In science, it’s intriguing to ask the question "What if?" Without assuming that we live in the "best of all possible worlds," we can ask what the universe would have been like if it’s core attributes had been different.
Asymmetry of one part in 109 gave rise to us and all the galaxies in the universe. If the asymmetry had been much smaller — one part in 1011 or less — there wouldn’t have been enough matter for galaxies to form. If the asymmetry had been much larger, the abundant matter would have congealed without forming stars and galaxies. Another dimensionless number from cosmology is the level of fluctuations in the microwave background, one part in 105, which is the "graininess" of the early universe. If it had been much smaller — one part in 106 or less — stars would not have formed. If it had been one part in 103 or more gravity would have formed massive black holes but not normal stars. Apparently the graininess of the early universe was "just so" to generate normal stars.
The cosmic expansion poses other puzzles. The geometry of space is very close to flat. As we saw when discussing the microwave radiation left over from the big bang, the characteristic speckles haven’t been magnified or de-magnified in their long travel through space, and that means space is flat to 1%. Flatness is caused by a dark matter density within a factor of three of a value that sets the current expansion rate near the boundary between endless expansion and future contraction. Very soon after the big bang, when space was curved and expanding insanely fast, there must have been exquisite fine-tuning towards this eventual situation. Much less dark matter and expansion would have been too fast for structure to form; much less and the universe would have re-collapsed before stellar evolution could get established. Dark energy plays a role in this delicate balance. It’s been dominating the expansion in the last five billion years; if it had been much stronger the rapid acceleration would have squelched all structure formation.
These hypothetical outcomes are as intriguing in the micro-world as they are in the macro-world. For example, if the strong force that binds quarks in atomic nuclei were a few percent stronger, quarks wouldn’t form protons, and if it were 5% weaker, stars couldn’t make heavy elements past hydrogen. If the weak force has been much stronger, the big bang would have cooked atoms all the way up to iron, and if it were weaker the stars would have converted all the mass into helium. A stronger gravity force would have led to stars as feeble red dwarfs; a weaker force would have led to fast-burning blue giants. Either way, normal main sequence stars like the Sun would be rare or absent. There are other mass relationships between elementary particles where matter would be unstable if the values or ratios were different. The mathematician Roger Penrose has estimated that the combined probability of all the physical constants having their measured values is 10 to the power 10 to the power 123, a phenomenally unlikely outcome.
The common feature of all these counterfactual universes is the fact that they’re physically sensible, even if they operate under altered laws of physics. However, in almost every case, matter collapses, or heavy elements can’t form, or stars can’t function, or the universe is very short or long-lived. Many scientists and philosophers have taken this situation to imply that the universe is somehow "finely tuned" or exhibits coincidences that have no natural explanation. They then go one further step and say that if the universe were slightly different it would be a sterile wasteland, devoid of life. Stephen Hawking puts it this way "...these numbers seem to have been very finely adjusted to make possible the development of life."
The evocative analogy of a firing squad has been used to understand these issues. You’ve been sentenced to death and are facing a squad of a hundred highly trained sharpshooters. There’s a deafening volley and a cloud and a cloud of smoke. Amazingly, you survive. How are you to react to this outcome? Clearly, you shouldn’t be surprised that you’re alive, since if you had been killed, that would be the end of the story. But you are alive and have survived in the face of overwhelming odds. It seems more likely that you survived for a definite reason than by sheer chance and luck. The low odds of your survival are an analogy to the fine-tuning of the universe and the improbability of life if the conditions had been slightly different.
Should we be seduced by these arguments or maintain a stout and healthy skepticism? Let’s be skeptical. As Hippocrates once said: "Men think epilepsy divine, merely because they do not understand it. But if they called everything divine which they do not understand, there would be no end of divine things." It’s likely there are natural explanations for many anthropoic coincidences, although some will only emerge from frontier theories that aren’t yet well-tested. Also, some instances of fine-tuning are not as fine as has been claimed, and they’re only remarkable in the context of a vast number of hypothetical physical realities. Most importantly, these anthropic arguments depend on knowing the full range of conditions for life and for intelligent observers to evolve. Without a general theory of biology we can only guess. If life elsewhere is stranger than we imagine, then a much wider range of hypothetical universes could support life and our surprise should be correspondingly reduced.