Periodic Reporting for period 4 - Bits2Cosmology (Time-domain Gibbs sampling: From bits to inflationary gravitational waves)
Reporting period: 2022-10-01 to 2023-09-30
This collaboration has been a resounding success, and the first major end-to-end Commander-based data release was presented to the public at the BeyondPlanck release conference on November 18-20, 2020. That release was based on a suite of 17 scientific papers describing both algorithmic breakthroughs and scientific applications. Prof. Hans Kristian Eriksen (PI of both bits2cosmology and BeyondPlanck) gave the opening talk at the conference, which was attended by more than 170 scientists from 27 institutions in 6 continents. Overall, the release conference was a resounding success for both BeyondPlanck, which now has delivered new state-of-the-art Planck LFI products to the community, and for the bits2cosmology project, which has managed to establish Commander3 as a new industry standard for end-to-end CMB analysis, and demonstrated this on a leading real-world data set.
In parallel, another ERC-funded project called Cosmoglobe (PI: Prof. Ingunn Kathrine Wehus at the University of Oslo) aims to combine all available state-of-the-art experiments into one coherent global model of the radio, microwave and sub-mm sky. As such, the algorithmic foundation developed in bits2cosmology represents an ideal framework for performing such work, and the work performed in bits2cosmology is already guaranteed to have a long-lasting impact because of the much larger Open Source Cosmoglobe project.
1) Development of a new and efficient Gibbs-based CMB mapmaking algorithm that for the first time supports end-to-end error propagation of correlated noise at full angular resolution. This is an extremely important example of the synergies arising from the BeyondPlanck/bits2cosmology collaboration: The original idea was proposed by Dr. Elina Keihänen (BeyondPlanck), and then implemented and optimized in Commander by the bits2cosmology team. The result was a speed-up of almost one order of magnitude compared to the idea that was originally outlined in the bits2cosmology proposal, which was relied on traditional Conjugate Gradient-based maximum-likelihood mapmaking.
2) In-memory data compression. For the first time in CMB history, we now store time-ordered data compressed in memory, using so-called Huffman compression, which saves about a factor of five in the total amount of RAM needed for the analysis. This translates directly into a lower computational costs, and we are now in fact able to analyze the entire Planck LFI data set on one single compute node!
3) A detailed time-dependent model of the Planck LFI instrument, in which the instrument noise properties are allowed to vary from hour to hour. This new instrument model has in turn resulted in a fundamentally new understanding of both systematic effects in the LFI data and better sky maps overall.
4) A new Planck LFI gain model that uses external data from NASA's WMAP satellite mission to break internal degeneracies in the Planck data set. Likewise, using the new Planck data, we have isolated poorly measured modes in WMAP that later can be solved by also analyzing WMAP in terms of time-ordered data. As a result of this joint analysis, both experiments improve.
5) A new sky model, including novel constraints on polarized synchrotron emission. Importantly, we limit any spatial variations in the spectral index of polarized synchrotron emission, and show that this quantity is similar in two of the brightest regions in the sky. This is an important conclusion for future B-mode experiments.
6) The first ever cosmological constraints with true end-to-end Bayesian error propagation.
Overall, this development and demonstration was precisely the main goal for the entire bits2cosmology.