SO:UK - A major UK contribution to the Simons Observatory

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During the last three decades, measurements of the Cosmic Microwave Background (CMB) have been the driving force in establishing the standard cosmological model. UK scientists have played a pivotal role, particularly in recent times with major roles in ESA's Planck mission. These advances have been hugely important but the CMB's greatest contributions to fundamental physics could well be yet to come. The primary science goals of future CMB experiments include (i) the search for curl or ``B-mode" fluctuations on large angular scales in the CMB polarisation field, a tell-tale signature of primordial gravitational waves from inflation, (ii) to search for new light relic particles beyond the Standard Model through their imprint on the CMB fluctuations on small angular scales, (iii) to use measurements of the gravitational lensing and Sunyaev-Zel'dovich (SZ) effects in the CMB to constrain the sum of the neutrino masses and (iv) to help understand the observed accelerated expansion of the Universe using the low-redshift probes of CMB lensing and SZ measurements. The high sensitivity and high angular resolution of future experiments will also facilitate a wide range of additional frontier science ranging from studies of the reionisation era to searching for additional solar system objects. Simons Observatory (SO) is a US-led international project to construct a group of CMB telescopes in the Atacama Desert in northern Chile. It has been designed to address these new science challenges, and is due to begin operations in 2023. Here we propose a major UK contribution, composed of three main components. Firstly, we will establish a UK-based data centre, which will play a lead role in delivering the primary data products from all of the SO telescopes. Secondly, we will pursue a program of algorithm development work, forming a major contribution to the SO data pipeline software infrastructure. The third strand of our programme is the provision of a single ultra-high-frequency (UHF) optics tube for the SO Large Aperture Telescope.Delivering the data centre, and the algorithms and processing functions needed for the data pipeline, will address a critical need within the SO project and will position SO:UK scientists optimally for taking lead roles in the subsequent headline science exploitation of the SO data. The UK-based data centre will also help facilitate joint analyses (by the wider UK cosmology and astrophysics community) of the SO data in combination with data from other flagship UK astronomy projects, including the Euclid satellite, the Vera Rubin Observatory and the Square Kilometre Array. The SO:UK instrument includes the development and demonstration of key technologies including Kinetic Inductance Detectors (KIDs) and meta-material (MM) quasi-optical components. Demonstrating the compelling advantages of these UK-driven technologies as part of the leading CMB experiment of the 2020s will be a powerful argument for their adoption in future CMB projects (including the ground-based CMB-S4 experiment and a possible future ESA-led satellite mission) as well as in future projects in other high-profile areas of extra-Galactic astronomy and cosmology.