“The poor man’s accelerator”, or how the primordial universe became a testing ground for particle physics

Among physicists and cosmologists, it is common practice to refer to the “cosmic laboratory” to describe how the study of the universe offers important insights into the inner constitution of matter. Yakov Zel’dovich, for instance, famously claimed that the “universe is the poor man’s accelerator.” The goal of this paper is to clarify the relationship between cosmology and particle physics by examining a case of crucial importance in connecting these fields: how cosmologists of the 1970s were able to limit the number of lepton families from the measure of the cosmic abundance of helium-4. My claim is that knowledge of the primordial universe enables cosmologists to conceive it as a natural experiment for testing hypotheses of particle physics, thereby supporting the analogy between the universe and particle colliders that is at the foundation of a recent area of cosmological research.