchrotron foregrounds, and Gaussian white noise. We use linear combinations (LCs) of the simulated multifrequency data to obtain maximum likelihood estimates of $r$, the relative scalar amplitude $s$, and LC coefficients. We find that for 10,000 simulations of a CLASS-like experiment using only measurements of the reionization peak ($\ell\leq23$), there is a 95% C.L. upper limit of $r<0.017$ in the case of no primordial gravitational waves. For simulations with $r=0.01$, we recover at 68% C.L. $r=0.012^{+0.011}_{-0.006}$. The reionization peak corresponds to a fraction of the multipole moments probed by CLASS, and simulations including $30\leq\ell\leq100$ further improve our upper limits to $r<0.008$ at 95% C.L. ($r=0.01^{+0.004}_{-0.004}$ for primordial gravitational waves with $r=0.01$). In addition to decreasing the current upper bound on $r$ by an order of magnitude, these foreground-cleaned low multipole data will achieve a cosmic variance limited measurement of the E-mode polarization's reionization peak.