The Simons Observatory, a new telescope in Chile, began scanning the sky in late 2023 to capture the oldest light in the universe—the cosmic microwave background (CMB). This matters because the CMB is a fossil record of the universe just after the Big Bang. Its patterns encode answers to fundamental questions that current physics cannot explain: what drove the universe’s rapid expansion in its first moments, what gives neutrinos their mass, and what mysterious force is now accelerating cosmic expansion. The observatory’s unprecedented combination of sensitivity, resolution, and sky coverage will allow researchers to extract signals that previous experiments could not detect. If successful, this project could detect primordial gravitational waves—ripples in spacetime from the Big Bang—which would confirm theories of cosmic inflation. It could also measure the total mass of neutrinos, a quantity that particle physics experiments have struggled to pin down, and place the tightest constraints yet on dark energy’s properties. This is fundamental science. It will not produce a new battery or smartphone sensor. But understanding the universe’s basic ingredients and laws has historically enabled unexpected technologies—from GPS (which relies on general relativity) to medical imaging (which uses principles from particle physics).
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The cosmic microwave background (CMB) has been a treasure trove for cosmology and fundamental physics. It is arguably the main driver behind our understanding of the Universe’s composition and evolution from its birth to today. This is thanks to three decades of improvement in sensitivity, in combination with the broad variety of physical signals that can be extracted from CMB observations, which are sensitive both to the physics of the early Universe, and to its large-scale structure and late-time expansion. The Simons Observatory (SO) represents the next step in the quest to constrain fundamental physics from cosmology with the CMB, given its unparalleled combination of sensitivity, resolution, and area coverage. Having seen first light at the end of 2023, the data taken in the next ~5 years will allow SO to place tight constraints on fundamental physical properties of key importance for the community. These include a potential discovery of primordial gravitational waves, detecting the mass of neutrinos, placing a tight bound on the abundance of unknown relativistic particles, constraining the statistics of the primordial metric fluctuations at high precision, and shedding light on the properties of Dark Energy. In this project, we will exploit the first three years of SO data to obtain transformative constraints on these fundamental parameters through a combination of observational probes: large-scale measurements of the B-mode polarisation, high-resolution measurements of the primary temperature and polarisation anisotropies, reconstructions of the gravitational lensing potential, and catalogues and maps of the most massive structures in the Universe using the Sunyaev-Zel’dovich effect. Achieving this goal requires the collaboration between a large team of scientists with expertise in different areas of CMB theory and data analysis, given the inter-connectedness and complementarity of the different CMB probes. Our team, composed of members from six different UK institutions, is optimally placed to carry out this work, given our complementary expertise and current scientific leadership in SO. The Large Awards scheme presents a timely opportunity to establish this coordinated effort, with the first SO science data available towards the start of this grant. Through this project, we will capitalise on the investment already made by STFC/UKRI in SO at the level of instrumentation and infrastructure. This will ensure a leading scientific role for the UK CMB community in one of the most exciting cosmology experiments in the next decade.
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