Galaxy clusters are the most massive gravitationally bound objects in the Universe today. Their alluring beauty so prominently on display in the present-day Universe - mega-parsec-sized structures containing up to thousands of galaxies residing in massive dark matter halos - belies what is a complex and prolonged evolutionary history. This seminar delves into fundamental questions about the early stages of galaxy cluster formation, specifically examining protoclusters during the Cosmic Dawn Era at redshifts greater than 6. Our recent investigation, based on the COSMOS2020 catalogue featuring ultra-deep photometry of approximately 10^6 sources across the ~2 square degree COSMOS area, reveals promising protocluster candidates at redshifts between 6 and 8. We derive their dark matter halo masses, and compare their abundances with expectations from current models of hierarchical structure formation. Utilizing Keck, we spectroscopically confirm two of our z=6 protocluster candidates, and find that they are indeed very massive and rich structures that most likely will end up as Coma-like clusters at the present day. Notably, these structures appear embedded at the intersection of large-scale filaments, supporting the hypothesis that protoclusters form at the nodes of such filaments. Studies such as this will soon be possible on a much larger scale through coordinated surveys with Euclid and ground-based telescopes. Combined with follow-up efforts with JWST and ALMA, and numerical simulations of increasing scope and complexity, we are now for the first time in a position to answer the most fundamental questions regarding the formation and evolution of galaxy clusters.
