SO:UK - A major UK contribution to Simons Observatory

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 contribution to fundamental physics could well be yet to come. The field is now turning to the search for primordial gravitational waves, which, if present, would imprint a very specific pattern (termed "B-modes") on the polarisation of the CMB. This signal is predicted to be present in many theories of inflation, a period of rapid expansion thought to have occurred at the very beginning of our Universe. Observing primordial B-modes would thus provide a probe of physics at very early times - or equivalently at very high energies, far beyond the energies accessible to ground-based particle physics experiments. A detection of the B-mode signal would open a unique observational window on fundamental physics and would almost certainly result in a third Nobel prize for the field of CMB studies. In addition, by measuring the effects of gravitational lensing on the CMB, future experiments will also provide unique insights into neutrino physics, cosmic acceleration & dark energy, the nature of dark matter, and the end of the dark ages.

The 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. Thirdly, we will build, deploy, commission and operate two small aperture telescopes (the SO:UK instrument) to complement the existing (US) SO instrument plans. The UK instrument will form a major component of the SO, providing up to 50% of its sensitivity to primordial B-modes. The additional sensitivity that will be provided by the SO:UK instrument has the potential to bring a compelling class of inflation models within our reach, for the first time.

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.

In building the instrument, we will incorporate a number of innovative features including the first use of a new detector technology (Kinetic Inductance Detectors, KIDs) in a CMB B-mode experiment. In addition to KIDs, the instrument work also includes the development and demonstration of novel meta-material (MM) quasi-optical components and high-performance detector readout technology. 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 $600M+ CMB-S4 project towards the end of this decade 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.