Where Did All the Energy Go?
This is an incorrect recollection! As the expansion of the universe proceeded, the ZPE built up with time. It did not decrease! That has always been the proposition. What is happening in the simplest terms is likened to a rubber band being expanded out or a balloon being inflated. This process puts energy into the fabric of the rubber band or balloon. In the case of expanding out the fabric of space (which is another topic itself!) this energy ultimately appears as the Zero Point Energy (ZPE). As long as the expansion went on, the ZPE built up. The observational evidence of hydrogen cloud separation suggests that expansion went on for some time, then slowed and came to a halt and that we have a static, but slightly oscillating universe since then. Because the transfer of energy to the vacuum was a two-step process, the ZPE continued to build for a little time even after the expansion ceased.
This building energy intrinsic to the vacuum affected (slowed) the speed of light and many atomic processes, like atomic clocks and radioactive decay. The reason is that the ZPE is electromagnetic in character and its strength affects the electric and magnetic properties of the vacuum. Since light is an electromagnetic phenomenon and because atomic interactions are basically electromagnetic in character, both will be affected by the ZPE build-up. In fact both will be slowed as space has become “thicker” with energy. Recent work on quantum mechanics shows that the ZPE supports atomic structures across the cosmos. In actual fact the strength of the ZPE determines the energy of atomic orbits. Thus when the ZPE strength was lower, atomic orbit energies were also lower. This means that all light emitted from atoms in the early days was less energetic or redder, and this gives us the redshift. In fact the redshift is one measure of the ZPE strength in the early universe.
Again, things are not correctly recalled. The energy intrinsic to the vacuum, the ZPE, has built up with time, not decreased. And, importantly, that energy is still there in the vacuum. Each cubic centimeter of the vacuum is acknowledged by secular science to contain about 10110 joules of energy. It can be measured indirectly by the Casimir effect. This is the energy which the Eternal put into the fabric of space when He stretched the heavens. This energy now supports all atomic structures throughout the cosmos. When Colossians and Hebrews state that He supports all things with the word of His own power, this is how He is doing it. Withdraw the ZPE for a moment, and every atomic structure throughout the universe would disappear in a flash of light. The result would be the fireball of 2 Peter 3:10.
So energy has been conserved; it is still there. And every atomic orbit has the energy it does because of this.
In reply let me assure you that your recollection here is basically correct. But we need to be a bit careful. In the early days when the ZPE strength was low, radioactive decay rates were indeed faster. Plasma physics shows that the planets were formed already layered and cool, and that the radioactive elements were largely concentrated towards the centers of the planets because of the sorting process of elements in plasma filaments called Marklund convection. This means that rapid radioactive heating then built up the temperatures of the planetary bodies to what we have today. This is not a problem for the model, but rather one point in its favor.
So in your final question/summary it is certainly true to say that with the ZPE model, energy is conserved.