A collaboration I’m part of submitted two papers for publication this week, containing the first batch of results from an ongoing effort to simulate interferometric observations of cosmic microwave background polarization.
If you don’t already know what all the words in that last sentence mean, you probably won’t be terribly interested in the papers. Here’s the executive summary of the project as a whole:
- The cosmic microwave background radiation, which is the oldest light in the Universe, contains bunches of information about what things were like shortly after the Big Bang.
- There are very good reasons to believe that highly sensitive measurements of the polarization of this radiation will give us extremely valuable information that we can’t get any other way, but sufficiently sensitive measurements haven’t been made yet.
- Consequently, lots of people are trying to make those sensitive measurements.
- We’re interested in the possibility that a kind of instrument called an interferometer may do better than traditional imaging telescopes at keeping various possible sources of error under control.
- To see whether this is true, and if so to try to convince other people (e.g., funding agencies) that this is a good way to make these measurements, we’re simulating the performance of these instruments.
As usual, the vast majority of the work that went into these papers was done by the young people, Brown graduate student Ata Karakci and Wisconsin postdoc Le Zhang.
Titles, abstracts, links for those who want them:
Karakci et al., Bayesian Inference of Polarized CMB Power Spectra from Interferometric Data, arXiv:1209.2930.
Detection of B-mode polarization of the cosmic microwave background (CMB) radiation is one of the frontiers of observational cosmology. Because they are an order of magnitude fainter than E-modes, it is quite a challenge to detect B-modes. Having more manageable systematics, interferometers prove to have a substantial advantage over imagers in detecting such faint signals. Here, we present a method for Bayesian inference of power spectra and signal reconstruction from interferometric data of the CMB polarization signal by using the technique of Gibbs sampling. We demonstrate the validity of the method in the flat-sky approximation for a simulation of an interferometric observation on a finite patch with incomplete uv-plane coverage, a finite beam size and a realistic noise model. With a computational complexity of O(n3/2), n being the data size, Gibbs sampling provides an efficient method for analyzing upcoming cosmology observations.
Zhang et al., Maximum likelihood analysis of systematic errors in interferometric observations of the cosmic microwave background, arxiv:1209.2676.
We investigate the impact of instrumental systematic errors in interferometric measurements of the cosmic microwave background (CMB) temperature and polarization power spectra. We simulate interferometric CMB observations to generate mock visibilities and estimate power spectra using the statistically optimal maximum likelihood technique. We define a quadratic error measure to determine allowable levels of systematic error that do not induce power spectrum errors beyond a given tolerance. As an example, in this study we focus on differential pointing errors. The effects of other systematics can be simulated by this pipeline in a straightforward manner. We find that, in order to accurately recover the underlying B-modes for r=0.01 at 28 < l < 384, Gaussian-distributed pointing errors must be controlled to 0.7 degrees rms for an interferometer with an antenna configuration similar to QUBIC, in agreement with analytical estimates. Only the statistical uncertainty for 28 < l < 88 would be changed at ~10% level. We also show that the impact of pointing errors on the TB and EB measurements is negligibly small.