The goal of this collaborative research is to conduct the first end-to-end simulations of systematic errors in Cosmic Microwave Background (CMB) experiments conducted with interferometers. A variety of cosmological processes have imprinted the CMB radiation with faint polarization signals, including Thomson scattering, gravitational lensing, and gravitational waves released during inflation. Measurements of these faint signals will determine key parts of the cosmological puzzle, among which are the energy scale of inflation and the masses of the neutrinos. Detecting these small signal levels is extremely challenging, and this program will help determine the best ways to accomplish such measurements.
The broader impacts of this program include training of undergraduate students in research, and public release of simulations which will be useful to future CMB experiments.
Precision measurements of the cosmic microwave background (CMB) radiation, the oldest light in the universe, allow us to probe conditions in the very early universe. Measurements of these faint signals will determine key parts of the cosmological puzzle: the energy scale of inflation, the masses of the neutrinos, etc. Measurements at these small signal levels are extremely challenging. Detector technology has matured to the point where raw sensitivity is no longer the main obstacle. Now the main challenge is systematic effects. One way to observe the CMB is to use a custom-built radio interferometer. Although radio interferometers are traditionally constructed to achieve high angular resolution, for these applications a more important feature is that interferometers have traditionally demonstrated excellent control of many types of measurement errors. To quantify these errors we have conducted end-to-end computer simulations of hypothetical radio interferometers designed to measure temperature and polarization fluctuations in the CMB. We developed a general purpose simulation code based on a type of Bayesian statistics called Gibbs sampling and used it to test systematic errors in a wide variety of interferometer implementations. Input parameters include models of the CMB sky, the primary antenna beam patterns, placement of antennas, scan strategy, and a noise model for the detectors. The first part of the code (Generalized Interferometer Simulator, or GIS) generates simulated interferometer signals from artificial CMB skies. A second code (called intergibber, for Interferometric Gibbs sampling) recovers a map of the sky along with the power spectrum (a measure of the fluctuations in the sky as a function of angular scale). We have used these codes to simulate many types of systematic errors and determined that most of them are far below the level at which they could contaminate the cosmological signal. These simulation tools can also be applied to a variety of realistic interferometer setups and observational goals. We have demonstrated that they can accurately recover images from any radio interferometer, not just CMB instruments. We have begun to extend them to analyze simulated 3-D maps of the cosmos from observations of the 21 cm line from neutral hydrogen gas over a large range of redshifts. Analysis of these ‘data cubes’ may allow even more precise measurements of cosmological parameters than can be achieved with the CMB alone.
