Sommaire:

The steady-state theory, which held that the density of the universe remains constant due to the continuous creation of matter during expansion, died during the 1960s. This death is commonly attributed to the discovery of the Cosmic Microwave Background (CMB) in 1965, which was interpreted as evidence that the universe had evolved from a superdense state—contradicting the core assumption of the steady-state model. Historian Stephen Brush argued that the theory's supporters, who embraced a Popperian view of science, abandoned it in light of the CMB's apparent falsification. However, this narrative has been challenged by other historians, notably Helge Kragh, who downplay both Popper’s influence and the CMB's role in the theory’s downfall. This talk reopens the cold case of the steady-state theory's death. Did the discovery of the CMB really shoot the fatal blow? Was Popper an (unwilling) accomplice? Is the real murderer still unidentified? Drawing on new testimonies from both theorists and observers, I propose an alternative explanation: it was not the CMB, nor Popper, that "killed" the steady-state model, but its declining fecundity during the 1960s. Using new tools of history and philosophy of science, I argue that while Popper played a significant role in the theory’s development during the 1950s, he had little influence on its fate in the 1960s. Likewise, the CMB was not seen as a straightforward falsification of the steady-state theory, nor as direct confirmation of its rival, the big bang model. Rather, it was perceived as a fertile phenomenon that opened new avenues for connecting nuclear and particle physics, general relativity, and radio astronomy.

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