I wanted to see if I could run the numbers and test how ridiculously contrived this hypothesis would have to be if it were true. I assumed that for the Earth to expand, the mantle and core would have to behave like an expanding fluid. Honestly, it's the only physical process I can think of that even barely works, but it still fails horrifically. Here's what the thermodynamics say:
1. The core of the Earth would have an average temperature of around 17,760 K (~3 times the surface temperature of the Sun). a) Temperatures like this aren't impossible to find in the universe, but they're not likely for a smaller planet like ours. To put it in perspective, it would be like stuffing Saturn's hot core into a planet the size of Mars! I’m no expert in geology, but I’d be amazed if extreme conditions like that didn’t leave long-lasting marks on our planet. b) If Earth's heat flow had been relatively consistent throughout its history, the cooldown time of the Earth's core to its current temperature would have taken more than three times the entire age of the universe!
2. An expanding Earth would also have started with a surface gravity of 33.8 m/s² (about 3.4 Gs, or 1.4 times Jupiter's surface gravity). I don't care how oxygen-dense the surface might be; the bones of megafauna would simply be unable to keep up. Hell, fuana on the humans scale would have struggled.
3. I found that the early thermal pressure of the core and mantle could provide up to 75% of the energy needed to break Earth's binding energy. For those who don’t know, energy amounts above Earth's binding energy are enough to physically obliterate the planet. That’s one Earth-Theia collision event away from shattering the planet into oblivion. a) Speaking of which, why can't we see any visible signs of inflation on the Moon? Old craters still retain their shape, marks, and ridges, yet expansion should suggest that craters would smooth out over time or at least be discontinuous. b) If I use the same process for Jupiter, it highly suggest that Jupiter should have started off as possible Brown Dwarf star. When fusion starts, it's hard to stop. Yet for some reason, Jupiter just decided to change career paths I guess.
4. I also found that the core density of an expanding Earth would have started at half the density of the Sun's core. Which brings an important question: What would even cause these insane initial conditions to form anyway? We're talking about stellar-level packing on a random dusty ball in the middle of nowhere space. a) We have a scenario where, for some reason, all the elements of the Earth are compressed beyond there natural sizes for a planet like ours. b) Conditions like this would have made it nearly impossible for complex life to form as early as it did. Part of me feels like it's a gateway to creationist Earth hypotheses (be it Scientology or whatnot) there are just to many unexplainable questions as to why Earth started off so unnaturally
5. Furthermore, why don’t we see evidence of planet expansion elsewhere in the universe? As we look into deeper regions of space, redshift should allow us to see the early universe. We should see exoplanets that are smaller on average than in the early universe vs. those in the later universe, but we don’t observe that trend at all. And if this mechanism works on our planet, why don’t we see it on larger-scale objects like galaxies? Don’t get it confused—the universe itself is expanding (i.e., the distances between objects), but gravitationally bound structures like galaxies don’t change size for no reason. These are also structures that tend to have more outward pressure, unlike Earth. But no expansion to speak of.
