鈥淲E LIVE in the best of all possible worlds.鈥� So insists Professor Pangloss against all evidence to the contrary in the 1759 satire Candide, anyway. The writer Voltaire intended the character to lampoon certain theologically tinged scientific thinkers of the day, who insisted that things couldn鈥檛 be bettered because a beneficent deity would hardly have willed them otherwise.
Gods and their intelligent designs are less in the mainstream of scientific thought now, yet similar ideas about an optimal universe still trickle through cosmology. That is principally down to some mysterious numbers that determine its workings. Tot them all up in the standard models of particle physics and cosmology, and you end up with about 30 constants of nature 鈥� numbers like the strengths of the fundamental forces and the masses of elementary particles that our theories can鈥檛 explain, but are just 鈥渢here鈥�.
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Change many of these constants, and nothing happens. 鈥淏ut with others, it鈥檚 drastic, not to say lethal,鈥� says cosmologist . Alter the relative strengths of gravity and electromagnetism just a little, say, and stars and galaxies can鈥檛 form. Flip the tiny difference in the proton and neutron鈥檚 masses to make the proton heavier, and you don鈥檛 even get stable atoms.
鈥淐hanging these numbers would probably preclude any life in the universe,鈥� says Davies. It isn鈥檛 a big leap to say it looks like the knobs have been twiddled 鈥� as if the universe were somehow fine-tuned for our existence.
Stuff and nonsense, says . 鈥淚t is sad that even good scientists fall in this trap,鈥� he says. 鈥淭he universe is not 鈥榡ust right鈥�, it is what it is.鈥� For him, to say the cosmos is fine-tuned is a failure of imagination coming from no one really having the slightest idea how the universe would be if its vital parameters were any different. 鈥淚t might be perhaps far more varied and interesting and with all sorts of strange, complex entities asking silly questions about how very, very just-right their universe is,鈥� says Rovelli.
Back in 2008, astronomer . He showed, for example, that if the strong nuclear force were just a little stronger, stars could synthesise carbon more efficiently, creating a wider time window for life to evolve. On that reading, our 鈥渇ine-tuned鈥� cosmos is a botch job.
For many cosmologists today, to think the universe is in some way special is to make a mistake that has long plagued human thought: to think we are special. 鈥淓very scientific development in cosmology, starting from Copernicus onward, has shown us that we are not,鈥� says . To extend that thinking to the wider cosmos is, in part, a balance-of-probabilities hunch. 鈥淵ou have to decide if the origin of the universe is a natural, or a supernatural, event,鈥� says Davies. 鈥淚f it is a natural event, you wouldn鈥檛 expect it to happen just once.鈥�
Such thinking is increasingly also an end point of many scientifically grounded hypotheses. One such is cosmic inflation, a flash of faster-than-light-speed expansion in the big bang鈥檚 first instant that, for most cosmologists, is the best way to explain many features of the universe. Inflation, if it happened, would have been very difficult to stop, creating infinite universes budding off from one another faster than light speed, losing all causal connection with one another 鈥� and probably developing their own properties and parameters.
鈥淚f they do, just by chance, here and there, the numbers will come out just right,鈥� says Davies. In such a multiverse of many possible universes, our just-right universe is simply a matter of what is known as anthropic selection 鈥� questioning life will find itself in a universe suitable for questioning life. We鈥檙e here because we鈥檙e here, in other words.
For many cosmologists, the multiverse is as natural an answer as any. 鈥淚鈥檓 really quite comfortable with it,鈥� says Natarajan. Others, such as Rovelli, are less impressed. Some think the multiverse absolves physicists of all responsibility for trying to explain why things are as they are. Many criticise the lack, as yet, of any direct way to test the idea. Still, whether ours is the best of all possible worlds or just one of many, it seems reasonable to be grateful for what we have got.
