BEYONDPLANCK
Authors:   BeyondPlanck Collaboration, Andersen K. J. , Aurlien R. , Banerji R. , Basyrov A. , Bersanelli M. , Bertocco S. , Brilenkov M. , Carbone M. , Colombo L. P. L. , Eriksen H. K. , Eskilt J. R. , Foss M. K. , Franceschet C. , Fuskeland U. , Galeotta S. , Galloway M. , Gerakakis S. , Gjerløw E. , Hensley B. , Herman D. , Iacobellis M. , Ieronymaki M. , Ihle H. T. , Jewell J. B. , Karakci A. , Keihänen E. , Keskitalo R. , Lunde J. G. S. , Maggio G. , Maino D. , Maris M. , Mennella A. , Paradiso S. , Partridge B. , Reinecke M. , San M. , Stutzer N.-O. , Suur-Uski A.-S. , Svalheim T. L. , Tavagnacco D. , Thommesen H. , Watts D. J. , Wehus I. K. , Zacchei A. 
Published: February 2023

We describe the BEYONDPLANCKproject in terms of our motivation, methodology, and main products, and provide a guide to a set of companion papers that describe each result in more detail. Building directly on experience from ESA’sPlanckmission, we implemented a complete end-to-end Bayesian analysis framework for thePlanckLow Frequency Instrument (LFI) observations. The primary product is a full joint posterior distributionP(ω ∣ d), whereωrepresents the set of all free instrumental (gain, correlated noise, bandpass, etc.), astrophysical (synchrotron, free-free, thermal dust emission, etc.), and cosmological (cosmic microwave background – CMB – map, power spectrum, etc.) parameters. Some notable advantages of this approach compared to a traditional pipeline procedure are seamless end-to-end propagation of uncertainties; accurate modeling of both astrophysical and instrumental effects in the most natural basis for each uncertain quantity; optimized computational costs with little or no need for intermediate human interaction between various analysis steps; and a complete overview of the entire analysis process within one single framework. As a practical demonstration of this framework, we focus in particular on low-ℓCMB polarization reconstruction withPlanckLFI. In this process, we identify several important new effects that have not been accounted for in previous pipelines, including gain over-smoothing and time-variable and non-1/fcorrelated noise in the 30 and 44 GHz channels. Modeling and mitigating both previously known and newly discovered systematic effects, we find that all results are consistent with the ΛCDM model, and we constrained the reionization optical depth toτ = 0.066 ± 0.013, with a low-resolution CMB-basedχ2probability to exceed of 32%. This uncertainty is about 30% larger than the official pipelines, arising from taking a more complete instrumental model into account. The marginal CMB solar dipole amplitude is 3362.7 ± 1.4 μK, where the error bar was derived directly from the posterior distribution without the need of any ad hoc instrumental corrections. We are currently not aware of any significant unmodeled systematic effects remaining in thePlanckLFI data, and, for the first time, the 44 GHz channel is fully exploited in the current analysis. We argue that this framework can play a central role in the analysis of many current and future high-sensitivity CMB experiments, including LiteBIRD, and it will serve as the computational foundation of the emerging community-wide COSMOGLOBEeffort, which aims to combine state-of-the-art radio, microwave, and submillimeter data sets into one global astrophysical model.