Direct astronomical observations allow us to probe the history of the Universe as far back as light is able to travel, and infer its evolution as far back as established physics is applicable. But during the very first second of the life of our Universe, conditions were so extreme that a description involves physics at the fundamental scale, in terms of quantum particles and fields. In this way, observations of the early Universe works in tandem with high-energy collider experiments on Earth to help us uncover the microscopic properties of matter, stress-testing General Relativity and exploring the origins of our Universe.