Preludes to dark energy: Zero-point energy and vacuum speculations

Although dark energy is a modern concept, some elements in it can be traced back to the early part of the twentieth century. This paper examines the origin of the idea of zero-point energy and in particular how it appeared in a cosmological context in a hypothesis proposed by Walther Nernst in 1916. The hypothesis of a zero-point vacuum energy attracted some attention in the 1920s, but without attempts to relate it to the cosmological constant that was discussed by Georges Lema\^itre in particular. Only in the late 1960s was it recognized that there is a connection between the cosmological constant and the quantum vacuum. As seen in retrospect, many of the steps that eventually led to the insight of a kind of dark energy occurred isolated and uncoordinated.