Over the past four centuries, astronomers have made enormous progress in unraveling the universe. At the end of the sixteenth century, the realization that the stars in the night sky were all sunlit, perhaps accompanied by planets such as the earth, was just beginning to dawn, and science still had to get used to the idea that man’s home planet was not a central position in the universe. The telescope had not yet been invented, no one had ever heard of star evolution, that radiation formation other than visible light existed was unknown, and the question of the origin of the cosmos lay in the field of religion.
At the beginning of the twenty-first century, telescopes orbit the Earth, and the universe is studied by colossal viewers, radio telescopes and X-ray satellites. Soft landings have been made on the moon, Venus and Mars, and all planets in the solar system have been closely visited by unmanned space probes. The course of life of stars has few secrets, the energy source of the sun is unraveled, and astronomers have discovered eccentric inhabitants of the Milky Way galaxy like white dwarfs and pulsars. It has been shown that other stars are accompanied by planets and that the countless galaxies in the universe are grouped in clusters and superclusters. Even the question of birth, life course and future of the cosmos as a whole has been answered to a significant extent.
Yet the universe continues to defy our imagination. How big is it? Where does it end? And what’s outside? A finite universe is hard to imagine: is there an edge somewhere, and what lies behind it? How many galaxies are there in such a finite universe, and how is that number determined? But an infinite universe also goes against our common sense. In what way could such an infinitely large space expand? And how could an infinite universe ever arise from nothing?
Equally difficult to imagine is the limit that the cosmos has in time: before the big bang there was no universe, so there was no space, no time, no matter and no energy. But why did that ever change? Why did the absolute Nothing turn into a richly structured Something? What caused the big bang? To what do natural laws owe their existence? And, as Einstein wondered, could the universe have been very different from what it is now?
The latter question in particular has important philosophical consequences, and cannot be satisfactorily answered even by professional astronomers. Nobody knows from which the natural constants derive their values, why the four forces of nature (gravity, electromagnetic force and strong and weak nuclear force) have the strengths they have, and which dictate the natural laws of the present universe. But it is known that a small change in those fundamental properties of nature would lead to a completely unlivable cosmos.
If gravity were slightly stronger or weaker, stars would never have formed, or all stars would turn into black holes shortly after birth. If the weak nuclear force had a slightly different value, all matter would have been converted to helium during the big bang, and other elements would never have emerged. If the mass ratio between a proton and an electron (the building blocks of hydrogen atoms) were different, no stable atoms could exist. In all those cases, life in the universe was never created.
Is the universe specially designed for life? Or is there just coincidence? Could it have been different? Maybe so – then we just wouldn’t have been surprised about that. Or is there a principle of physics that has not yet been discovered from which all natural laws, forces and constants logically result? And why would this all-encompassing theory precisely prescribe a universe in which complex structures such as galaxies, stars, planets, organic molecules and living beings can arise?
Some theorists have no peace with such inexplicable coincidence. They imagine that there are several universes, “parallel” to ours, in which the natural constants have very different values. Every possible combination would have been realized somewhere, and it would seem that we are in the universe where the circumstances are favorable for the emergence of life.
The universe is a wonderful place. The better we get to know the cosmos, the more mysterious it becomes. The scientific exploration of the past centuries has yielded many new insights, but at least as many riddles uncovered. And it will stay that way in the coming period. The adventure has only just begun.