  Here Bondi speaks of Milne’s cosmological principle. According to Milne’s principle, every observer in the universe should get the same world picture, that is, should make precisely the same observations of the universe at the same moment as any other observer (Milne 1934b). Uniformity over spatial slices is guaranteed by Milne’s invoking of the principle. Yet Milne’s universe evolves, it changes its form over time. Hence it has no temporal uniformity. Bondi felt that this raised the possibility that physics itself might change over time. Because of this risk, Bondi generalized Milne’s cosmological principle into what he called the perfect cosmological principle [= PCP]. According to this principle, all observers at all places and at all times will look out upon the same unchanging, unevolving, universe. Such a universe is a universe in a steady-state—hence the name.