The Atacama B-Mode Search: Instrumentation and Observations

Abstract

The Atacama B-Mode Search (ABS) is an experiment designed to

probe the physics of the early universe by searching for the

signature of gravitational waves in the Cosmic Microwave Background (CMB). Primordial gravitational waves, if present, imprint a signal onto the CMB, the so-called ``B-mode" polarization.

The challenge to measuring the CMB B-mode signal is its diminutive amplitude, currently constrained to be less than $\sim\!50$\,nK on the sky, some 2000 times below that of the CMB temperature anisotropy.

To measure or further constrain the B-mode signal requires

high sensitivity and control of systematic error.

To tackle this problem, ABS observes with 480 transition-edge sensor bolometers. The detectors are coupled to the sky through a fully cryogenic all-reflective telescope optimized for fidelity of the polarization signal.

For a ground-based experiment, such as ABS, the measurement

is further complicated by emission from water vapor in the atmosphere.

To minimize atmospheric contamination, ABS observes from

Cerro Toco in the Atacama desert, an excellent observing site due to the

high altitude (5190\,m) and the low levels of water vapor.

ABS additionally pioneered the use of an ambient-temperature continuously rotating half wave plate to modulate the celestial polarization signal, enabling rejection of the fluctuating unpolarized atmosphere.

This thesis describes the ABS instrumentation and observations.