The cosmic coincidences that made our universe – and us – possible

All of a sudden, the sun went out. That is what it felt like to witness last month’s total solar eclipse from a ship off the east coast of Greenland. Sure, there was a bit of a slow fade as the moon moved into position over the course of an hour, but the transition from a little bit of sunlight to none at all was stark. What makes these astonishing cosmic shows possible is a coincidence that is perhaps even more astonishing: the sun is almost exactly 400 times bigger than the moon, and it is also almost exactly 400 times further away. As we look into the sky, that translates to the two of them appearing just about the same size, which allows the moon to cover the disc of the sun, plunging the area under the shadow into sudden darkness. It is another cosmic coincidence that we happen to be alive at just the right time to witness the wonder of a total solar eclipse. The moon is slowly receding from Earth. It is moving less than 4 centimetres a year, but, over the aeons, that time adds up: in a little over 600 million years, eclipses will start to become more and more rare. By 1.2 billion years from now, Earth’s surface will have seen its last total solar eclipse. All this got me thinking about how extraordinary it is that we are here at all. I don’t necessarily mean me in particular, or even humanity – the twin marvels of life arising from non-living matter and evolution are a bit outside my wheelhouse, and frankly too complicated for me to fully comprehend – but the conditions of the universe that make life, and planets, and suns, and even something as fundamental as matter possible. Advertisement This is known as the fine-tuning problem in cosmology. There are dozens of fundamental constants – the strength of gravity, the masses and electric charges of fundamental particles like protons or quarks, and even the density of the universe – and all of them seem to have values that are exactly right for life to arise. In A Brief History of Time, Stephen Hawking summarised this almost-unbelievable coincidence: “The remarkable fact is that the values of these numbers seem to have been very finely adjusted to make possible the development of life… It seems clear that there are relatively few ranges of values for the numbers that would allow for development of any form of intelligent life.” If the expansion rate of the universe were just a little bit higher, it would pull clouds of cosmic gas apart too fast for stars to form. If there were a little more mass in the universe, or if gravity were a tiny bit stronger, everything would have crunched in on itself long ago. If the strong nuclear force were just a little bit stronger, the universe wouldn’t have enough hydrogen, carbon or oxygen for life to arise. I could go on and on, but you get the point: the cosmos is balanced on a razor’s edge where a tiny push in either direction would have rendered it uninhabitable by the likes of us. Naturally, scientists have been arguing over why this might be for ages. It’s what scientists do. The fine-tuning problem has become one of the premier arguments for a multiverse in which infinite universes have infinite combinations of different fundamental constants, so the emergence of at least one with the right constants for life is inevitable. The fact that we happen to be in that one fine-tuned universe then makes a kind of sense, because if it were any of the others, we wouldn’t exist there to observe it. A variant of this notion is the idea that the universe has had many different histories, with different fundamental constants, all in quantum superposition with one another, and the history that applies to us depends on us as the observers. Then, of course, there is the notion that the universe was created by some higher power or technologically advanced master of simulation, and they made it this way on purpose, although that is more the realm of philosophers and theologians than physicists and cosmologists. To me, all of these arguments have always seemed a bit circular. They say “it is how it is, and therefore there was no chance it could have been otherwise”, which is a version of the anthropic principle. But hindsight is 20/20. Now that we are here, the chances of our existence – and of the sun and moon being arranged just right for solar eclipses – are 100 per cent. Still, that doesn’t mean this has always been so. Maybe, at the big bang, there was a sort of crossroads, a moment when things could have been pushed off that razor’s edge. Maybe there could have been a pinch more antimatter to annihilate with matter, and then the universe would have been made of pure radiation. Maybe gravity could have been a skosh weaker, and then there would have been no sun, no moon for it to hide behind, no Earth to watch from. It is the job of science to explain things. But some things are simply inexplicable. And maybe I’m betraying my scientific background and sensibility by saying it, but I don’t think seeing those things as just a wonderful coincidence is all that bad.