Oh, my goodness, Davian, where to begin? There are literally thousands of factors and characteristics to the universe that have to be just right in order for life to exist right here on Earth. Obviously whole books could be and have been written that explicate them in great detail, and just as obviously, I'm not going to be able to do justice to that here. But I realize it wouldn't do to simply leave it at that, so I'll try to offer at least a few examples of the incredible fine-tuning that exists in the cosmos.
For one, even the total density of stars is evidence of fine-tuning. If there were slightly less stars in the universe, nuclear fusion would be too inefficient so that not enough of the heavier elements necessary for life to exist would have formed; there would be an abundance of hydrogen and helium, but far too little if anything of everything else. If there were slightly more stars, there would be too much fusion so that the resulting elements that would exist would all be heavier than iron, with none of the vital lighter elements like carbon, phosphorous, nitrogen, and oxygen.
As well, whereas the density of stars in the universe is just right, as youre likely already aware, of the total mass in the universe stars actually make up a fairly meagre amount of it with about 25% of the universes mass coming from dark matter. It turns out that the amount of dark matter is just right as well, for it kept the universe from expanding too fast after the Big Bang. Otherwise, even with the right number of stars (or rather, the gas that would have coalesced into stars), any nuclear fusion would again have been too inefficient to produce anything but hydrogen that is, the universe would have been nothing but a giant diffusion of hydrogen gas. (And, of course, the corollary would be the case if there were more dark matter in the universe.)
Closer to home, the four outer gas-giant planets in our solar system are just the right size, are spaced out both from each other and from us, and are at just the right distance so that they act to "sweep" all the really dangerous comets and debris away from us. (E.g. from only a few years ago, if virtually any one of the fragments from Comet Shoemaker-Levy 9 had struck Earth rather than Jupiter never mind all twenty-plus of them suffice to say none of us would be here right now to discuss this).
But at the same time, especially during the early formation of the solar system, the gas giants allowed enough to get through so that those objects that did reach Earth brought us those vitally important heavier elements we needed for life here. And they also brought us the crucially important amount of water. Not only is water vital to life, but so also is our planet's ability to sustain water in all three of its states simultaneously. (We used to think water was quite rare in the universe, but more recent discoveries have shown that there is in fact an abundance of it. The problem has been that, of all the planets that astronomers have found and there are even some that are roughly the same size as Earth at roughly the same distance from its star it appears that Earth is so far the only one capable of sustaining water in its three states all at once. What accounts for this? Well
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Anywhere from 30 - 100 million years after Earth formed, a roughly Mars-sized planetoid, called Theia, collided with our planet, thus tilting it on its axis at just the right angle (23.5 degrees relative to the plane of our orbit around the Sun) to give us our life-sustaining seasons. Much more or less and the changes in temperature would be simply too extreme for life.
Additionally, geological evidence indicates that Theia also brought with it a simply huge amount of the elements uranium and thorium. In fact, research shows that because of this collision, Earth now contains at least 16,000 times more uranium than any other detected planet, and at least 23,000 times more thorium. These amounts are necessary for the Earths development of its electro-magnetic field, which, among other things, keeps our atmosphere from dispersing into space, as well as acting as a continual shield against the Suns frequent cosmic radiation blasts that would otherwise have rendered our planet as barren a wasteland as Mars.
And speaking of the atmosphere, the impact of this collision also drove Earth's early atmosphere into space and allowed a more life-friendly atmosphere to form. Theia also increased our mass to just the right size and brought us the balance of the water we needed. And after striking Earth its remains then drifted off to become our moon, which is also just the right size and at just the right distance away to give us the necessary tides to keep our oceans, seas, and lakes clean and life-sustaining. If our moon were just a little closer or bigger in size, the tides would be continuous tsunamis. On the other hand, if the moon were just a little further away or smaller, our oceans and seas would have become brackish and hostile to life aeons ago.
Theia also knocked the Earth into just the right orbit called the Sweet Spot" or the Goldilocks zone (or, less creatively, the habitability zone) where if it were slightly farther from the Sun, it would be too cold, and much closer and the planet would be too hot; similar to if the Sun was slightly smaller or bigger than it is.
I could also mention the fine-tuning of the force of gravity relative to the electric (Coulomb) force. If it were slightly stronger, stars would be smaller and have shorter lifespans. Our Sun is about 4.6 billion-yrs old; about halfway through its life. Thus, if the gravitational force were any stronger, the Sun would have burned itself out by now, cutting too short the necessary time for the evolution of life on the planet.
Furthermore, the planets too would be adversely affected in that they too would likewise be smaller and denser, resulting in higher surface gravities. Even if any life forms were to evolve, because of the shorter time period and greater gravity, they would not evolve much beyond the single-cellular stage. (And thats a huge if!)
It would affect the formation of galaxies, too. They would be smaller and more compact. With stars too close together, competing gravitational forces would tear solar systems apart. Without stable solar orbits, life could not even get started, much less evolve.
On the other hand, if the gravitational force relative to the Coulomb force were much weaker, far fewer stars would form, leaving far fewer planets on which any life could arise.
But truly amazing evidence of fine-tuning takes us back to dark matter. There is about five or more times as much dark matter as ordinary matter (i.e. stars, planets, and gas and dust clouds). Both exert gravity, which causes an attractive tug on the expanding universe. Knowing how rapidly the universe is expanding, its possible to calculate just how much matter of both types there would have to be to stop the universe from expanding. That amount of matter is called the critical density; the amount that is just enough to cause a re-collapse of the universe. If the universe had on average the critical density, it would expand ever more slowly, stopping altogether at some point in the distant future. More than the critical density and the universe would eventually collapse upon itself.
As it turns out, the actual current density is about 30% of the critical density. But if you extrapolate back in time, 30% of critical density today translates into a 99.999999999999999% of critical density one second after the Big Bang. Obviously this is incredibly close to 100%; in fact, one part in a million billion according to British cosmologist Sir Martin Rees.
Here is amazing fine-tuning. If the density immediately after the Big Bang had been only 99.9% of critical, without all the other decimal-place nines, the density today would be many orders of magnitude less. This would have the consequence that the universe would be expanding too fast for stars and galaxies to form.
And had it been just a fraction of a percent greater than 100% of the critical density at the very beginning, the universe would have collapsed long ago.
Thus we have the situation that if the overall density of matter in the universe had been higher or lower than it was at the very beginning, and by an utterly infinitesimal percentage, we would either have a completely lifeless universe with black holes instead of stars, or else nothing but a tenuous gas filling the universe instead of stars and planets. A millionth of a millionth percent difference either way at the time of the Big Bang would have doomed, not just life, but the universe itself one way or the other. THAT is an impressive fine-tuning.
I havent even mentioned the fine-tuning involved concerning the weak and strong nuclear forces, the even bigger mystery of dark energy, quantum clumps, the chemistry of life (re: carbon and oxygen), the extraordinary properties of water, the relative heaviness of neutrons to protons, the role of anti-matter (or rather the lack of it), and I could go on and on.
In the face of all this, even atheist astrophysicist Sir Fred Hoyle calculated that the chance of all the factors necessary for life to have worked themselves out purely via blind, purposeless chance would be the mathematical equivalent of a tornado touching down in a junk yard and leaving in its wake a complete and fully functioning Boeing 747. Now, Im no math genius, but Ive got to think those are some pretty long odds.
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