Is the speculation that the basic architecture of the Universe should be as simple as possible. It's a sort of Occam's Razor for the Universe, not necessarily true, but an explanation with initial, intuitive appeal for scientists. The PCP states the Universe on its largest scale should be as uniform as possible. Sure, at the local level, one would see variation and heterogeneity, but if you looked at a big enough piece of it, it would be the same as a similar sized piece of it anywhere or anywhen else. That is, no matter where, or when, you were located, if you looked around you would see the same thing: an expanding universe of galaxies receding into the infinite. No place or time in the universe would be special, or unique. The PCP was the perfect expression of the Principle of Mediocrity; that there is no place or time in the universe fundamentally different from any other. It is the logical extension of the universality of natural law, the idea that the laws of physics are the same anywhere and anywhen. That's the closest thing to religious faith a cosmologist can allow himself.

When Hubble showed observationally through the measurement of the Red Shift that the universe was expanding, a modification to the PCP had to be proposed to save the theory from the universe's eventual death by dilution, and its birth in a singularity. This would mean the universe was finite in space and time and was evolving into a different condition. It had a destiny and a history, which violated the PCP and every prejudice and intuition about the universe held by scientists. Creation, evolution, perhaps death, a finite age, perhaps even a finite size, no, it would just not do. It sounded too much like all that god nonsense.

To preserve the PCP, new matter had to be spontaneously created to maintain the mean density of matter constant as the universe expanded into vacuum. Spontaneous creation of matter from nothing isn't really such a radical idea, after all, either it is being continuously created to maintain the PCP, or it was all created at once in the singularity. Take your pick. The necessary creation rate was calculated to be quite modest: 1 atom of hydrogen per cubic meter of space per century. This would provide enough new matter to maintain a constant density as the universe expanded forever into infinity, and it was safely protected from pesky challenge by observational astronomy and experimental physics. Such a rate of spontaneous creation could never be detected in the laboratory or observed at the telescope. It was just too tiny.

The major objection to this idea was "Where do the elements originate?" The earliest proponents of the Big Bang theories argued you needed a singularity to create a fireball massive enough to create the periodic table by hydrogen fusion. Fred Hoyle, a British astronomer who was a champion of the Big Bang's rival theory, the Steady State Theory, came up with a solution, and it just happened to be true. He proposed the chemical elements were produced by thermonuclear reactions inside stars, and returned to the interstellar medium by supernovae and outgassing by dying stars. This eliminated the need for a fireball at the beginning of time to create them. The only creation you needed was this magical introduction of hydrogen from nowhere (1 atom/M^3/century). Everything could be explained with only one a priori (or is it ad hoc?) assumption.

Unfortunately, in the 60s incontrovertible observational evidence of the Big Bang was revealed, the cosmic microwave background radiation, and it no longer was possible to deny the Big Bang, there apparently was NO Perfect Cosmological Principle. Other evidence, concerning isotopic abundances, soon confirmed this. This debate was still going on when I started my undergraduate work in astronomy, a coincidence of truly cosmic proportions. I can rightly claim I was present at the birth of the universe as we know it.