To achieve the objectives of the action, we pursued a number of profitable methodological directions, all aligned with the ultimate goal of building a robust end-to-end inference pipeline for 21cm cosmology. Forward-models of high-redshift 21cm observations comprise a number of distinct steps, from modeling and simulating the underlying cosmological signal, through modeling the foregrounds, computing the ideal instrumental response, adding instrumental systematics and noise, and finally performing data reduction and statistical inference. This project advanced the state-of-the-art in each of these steps. We developed a novel modeling technique for simulating the 21cm signal that includes the stochastic contributions of discrete populations of galaxies, implemented within the popular 21cmFAST simulation code. This allows for more accurate predictions of the 21cm signal, but also crucially enables joint analysis with other observations from Cosmic Dawn such as high-redshift galaxy observations from the James Webb Space Telescope. Computing the instrumental response of radio interferometers is computationally expensive, so in this project we developed two new algorithms to accelerate this step: a GPU-accelerated visibility simulator (matvis), and a novel method that exploits Non-Uniform Fast Fourier Transforms (NUFFTs) to compute visibilities more efficiently (fftvis). Both methods reduce computational cost of visibility simulation by up to a factor of 1000 and make complementary trade-offs. We also developed new models for the dominant instrumental systematic affecting HERA: mutual coupling between antennas. We implemented these models within the open-source simulation pipeline hera_sim. We developed a new open-source noise estimator for 21cm power spectrum measurements, 21cmSense v2, which enables fast and flexible forecasting of experimental sensitivities. Notably, the action supported the the PI's work as corresponding author on the very first multi-redshift power spectrum upper limits from HERA Phase II---a significant achievement that has placed the strongest limits on the signal during Cosmic Dawn from any experiment, and establishes HERA's end-to-end analysis pipeline as providing unbiased estimates of the signal over a wide range of scales. This result has been submitted to be published in a peer-reviewed journal. Supporting this significant result, we produced the largest-ever end-to-end instrumental simulations using the tools developed in this action: fftvis and hera_sim. To interpret the result in terms of astrophysics, we used a Bayesian inference tool also developed within this action: 21cmEMU. This is a novel machine-learning based emulator for 21cm power spectra, which enables fast parameter inference over different subsets of data.
Gains were also made in the analysis of EDGES data to validate the first claimed detection of the 21cm global signal. We developed a new data analysis pipeline that incorporates components that can be used in a forward-modeling framework, and this work is close to completion. Simultaneously, we have been applying this pipeline towards two important goals: exploring the impact of various analysis choices on the seminal results of the EDGES-2 experiment (which claimed the first detection of the global 21cm signal from Cosmic Dawn in 2018), and the first processing of the next-generation EDGES-3 experiment. These results are all in the final stages of preparation.
The action and the collaborations it fostered have supported a substantial driving contribution to 9 publications in peer-reviewed journals so far, with three more already submitted, and five more in the latter stages of preparation.
