Pope Pius XII was no supporter of YEC. Pius XII was an enthusiastic advocate of the Big Bang theory, which originated with Abbé Lemaître.
Here is an excerpt from the
Address of Pope Pius XII to the Pontifical Academy of Sciences, November 22, 1951 which by its nature suggests just how truly alien YEC thinking is to the Catholic Church:
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C. THE UNIVERSE AND ITS DEVELOPMENT
In the future:
31. If the scientist turns his attention from the present state of the universe to the future, even the very remote future, he finds himself constrained to recognize, both in the macrocosm and in the microcosm, that the world is growing old. In the course of billions of years, even the apparently inexhaustible quantities of atomic nuclei lost utilizable energy and, so to speak, matter becomes like an extinct and scoriform volcano. And the thought comes spontaneously that if this present cosmos, today so pulsating with rhythm and life is, as we have seen, insufficient to explain itself, with still less reason, will any such explanation be forthcoming from the cosmos over which, in its own way, the shadow of death will have passed.
In the past:
32. Let us now turn our attention to the past. The farther back we go, the more matter presents itself as always more enriched with free energy, and as a theater of vast cosmic disturbances. Thus everything seems to indicate that the material universe had in finite times a mighty beginning, provided as it was with an indescribably vast abundance of energy reserves, in virture of which, at first rapidly and then with increasing slowness, it evolved into its present state.
33. This naturally brings to mind two questions:
Is science in a position to state when this mighty beginning of the cosmos took place? And, secondly, what was the initial or primitive state of the universe?
34. The most competent experts in atomic physics, in collaboration with astronomers and astrophysicists, have attempted to shed light on these two difficult but extremely interesting problems.
D. THE BEGINNING IN TIME
35. First of all, to quote some figures-which aim at nothing else than to give an order of magnitude fixing the dawn of our universe, that is to say, to its beginning in time- science has at its disposal various means, each of which is more or less independent from the other, although all converge. We point them out briefly:
(1) recession of the spiral nebulae or galaxies:
36. The examination of various spiral nebulae, especially as carried out by Edwin W. Hubble at the Mount Wilson Observatory, has led to the significant conclusion, presented with all due reservations, that these distant systems of galaxies tend to move away from one another with such velocity that, in the space of 1,300 million years, the distance between such spiral nebulae is doubled. If we look back into the past at the time required for this process of the “expanding universe,” it follows that, from one to ten billion years ago, the matter of the spiral nebulae was compressed into a relatively restricted space, at the time the cosmic processes had their beginning.
(2) The age of the solid crust of the earth:
37. To calculate the age of original radioactive substances, very approximate data are taken from the transformation of the isotope of uranium 238 into an isotope of lead (RaG), or of an isotope of uranium 235 into actinium D (AcD), and of the isotope of thorium 232 into thorium D (ThD). The mass of helium thereby formed can serve as a means of control. This leads to the conclusion that the average age of the oldest minerals is at the most five billion years.
(3) The age of meteorites:
38. The preceding method adopted to determine the age of meteorites has led to practically the same figure of five billion years. This is a result which acquires special importance by reason of the fact that the meteorites come from outside our earth and, apart from the terrestrial minerals are the only examples of celestial bodies which can be studied in scientific laboratories.
(4) The stability of the systems of double stars and starry masses:
39. The oscillations of gravitation between these systems, as also the attrition resulting from tides, again limit their stability within a period of from five to ten billion years. 40. Although these figures may seem astounding, nevertheless, even to the simplest of the faithful, they bring no new or different concept from the one they learned in the opening words of Genesis: “In the beginning . . .,” that is to say; at the beginning of things in time. The figures We have quoted clothe these words in a concrete and almost mathematical expression, while from them there springs forth a new source of consolation for those who share the esteem of the Apostle for that divinely inspired Scripture which is always useful “for teaching, for reproving, for correcting, for instructing” (2 Tim., 3, 16).
There are different theories that attempt to explain design. Some theories just have it all mixxxxed up philosophically.