A New Upper Limit on the Polarization of the Cosmic Microwave Background Radiation

TL;DR

This paper reports a new upper limit on the polarization of the Cosmic Microwave Background at large angular scales, using a sensitive experiment that constrains E- and B-type polarization to 10 μK at 95% confidence.

Contribution

It presents the first high-sensitivity measurement setting an upper limit on CMB polarization at large scales, advancing the search for primordial gravitational waves.

Findings

Upper limit on CMB polarization at large scales is 10 μK (95% confidence).

If B-polarization is zero, E-type polarization limit is 8 μK.

The experiment demonstrates the potential of new sensitive instruments for future polarization mapping.

Abstract

The Cosmic Microwave Background Radiation (CMB) is an invaluable probe of the conditions of the early universe. Recent measurements of its spatial anisotropy have allowed accurate determinations of several fundamental cosmological parameters, such as the curvature of the universe, the shape of the spectrum of primordial density fluctuations, and the contribution of baryons, dark matter, and dark energy to the overall energy density of the universe. In addition to being spatially non-uniform, the CMB is theorized to be slightly polarized. Measurements of this polarization, particularly at large angular scales, have the potential to provide information on primordial gravitational waves, theories of inflation, and the ionization history of the universe, as well as help further constrain cosmological parameters. Polarization has not yet been detected in the CMB. This thesis describes a recent search for CMB polarization at large angular scales, conducted in the spring of 2000 at the University of Wisconsin-Madison. After a general introduction on both CMB polarization and general microwave polarimetry, details of the experiment itself are given, as well as a full description of the data selection and analysis techniques. Using these techniques, our data lead to a new upper limit on CMB polarization at large angular scales of 10 $\mu$K in both E- and B-type polarization at 95% confidence. If B-polarization is assumed to be zero, the limit for E-type polarization is lowered to 8 $\mu$K. This experiment is the first of a new breed of highly-sensitive instruments that will one day map out this interesting property of the Cosmic Microwave Background Radiation.