Life, the universe and everything
Evolution by natural selection can explain how, starting from simple self-replicating entities, complex life forms came about. Not all the details are in place, but plenty is known and there are good theories to explain most of the gaps.
Physicists face a different conundrum. Why is the universe so remarkably suited to life? So many factors needed to make life possible – the value of physical constants for example – are just right; a little higher or a little lower and life would be impossible. It is almost as if the universe is designed for life…
The classic example is 'dark energy', a mysterious, hypothetical substance that is supposed to be responsible for most of the universe's energy density. It has 'negative pressure' and explains why the expansion of the universe is accelerating even though there does not appear to be enough matter in the universe to drive expansion.
Dark energy is thought to make up around three-quarters of the density of the universe (even though it is not actually very dense - just 10–29 grams per cubic centimetre). But there is a big problem with dark energy: current theories predict a value that is too large by the small matter of 10120.
For galaxies, planets and life to exist, its value has to be cancelled out almost exactly. That would be a pretty extraordinary coincidence…
Anthropic principle
How do we deal with the extraordinary good fortune that makes the universe suitable for human life? One answer is that it also implies the existence of a designer. As with intelligent design and biology, this is not a scientific approach.
An alternative view is that there is still some yet to be discovered physics that could supply the answer. Certainly, no one claims our knowledge of physics is complete.
A third view is that if things were any other way we would not be around to know it. The fundamental constants of nature just happen to be consistent with the emergence of some complex chemistry in an obscure corner of the universe. Because we are here, the values had to have the values they do. This is known as the anthropic principle.
The anthropic principle is an answer of sorts, but feels inadequate. As some physicists put it, it's an observation not an explanation.
A similar argument can be applied to the long odds against human life evolving on Earth – if it had not happened we would not be here to witness it. As covered elsewhere, this is a false analogy in biology because, although based on random chance, natural selection provides a mechanism that can lead to many forms of life in a series of sequential steps.
Multiverses
So what might be the cosmological equivalent of natural selection? What could change an astronomical fluke to a virtual certainty? Our understanding of cosmological physics is far from complete, so the answer may still be hidden, but there is at least one possible solution: the multiverse theory.
Although there are a number of variations of this theory, the general idea is that there is not one universe but many, perhaps an infinite number. Some theories of quantum gravity, for example, suggest there are 10500 universes, with fundamental constants different in each. We occupy one of the 10500 universes in which the fundamental constants are compatible with human life.
Some version of the currently fashionable superstring theory, which sees the physical world as made up of vibrating subatomic strings that exist in 10 or 11 dimensions, also generate the possibility of multiverses. In the 'landscape' model, the value of fundamental constants can vary at different locations; we happen to live in an area where they are just right for life to emerge. For some physicists this sounds alarmingly like a version of the anthropic principle.
This area is another of active debate. The mystery of our universe is far from solved. It is also challenging to know how it will be solved. One of the criticisms of superstring theory is that it is difficult to test experimentally – and so can it really be considered science at all?
One interesting consequence of some multiverse models is that there are parallel universes almost exactly like ours, containing other versions of ourselves. It might be a relief to know that the nearest other 'you' is 10 to the power of 1029 metres away.